Cid Ramon_Bermejo M, CC, DUN INESTABLE ELEMENTO

Cid Ramon_Bermejo M, CC, DUN INESTABLE ELEMENTO

DUN INESTABLE ELEMENTO A UN ELEMENTO
INESTABLE: LEMBRANDO A MENDELEEV.
Manuel R. Bermejo Patiño
Dpto Química inorgánica (USC)
Ramón Cid Manzano
I.E.S. de SAR (Santiago)
I.
INTRODUCCIÓN
Na celebración do centenario da morte de Mendeleev, pareceuno
apropiado achegarmos ao persoaxe desde unha perspectiva diferente ao que
sería unha descrición biográfica máis ou menos tradicional. Tentaremos unha
aproximación desde tres ángulos diferentes, pero que abranguen tanto a
dimensión científica como humana do científico ruso. Mendeleev, percorre
connosco tres escenarios distintos pero que representan de xeito significativo
importantes momentos relacionados coa vida e obra dun home de ciencia desa
envergadura.
Un congreso que cambiou o destino da Química para sempre; a
débeda contraída con el pola comunidade científica por non ter sido
galardoado co Premio Nobel de Química; e a súa entrada de pleno dereito na
“súa” Táboa Periódica dos elementos químicos, ocupando o lugar 101. Eses
son os tres escenarios.
Naturalmente, e máis aló dunha mera lembranza do químico ruso,
preténdese aportar datos de interese sobre os tres tópicos antes sinalados, e
así axudarao profesor ou profesora interesados a coñecer mellor esoutros
aspectos da Ciencia.
II.
O CONGRESO DE KARLSRUHE.
O Congreso de Química celebrado en 1860 en Karlsruhe (Alemania)
foi determinante para que Mendeleev, cando volve a San Petersburgo o ano
seguinte, se dedique de forma definitiva á búsqueda dunha clasificación para
os elementos químico. As cuestións fundamentais presentadas para debater no
Congreso foron:
- Definición de nocións químicas importantes como aquelas que son
expresadas polas palabras:átomo, molécula, equivalente, peso atómico e
básico.
- Exame da cuestións de equivalentes e fórmulas químicas
- Establecer unha notación e unha nomenclatura axeitadas.
Atendendo a invitación formulada desde Karlsruhe asistiron 127
químicos de 12 países. Cremos que é interesante facer mención do número de
asistentes por pais pois así poderemos decatarnos da presenza da Química
como ciencia en cada pais:
Alemania (57), Francia (21), Gran Bretaña (17), Austria (7), Rusia
(7), Suiza (6), Suecia (3), Bélxica (3), Italia (2), España (1), Portugal (1) e
México (1).
Pág 1 de 26
Brevemente facemos referencia, por razóns obvias, do representante
español e portugués.
Así, Ramón Torres Muñoz de Luna (1822-1890), catedrático de la
Universidad Central de Madrid, foi discípulo de Liebig (traducindo cinco das
súas obras) e de Wurtz. É un dos máis destacados químicos españoles do
século XIX, con 22 obras originales publicadas, entre as que cabe destacar
“Leccións elementales de Química General” e unha breve biografía de Liebig.
Ademais, a el débese as especialidades farmacéuticas “Bicarbonato sódico
Torres Muñoz, comprimidos” e “Bicarbonato sódico Torres Muñoz, pó”.
Por outra parte, J. Augusto Simões-Carvalho (1822-1902), foi
profesor de Química na Universidade de Coimbra sendo autor do libro “Lições
de Philosophia Chimica”. Pódese considerar como o introductor em Portugal
de tódalas ideas trascendentes da primeria metade do século XIX no campo da
Química.
O gran protagonista do Congreso foi Stanislao Cannizzaro (18261910), cunhas intervencións cheas de entusiasmo e elocuencia. Tiña publicado
un estudo sobre a natureza atómica da química titulado "Sunto di un corso di
Filosofia chimica" (1858), e nas sesións insistíu na distinción, antes
hipotetizada por Avogadro, entre pesos moleculares e atómicos, que estivera
olvidada durante medio século (Avogadro morrera en 1856). A hipótese de
Avogadro podía usarse para determinar o peso molecular de varios gases,
podéndose así determinar a composición dos gases a partir do seu peso
molecular. Deu unha brilante conferencia sobre a hipótese de Avogadro,
describindo a forma de usala e explicando a necesidade dunha distinción clara
entre átomos e moléculas.
En 1860, Meendeleev era un xove bolseiro ruso de 26 anos que estaba
en Heildelberg traballando con Kirchhoff y Bunsen. Dada a proximidade entre
Heilderberg e Karlsruhe, e dado que Bunsen ía estar presente no Congreso,
non é estrano que o químico ruso tamén asisitise. Non obstante, non consta
que Mendeleev intervira, alomenos de xeito notorio. Pero si foi certo que
quedou impresionado pola exposición que fixo Cannizzaro da súas ideas sobre
os pesos atómicos. Lonxe estaba o xove químico ruso de aqueloutro que se
negaría a existencia do electrón, a radiactividade e outras outras moitas
cousas novas, levantando polémicas e abandoando furioso laboratorios e
conferencias por toda Europa.
Esa fonda pegada que Cannizaro causa en Mendeleev queda recollida
nas súas propias palabras:
“…En primeiro lugar, foi nesa época cando os valores numéricos dos pesos atómicos foron
definitivamente coñecidos. Dez anos antes non exisitía tal coñecemento …químcos de todas as partes
do undo foron a Karlsruhe para chegar a algún acordom senón respecto aos puntos de vista sobre os
átomos si nalgunha medida em relación á súa definitiva representación....Moitos deles
probablemente lembren ... canto terreo foi gañado no Congreso polos seguidores da teoría tan
brilantemente representada por Cannizzaro. Eu nidiamente recordo a impresión producida polas
súas intervencións ... baseadas nas concepcións de Avogadro, Gerhardt e Regnault, as cales estaban
naquela época lonxe de seren recoñecidas de xeito xeralizado.”
III.
O NOBEL NON CONCEDIDO.
O 30 de Agosto de 1997 a IUPAC emitía o seguinte comunicado:
"O Consello da Unión Internacional de Química Pura e Aplicada
aprobou hoxe as recomendacións finais para os nomes dos elementos 101
a 109. Nunha votación de 64 a 5 (con 12 abstencións) delegados de 40
países aceptaron o informe do Comité de Nomenclatura de Química
Pág 2 de 26
Inorgánica (CNIC), finalizando, xa que logo, tres anos de controversia
en relación cos nomes deses elementos de corta vida producidos
artificialmente."
O elemento 101 é o Mendelevio, en honra de Dmitri Ivanovich
Mendeleev. É certo que o nome deste elemento xa estaba en tódalas táboas
periódicas moito antes de 1997, pero digamos que de forma oficial e definitiva
quedou fixado hai dez anos.
As razóns desta demora foron debidas a disputas de tipo político e de
rivalidades persoais entre investigadores e entre centros de investigacións.
Así, os elementos 99 (Es) ou o 100 (Fm) foron identificados nos restos que
deixaban as probas das armas nucleares, polo que non sorprenden as
controversias entre a comunidade científica á hora de admitir os nomes para
estes elementos e os posteriores.
Pero no caso do Mendeleevio, non se pode aceptar que houbese que
esperar tanto tempo. Se se pedise o nome dun químicos ilustre en calquera
parte do mundo ao longo da historia, seguro que un dos máis citados,
seguramente o primeiro, sería Mendeleev. O químico ruso falecera o 2 de
febreiro de 1907, polo que noventa anos foron precisos para que o pai da
Táboa Periódica fose obxecto do oficial recoñecemento mundial por parte da
Química.
Non menos incomprensible é que non tivese recibido un dos
primeiros Premios Nobel, senón o primeiro, concedidos en Química. Os
premiados nos anos anteriores ao pasamento de Mendeleev, foron:
1901: Jacobus Henricus Van't Hoff polo descubrimento das leis
da dinámica química e da presión osmótica nas solucións químicas.
1902: Hermann Emil Fischer en recoñecemento polos seus
traballos no campo dos glícidos e polos seus estudios de síntese no
grupo da purina.
1903: Svante August Arrhenius en recoñecemento polos seus
extraordinarios servizos para o avance da Química a partir da súa
teoría electrolítica da disociación.
1904: William Ramsay en recoñecemento do seu papel no
descubrimento dos gases inertes e a determinación da súa posición no
sistema periódico.
1905:
Johann
Friedrich
Wilhelm
Adolf
Von
Baeyer
en
recoñecemento dos seus servizos no avance da química orgánica e da
industria química.
1906: Henri Moissan in recoñecemento polo illamento do Flúor
e por ter ideado o arco de forno que leva o seu nome.
Sen cuestionar os méritos de ningún dos científicos anteriores,
cremos que soamente cos traballos publicados por Mendeleev en 1869 e en
1871, nos que se presenta a primeira Táboa Periódica e onde se indican a
existencia de ocos onde irían elementos aínda sen descubrir (predecíndose
as súas propiedades), sería de abondo para ter sido merecente dun premio
Nobel.
IV.
O M ENDELEVIO NA TÁBOA PERIÓDICA.
O procedemento para a obtención destes transuránidos coñécese como
“Síntese neutrónica”. Trátase de bombardear con neutróns núcleos pesados e
Pág 3 de 26
agardar a que a desintegración radiactiva dos núcleos inestables actúe. Era,
pois, preciso bombardear Uranio con neutróns, polo que a partir de 1946
comezou a súa búsqueda sistemática aproveitando o funcionamento de
reactores nucleares -que como sabemos xeran intensos fluxos neutrónicos e
"queiman" Uranio- baixo a dirección de I.V.Kurchátov, en Rusia, e de
G.T.Seaborg en EEUU.
Paralelamente á síntese neutrónica, comezouse a utilizar outra
estratexia: bombardear brancos de elementos pesados con ións acelerados.
Por razóns obvias, comezouse coas partículas máis fáciles de
acelerar: os núcleos de deuterio ou helio. Desta forma foron sintetizados os
elementos 94, 96, 97 e 98 e o último en ser obtido foi o 101 - o Mendeleeviono ano 1955:
Neste experimento, como branco servía unha capa fina duns mil
millóns de átomos de Es–253 -imperceptible para os ollos- aplicada sobre
unha folla finísima de ouro. Esta folla foi irradiada no acelerador de Berkeley
(California) con partículas alfa. Estas partículas movíanse cunha velocidade
de preto de 50000 km/s e expulsaban aos núcleos de Einstenio cos que
colisionaban. Estes núcleos chegaban a unha segunda folla de ouro situada
detrás da primeira e alí “asentábanse” pero xa como Md-256. Foron
recollidos soamente 17 átomos, pero os “artistas” chamados Albert Ghiorso,
Bernard G. Harvey, Gregory R. Choppin, Stanley G. Thompson, and Glenn
T.Seaborg foron quen de “identificalos”.
Unhas poucas semanas antes de cumprirse o centenario da morte de
Mendeleev, a revista Physical Reviews C publicaba un artículo no que se daba
a coñecer a síntese do último dos elementos en entrar no sistema periódico, o
elemento 118. Tras miles de horas de bombardear un branco de californio
enriquecido cun feixe acelerado de ions calcio, detectábanse tres átomos deste
novo elemento.
249
Cf
+
48
Ca 297
118
Nesta ocasión, foron dous equipos de 20 científicos rusos de Dubna e
10 norteamericanos de Berkeley quen lograron esta proeza científica. Unha
vez máis, a Táboa Periódica proposta por Mendeleeev en 1869 saía triunfante.
V.
FINALIZANDO.
Seguramente Mendeleev estaría encantado se soubera que o seu nome
está presenta na Táboa Periódica, e quizáis non lle dera demasiada
importancia a non ter sido premiado co Nobel. A fin de contas, o clube dos
que están nesa Táboa e “moito máis selecto”, pois hai unha decena de nomes
de persoas no sistema periódico e uns dous centos na lista de premiados en
Química pola academia sueca (algo parecido seguro que pensaría Lise
Meitner).
Pág 4 de 26
Por último, desexamos reiterar a importancia dos Congresos
(Reunións, Xornadas, Encontros, etc) no avance do coñecemento. Lugar onde,
máis veces das que se pensa, xurden iniciativas, ideas, accións, desatáscanse
vellas propostas, renóvanse compromisos ou se impulsan novos proxectos.
Sirva tamén, así, de pequena homenaxe a tódolos que teñen colaborado nos
CONGRESOS DE ENCIGA agora que vimos de celebrar o VIXÉSIMO.
Para os que pensan que a xente vai aos congresos simplemente “a
pasalo ben” hai que lembrarlles a famosa frse de Chesterton: “O contrario de
serio non é divertido, o contrario de serio é aburrido”.
Que poidamos seguir
congresos máis, polo menos.
celebrando,
divertidamente,
VI.
outros
vinte
BIBLIOGRAFÍA
1. ACCOUNT OF THE SESSIONS OF THE INTERN. CONGRESS OF CHEMISTS
IN KARLSRUHE. Tomado de SELECTED CLASSIC PAPERS
(http://web.lemoyne.edu/~GIUNTA/papers.html).
2. BERMEJO, M.R., GONZALEZ-NOYA, A.M. E VAZQUEZ M. (2006). “O nome
e o símbolo dos elementos químicos”. XUNTA DE GALICIA (Centro Ramón
Piñeiro para a Investigación en Humanidades). SANTIAGO.
3. BERMEJO M.R., e CID R., (1998). “Sobre a I.U.P.A.C., os nomes dos
novos elementos químicos e outras cousas”. Boletín das Ciencias (XII
Congreso de ENCIGA). Santiago.
4. BROCK, W.H. (1998). “Historia de la Química”. ALIANZA EDITORIAL
(CIENCIA Y TECNOLOGÍA). MADRID.
5. CID R. (1998). “Os últimos elementos da Táboa Periódica”. BOLETIN DAS
CIENCIAS, 33.
6. DE MILT CLARA (1951) “The Congress at Karlsruhe”. J. Chem. Educ., 28,
pp. 421-425.
7. MENDELEEV (1869). “The relation between the properties and atomic
weights of the elements”. Tomado en
http://www.rod.beavon.clara.net/periodic1.htm
8. MENDELEEV (1871). “A natural system of the elements and its use in
predicting the properties of undiscovered elements”. Tomado en
http://www.rod.beavon.clara.net/neweleme.htm.
9. MENDELEEV, D.I. (1889). “The Periodic Law of the Chemical Elements”.
Journal of the Chemical Society (London) 55, 634-656. Tomado en
http://web.lemoyne.edu/~GIUNTA/EA/ MENDELann.HTML.
10.
OGANESSIAN Yu. Ts., K MOODY. J., et al (2006).“Synthesis of the
isotopes of elements 118 and 116 in the 249Cf and 245Cm+48Ca fusion
reactions”. Phys. Rev. C 74, 044602
11.
ROMÁN POLO P. (2000), “El congreso de Karlsruhe y sus personajes”
Anales de la Real Sociedad Española de Química, 96 (4), pp 45-53.
12.
STRATHERN, P. (2000). “El Sueño de Mendeleiev. De la Alquimia a la
Quimica”. Siglo XXI de España Editores. Madrid.
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ANEXO I
Stanislao Cannizzaro, F.R.S. (1826-1910) and the First International Chemical
Conference at Karlsruhe in 1860 Harold Hartley
Notes and Records of the Royal Society of London, Vol. 21, No. 1 (Jun., 1966), pp. 56-63
Pág 6 de 26
ANEXO II
ACCOUNT OF THE SESSIONS OF THE INTERNATIONAL CONGRESS
CHEMISTS IN KARLSRUHE, ON 3, 4, AND 5 SEPTEMBER 1860
OF
originally published in Richard Anschütz, August Kekulé, 2 vols. (Berlin:
Verlag Chemie, 1929) as Appendix VIII (pp. 671-88 of vol. 1); English
translation by John Greenberg and William Clark published in Mary Jo
Nye, The Question of the Atom (Los Angeles: Tomash, 1984)]
It was Mr. Kekulé's idea to bring about an international meeting of chemists. During the fall of 1859 he
had an opportunity to make the initial overtures in this regard--first to Mr. Weltzien, then to Mr. Wurtz.
At the end of March 1860, these three scientists, all in Paris at that moment, devised the initial steps to be
taken in order to carry out the plan in question. An initial circular was composed, which had as its aim
winning the support of the most outstanding men of the science. It noted, in general terms, the differences
that had arisen between the theoretical views of chemists and the urgency of putting an end to these
differences by a common agreement, at least where certain questions were concerned.
The first appeal having been favorably received, an understanding was reached on the time and place of
the meeting, and printing of a second circular addressed to all European chemists, which explained the
objectives and goals of an international congress in the following terms was agreed upon:[2]
Paris, 15 June 1860
Dear Distinguished Colleague,
The great development that has taken place in chemistry in recent years, and the
differences in theoretical opinions that have emerged, make a Congress, whose goal is the
discussion of some important questions as seen from the standpoint of the future progress
of the science, both timely and useful.
The undersigned invite to this meeting all chemists authorized by their work or position to
express an opinion in a scientific discussion.
Such an assembly cannot deliberate on behalf of everyone, nor can it pass resolutions by
which everyone must abide, but by means of a free and thorough discussion, certain
misunderstandings could be eliminated, and a common agreement facilitated on some of
the following points: the definition of important chemical notions, such as those expressed
by the words atom, molecule, equivalent, atomic, basic; the examination of the question of
equivalents and of chemical formulae; the institution of a notation and of a uniform
nomenclature.
Knowing that the assembly's deliberations would not be of a nature such as to reconcile all
opinions and eliminate all disagreements immediately, the undersigned believe,
nevertheless, that such works could pave the way for a much desired agreement between
chemists in the future, at least where the most important questions are concerned. A
commission could be charged to continue the investigation of these questions and to
interest in them learned academies or societies with the necessary material means for
resolving them.
The Congress will convene in Karlsruhe on 3 September 1860.
Pág 7 de 26
Our colleague, Mr. Weltzien, Professor at the Polytechnic School in this city, wishes to
take on the duties of General Commissioner. In this capacity, he will be in charge of
registering prospective members for the Congress and will open the assembly at nine
o'clock in the morning on the day indicated.
In conclusion, and with the aim of avoiding any unfortunate omissions, the undersigned
request that the individuals to whom this circular will be sent please communicate it to
their scientist friends who are duly authorized to attend the planned conference.
Babo de, Freiburg
Fremy, Paris
Pelouze, Paris
Fritzsche,
Balard, Paris
St.
Petersburg
Hofmann,
Bekétoff, Kasan
A.
W.,
London
Piria, Turin
Regnault, V., Paris
Boussingault, Paris
Kekulé, Ghent
Brodie, Oxford
Kopp, H., Geissen
Bunsen, Heidelberg
Hlasiwetz, Innsbruck
Bussy, Paris
Liebig, J. de, Munich
Staedler, Zurich
Cahours, Paris
Malaguti, Rennes
Stas, Brussels
Cannizzaro, Genoa
Marignac, Geneva
Strecker, Tübingen
Deville, H., Paris
Mitscherlich, Berlin
Dumas, Paris
Odling, London
Engelhardt,
St.
Petersburg
Erdmann,
O.
Leipzig
L.,
Pasteur, Paris
Payen, Paris
Roscoe, Manchester
Schroetter,
A.,
Vienna
Socoloff,
St.
Petersburg
Weltzien,
C.,
Karlsruhe
Will, H., Giessen
Williamson,
W.,
London
Wöhler,
F.,
Göttingen
Fehling de, Stuttgart
Pebal, Vienna
Wurtz, Ad., Paris
Frankland, London
Peligot, Paris
Zinin, St. Petersburg
Nota Bene: You can sign up for the conference either directly with Mr. Weltzien,
Polytechnic School, Karlsruhe, or with Mr. A. Kekulé, Professor of Chemistry at the
University of Ghent, who will pass it on to Mr. Weltzien.
Pág 8 de 26
The number of people who wanted to participate was considerable, and on 3 September 1860, 140
chemists[3] met together in the meeting room of the second Chamber of State, which was made available
by the Archduke of Baden.
The list of the chemists in attendance follows:[4]
I. BELGIUM. Brussels: Stas; Ghent: Donny, A. Kekulé.
II. GERMANY. Berlin: Ad. Baeyer, G. Quinke; Bonn: Landolt; Breslau:
Lothar Meyer; Kassel: Guckelberger; Klausthal: Streng; Darmstadt:
E. Winkler; Erlangen: v. Gorup-Besanez; Freiburg i.B.: v. Babo,
Schneyder; Giessen: Boeckmann, H. Kopp, H. Will; Göttingen: F.
Beilstein; Halle a.S.: W. Heintz; Hanover: Heeren; Heidelberg:
Becker, O. Braun, R. Bunsen, L. Carius, E. Erlenmeyer, O. Mendius,
Schiel; Jena: Lehmann, H. Ludwig; Karlsruhe: A. Klemm, R. Muller,
J. Nessler, Petersen, K. Seubert, Weltzien; Leipzig: O. L. Erdmann,
Hirzel, Knop, Kuhn; Mannheim: Gundelach, Schroeder; Marburg
a.L.: R. Schmidt, Zwenger; Munich: Geiger; Nuremberg: v. Bibra;
Offenbach: Grimm; Rappenau: Finck; Schönberg: R. Hoffmann;
Speyer: Keller, Mühlhaüser; Stuttgart: v. Fehling, W. Hallwachs;
Tübingen: Finckh, A. Naumann, A. Strecker; Wiesbaden: Kasselmann,
R. Fresenius, C. Neubauer; Würzburg: Scherer, v. Schwarzenbach.
III. ENGLAND. Dublin: Apjohn; Edinburgh: Al. Crum Brown, Wanklyn, F.
Guthrie; Glasgow: Anderson; London: B. J. Duppa, G. C. Foster,
Gladstone, Müller, Noad, A. Normandy, Odling; Manchester: Roscoë;
Oxford: Daubeny, G. Griffeth, F. Schickendantz; Woolwich: Abel.
IV. FRANCE. Montpellier: A. Béchamp, A. Gautier, C. G. Reichauer;
Mülhousen i.E.: Th. Schneider; Nancy: J. Nicklès; Paris:
Boussingault, Dumas, C. Friedel, L. Grandeau, Le Canu, Persoz, Alf.
Riche, P. Thénard, Verdét, Wurtz; Strasbourg i.E.: Jacquemin,
Oppermann, F. Schlagdenhaussen, Schützenberger; Tann: Ch.
Kestner, Scheurer-Kestner.
V. ITALY. Genoa: Cannizzaro; Pavia: Pavesi.
VI. MEXICO. Posselt.
VII. AUSTRIA. Innsbruck: Hlasiwetz; Lemberg: Pebal; Pesth: Th.
Wertheim; Vienna: V. v. Lang, A. Lieben, Folwarezny, F. Schneider.
VIII. PORTUGAL. Coïmbra: Mide Carvalho.
IX. RUSSIA. Kharkov: Sawitsch; St. Petersburg: Borodin, Mendelyeev; L.
Schischkoff, Zinin; Warsaw: T. Lesinski, J. Natanson.
X. SWEDEN. Harpenden: J. H. Gilbert; Lund: Berlin, C. W. Blomstrand;
Stockholm: Bahr.
XI. SWITZERLAND. Bern: C. Brunner, H. Schiff; Geneva: C. Marignac;
Lausanne: Bischoff; Reichenau bei Chur: A. v. Planta; Zurich: J.
Wislicenus.
XII. SPAIN. Madrid: R. de Suna.
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First Session of the Congress
Mr. Weltzien, General Commissioner, opened the first session
with the following speech:
Gentlemen:
As provisional chairman, I have the honor to inaugurate a Congress which has no
precedent for its kind, the nature of which has never before met. To be sure, German
Natural Scientists and Physicians, upon the instigation of Oken, and emulating their Swiss
Colleagues, have assembled almost annually for scientific conferences in various cities of
the Fatherland since 1822. Following the lead of such congresses, English, French, and
during the past several years, also Scandinavian Natural Scientists have convened for a
similar purpose. Devotees of the different branches of Natural Science and Medicine were
regularly present, although all participants were invariably of the same nationality. The
business of these congresses was for the most part characterized by reports, the topics of
which were not integrated into any previously arranged program, but rather, as
presentations of work in progress, left to the discretion of each individual. A lively and
amicable intercourse, flavored by a sequence of festivities, united the ethnically and
linguistically related Natural Scientists and Physicians for several days.
Not so for the Congress convened here today. For the first time, the representatives of a
single, and indeed the newest Natural Science have assembled. These representatives
belong, however, to nearly every nationality. We may be of differing ethnic origin and
speak different languages, but we are related by professional specialty, are bound by
scientific interest, and are united by the same design. We are assembled for the specific
goal of attempting to initiate unification around points of vital concern for our beautiful
science. Due to the extraordinarily swift development of Chemistry, and especially because
of the massive accumulation of factual materials, the theoretical standpoints of researchers
and the means of expression, both in words and symbols, have begun to diverge more than
is expedient for mutual understanding, and, especially, more than is suitable for
instruction. Considering the importance of Chemistry for other natural Sciences and its
indispensability for technology, it seems exceedingly desirable and advisable to cast our
science in a more rigorous form, so that it will be possible to communicate it in a relatively
more concise manner.
In order to achieve this, we should not be constrained to only review various viewpoints
and writing conventions, the variety of which offers little of importance; and we should not
be burdened with a nomenclature, which in view of a plethora of unnecessary symbols
lacks any rational basis, and which, making matters worse, is derived, for the most part,
from a theory whose validity can hardly be maintained today. The ample attendance at this
Congress is surely a clear indication that these nuisances are universally recognized and
that their removal from the path toward unification appears desirable. The achievement of
this end is a prize of such beauty that it is well worth the effort to undertake the task here.
The original notion of a Chemistry-Congress was communicated to me some time ago by
our colleague Kekulé. Early this year, I initiated the first steps toward its realization. The
ripeness of this undertaking was manifoldly acknowledged; I obtained unsolicited support
from all quarters. Because of this, I do not doubt that this Congress will be called upon to
lay the foundations for a not unimportant epoch in the history of our science.
The city of Karlsruhe, which two years ago had the great fortune to host one of the most
splendid congresses of German Natural Scientists and Physicians, has now the honor of
Pág 10 de 26
seeing the first international Chemistry-Convention within its city walls. Karlsruhe is the
capital of a small but blessed province, in which, under the auspices of a noble prince and a
liberal government, the Arts and Sciences flourish, and where its devotees, esteemed and
sustained, can follow their calling with devotion and good cheer. Since it is my pleasure to
bid you a hearty welcome to this city, I expect that the same good cheer will permeate our
Congress, and hope that our science will one day look back with satisfaction upon our
assembly.
After this speech the general commissioner first asks Mr. Bunsen to preside, but the latter refuses and asks
the assembly to encourage Mr. Weltzien to direct the deliberations during the first session. Mr. Weltzien
is named President. Messrs. Wurtz, Strecker, Kekulé, Odling, Roscoe, and Schischkoff are appointed to
serve as Secretaries.
At the suggestion of Mr. Kekulé, the assembly decides that a commission will be put in charge of drawing
up a list of questions that will be submitted to the Congress for deliberation.
Mr. Kekulé takes the floor in order to explain the schedule of questions. (Then an analysis of Mr. Kekulé's
speech follows.[5])
Mr. Erdmann emphasizes the necessity of directing the assembly's deliberations and resolutions towards
questions of form, rather than doctrinal points.
A discussion begins over the question of whether the Commission will hold its sessions behind closed
doors or during plenary sessions of the entire assembly.
After some words among Messrs. Fresenius, Kekulé, Wurtz, Boussingault, and H. Kopp, the assembly
decides, upon the motion made by the last individual named, that the sessions will take place behind
closed doors.
First Session of the Commission
The commission met on 3 September at 11 A.M., Mr. H. Kopp
presiding.
The chairman suggests that the discussion begin with the notions of molecule and atom, and he asks Mr.
Kekulé and Mr. Cannizzaro, whose studies have especially encompassed this issue, to take the floor.
Mr. Kekulé emphasizes the need to distinguish between the molecule and atom, and, in principle at least,
the physical molecule and the chemical molecule.
Mr. Cannizzaro is unable to conceive of the notion of the chemical molecule. For him there are only
physical molecules, and the Ampère-Avogadro law is the basis for considerations relating to the chemical
molecule. The latter is nothing other than the gaseous molecule.
Mr. Kekulé thinks, on the contrary, that the chemical facts must serve as the basis for the definition and
determination of the (chemical) molecule and that physical considerations should only be invoked as a
check.
Mr. Strecker points out that in certain cases the atom and molecule are identical, as in the case of
ethylene.
Mr. Wurtz says that a certain difficulty can be sensed in defining the chemical molecule of oxygen and the
diatomic elements in general, which are comparable to ethylene. The view of these as molecules formed
of two atoms derived from physical considerations[6] but until now no chemical fact appears to militate in
favor of this doubling.
Mr. H. Kopp, summarizing the discussion, says that the need to separate the idea of the molecule from
that of the atom appears to be established; that the notion of the molecule can be fixed with the help of
purely chemical considerations; that the definition does not have to involve density alone; and, finally,
that it appears natural to call the largest quantity the molecule, and the smallest quantity the atom. In
concluding, the speaker formulates the first question to be put to the assembly. This question is as
follows: "Is it appropriate to establish a distinction between the terms molecule and atom, and to call
molecules, which are comparable as far as physical properties go, the smallest quantities of bodies which
enter into or come out of a reaction, and to call atoms the smallest quantities of bodies which are
contained in these molecules?"
Pág 11 de 26
Mr. Fresenius calls attention to the expression compound atom and said that these two words entail a
contradiction. Mr. Fresenius's remark motivates the drawing up of a second question to be submitted to
the assembly, which is as follows: "Can the expression compound atom be eliminated and replaced by the
expressions radical or residue?"
Mr. Kopp goes back to the program explained by Mr. Kekulé, and he calls attention to the definition of
the word equivalent. It seems to him that the notion of equivalent is perfectly clear and is sharply
distinguished from the notion of molecule and that of atom. Consequently, the commission adopts,
without discussion, the third proposition to be submitted to the assembly, which is as follows: "The notion
of equivalents is empirical and independent of the idea of molecule and that of atom."
The session continues, with Mr. Erdmann presiding.[7] The discussion on notation gets underway: Mr.
Kekulé points out that molecular and atomic notation, or the notation of equivalents, can be employed, but
that as in any system it is necessary to stick rigorously to the particular notation, whichever it be, once
adopted.
The meaning of the word "equivalent" is the object of several remarks. Mr. Béchamps says that
equivalence can only be assumed in cases where the functions of bodies are identical.
Mr. Schischkoff is not of the same opinion. He thinks that the notation for equivalence and equivalent
quantities is independent of chemical functions. Everyone assumes an equivalence between chlorine and
hydrogen. After a few observations presented by other members on the same subject, the session was
adjourned.
Second Session of the Congress
Mr. Boussingault presiding
In taking the chair, Mr. Boussingault thanks the Congress for having bestowed the honor of presiding
upon a scientist whose studies have had matters of applied chemistry as their object, rather than points of
abstract theory. The chairman sees in this choice that the Congress wanted to testify to the unity of the socalled old and new chemistries. He protests against these terms, and remarks that it is not chemistry that
grows old, but chemists.
The chairman announces that the work of the Commission is not ready, but that it has agreed upon the
drawing up of three questions to be submitted to the assembly for deliberation. He asks one of the
Secretaries to make these known to the assembly.
Mr. Strecker takes the floor and reads to the assembly the questions drafted by the commission and
indicated above.
Mr. Kekulé enlarges upon the points specified in the first question. Concerning the fundamental
hypothesis which can be made about the nature of matter, the speaker wonders if it is necessary to adopt
the atomic hypothesis or if a dynamical hypothesis is enough. The first alternative seems preferable to
him. Dalton's hypothesis was verified by everything known about the nature of gases. One is authorized to
assume small units or small components in gases, and when the same body can affect the gaseous state,
the solid state, and the crystalline state, it is possible that the crystalline molecules are precisely the small
gaseous components in question, or that these are a fraction of others. But the nature of these relations
cannot be specified. What is certain is that in chemical reactions there exists a quantity that enters into or
comes out of reaction in the smallest proportion, and never as a fraction of this proportion. These
quantities are the smallest that can exist in a free state. These are the molecules defined chemically. But
these quantities are not indivisible; chemical reactions succeed in cutting them and resolving them into
absolutely indivisible particles. These particles are atoms. The elements themselves, when they are free,
consist of molecules formed of atoms.[8] Thus the free chlorine molecule is formed from two atoms. This
leads one to assume different molecular and atomic units:
1. physical molecules
2. chemical molecules
3. atoms
Pág 12 de 26
The gaseous physical molecules have not been shown to be identical with the physical molecules of solids
and liquids. Secondly, the chemical molecules have not been shown to be identical with gaseous
molecules. Thus it is not established if the smallest quantity of a substance that enters into a reaction is
also the smallest quantity of this substance that plays a role in heat phenomena.
It must be said, however, that the chemical molecule is normally identical to the physical molecule. It has
even been maintained that the first never represents anything more than the second. For the speaker it is
not like that. The chemical molecule has an independent existence, and in order to allow the distinction in
question to be assumed, it is enough to demonstrate its reality in a few cases. But that is easy. Has it not
been shown that, for the density of sulphur vapor, the chemical molecules do not always completely
separate from one another, but remain fused together in certain conditions (at 500°) to form physical
molecules?
The speaker adds that the existence and magnitude of chemical molecules can and must be determined by
chemical demonstrations and that the physical facts are not enough to achieve this result.[9] With the help
of physical considerations, how could it be shown that hydrochloric acid is formed of a single hydrogen
atom and a single chlorine atom? Would it not be enough to multiply the formula HCl by a certain
coefficient, and to do the same for all other formulae, in order to establish a perfect agreement between
physical properties?
Mr. Cannizzaro takes the floor in order to point out that the distinction between physical and chemical
molecules appears to him neither to be necessary nor clearly established.
Mr. Wurtz expresses the opinion that this is a secondary point and can be reserved. It seems to him, on the
contrary, that the question relative to establishing the distinction between the terms "molecule" and
"atom" has nearly been concluded, and that everyone seems to recognize the utility of such a distinction. It
is a matter of clarifying the sense of words generally in common use; it is purely a matter of a definition,
and the speaker thinks that in a question of this type, there would perhaps be propriety and usefulness in
the expression, by the assembly, of an opinion following the discussion. This opinion, moreover, would
not bind anyone, and there would be nothing obligatory about it.
Discussion of the second question relative to the words "compound radical" begins. Mr. Miller thinks that
scientific language could not do without the words "compound atom." There are atoms of simple
substances; there are atoms of compound substances.
Messrs. Kekulé, Natanson, Strecker, Ramon Torres de Luna, Nicklès, Béchamps and other members
present varied observations in one direction or another, but the discussion of this question, like that of the
preceding one, leads to no resolution from the assembly.
Second Session of the Commission
Mr. H. Kopp presiding
Mr. Kekulé explains his ideas on chemical notation. He points out that either an atomic-molecular
notation or a notation in equivalents can be employed. In the first case, the chemical formula represents
the molecule; in the second, it represents equivalence. The following examples illustrate this distinction:
atomic molecular notation
notation
in
||
equivalents
H Cl
H Cl
H2O
HO
H3Az
H az[10]
The important thing is not to mix these notations and get them confused, as is often done. They get mixed
up if water is written HO = 9 and ammonia, H3Az = 17, etc.
Mr. Cannizzaro stresses the importance of considerations relating to volumes in the question of notation.
The arguments elaborated by the speaker are reproduced in extenso in the report of the third session of the
Congress (see ahead).
Pág 13 de 26
The chairman draws attention to the overly detailed course of the discussion, and he points out that,
within the Commission, the questions should be indicated rather than investigated. He also deems that the
discussion relative to notation in equivalents, such as had just been formulated by Mr. Kekulé, can be left
aside. No one is served by it. The speaker believes that it would be proper not to get too attached to
theoretical matters concerning the content of things, and to stick to questions of form. Several members
express a similar opinion, and Mr. Erdmann, in particular, draws attention to the urgency of adopting a
notation whose symbols always represent one and the same given value.
Summarizing the discussion, the chairman acknowledges that given the recent advances in the science, it
is likely that certain atomic weights ought to be doubled, but that it would be useful to take into
consideration the notation that has, until now, generally been employed in introducing notation to
represent these double weights and not to adhere too rigorously to the symbols in the latest notation
representing different values. As a transitional measure and to avoid confusion, he thinks it convenient to
adopt certain signs to indicate the differences in question. Consequently, the chairman approves of the
habit of some chemists, that of barring double atomic weights. In concluding, he formulates the question
to be submitted before the Congress in the following manner:
"Is it desirable to harmonize chemical notation with recent advances in the science by doubling a certain
number of atomic weights?"
Third Session of the Commission
Mr. Dumas presiding
Mr. Kekulé summaries the discussion of the preceding session and repeats the question, announced by
Mr. Kopp, in a slightly mitigated form. According to the speaker, this question should be posed in the
following way:
"Do the recent advances in science warrant a change in notation?"
Mr. Strecker proposes that atomic notation be adopted in principle.
The chairman stresses forcefully the disadvantages that result from current confusion. He points out that
this state of affairs, were it to continue, would be such as to undermine not only the proper direction of
teaching and advances in science, but the reliability of industrial work as well. Let us think back, says the
chairman, to what we remember from twenty years earlier. Berzelius's table of atomic weights was both
the underlying support for the whole science of chemistry and the infallible guide to industrial operations.
There is nothing today to replace this universally acknowledged authority, and we have to be careful that
chemistry does not fall from the high rank that it has enjoyed among the sciences until now.
Mr. Wurtz is pleased to acknowledge that Mr. Dumas has gotten to the core of the issue, and he thinks it
necessary to return to the principles of atomic weights and to Berzelius's notation. According to the
speaker, marginal changes in the interpretation of some facts would suffice to bring the principles and this
notation into harmony with the requirements of modern science. The notation suitable for adoption today
is not exactly Gerhardt's. Gerhardt rendered enormous services to the science. Today he is dead, and his
name, the speaker says, should only be spoken with respect. However, it seems that this chemist made two
mistakes. One concerns form alone, the other is inherent in the root of things.
First, instead of presenting his notation as founded upon new principles, he more moderately linked it to
Berzelius's principles, thus sheltering his innovation under the authority of this great name. Secondly, it
seems that Gerhardt made a mistake in likening all of the oxides of inorganic chemistry to silver oxide
and to anhydrous potassium oxide, and in attributing to them, as in the case of the latter, the formula
.
In organic chemistry there should be oxides corresponding to ethylene oxide, just as there are
representatives of ethyl oxide and of others that correspond to glycerol oxide, and if potassium hydrate,
for example, can be compared to alcohol, then other hydrates should be compared with glycol and
glycerine. It is understandable that these considerations are such as to prompt and justify some changes in
Gerhardt's notation and in the atomic weights that he attributed to certain metals.
After a discussion in which Messrs. Cannizzaro, Wurtz, and Kekulé take part, Mr. Kekulé opines that the
question is well enough prepared to submit to the Congress for deliberation, and he asks that the writing
of this question be entrusted to the Secretaries.
Pág 14 de 26
This proposition is adopted by the Commission.
Third Session of the Congress
Mr. Dumas presiding
In taking the chair, the chairman addresses some words of thanks to the assembly and expresses the hope
that a common agreement on some of the questions aired before the Congress will be reached.
Next, the Chairman suggests to the assembly that two Vice-chairmen be designated. Messrs. Will and
Miller are appointed to these positions. Mr. Odling replaces Mr. Roscoe, who had to leave, as Secretary.
Next, the Secretaries read the questions, whose writing had been entrusted to hem, and which have been
worked out by the Commission. These questions are conceived as follows:
"Is it desirable to harmonize chemical notation with advances in the science?"
"Is it appropriate to adopt the principles of Berzelius again, where notation is concerned, in bringing about
some modifications to these principles?"
"Is it desirable to distinguish new chemical symbols from those which were generally in use fifteen years
ago with the help of particular signs?"
Mr. Cannizzaro takes the floor in order to oppose the second proposition. It scarcely appears fitting or
logical to him to move science back to the time of Berzelius, so as to make chemistry again cover the path
that it has already taken. In effect, Berzelius's system has already undergone successive modifications, and
these modifications have led to Gerhardt's system of formulae. And these changes were not at all
introduced abruptly or without transitions into the system or without transitions into the system; they were
the result of successive advances. If Gerhardt had not proposed them, Mr. Williamson or Mr. Odling, or
another chemist who had taken part in the evolution of science, would have done so.
"The source that Gerhardt's system goes back to is the Avogadro-Ampère theory of the uniform
constitution of substances in the gaseous state. This theory leads us to view the molecules of certain
simple substances as susceptible to division in the future. Mr. Dumas understood the importance of
Avogadro's theory and all of its consequences. He posed this question: Is there agreement between the
results of Avogadro's theory and the results deduced by means of other methods used to determine the
relative weights of molecules? Realizing that science was still short of experimental results of this kind,
he wanted to gather together the largest possible number before risking any general conclusion on this
subject. Thus he got down to work, and with the help of the method which he applied to determine vapor
densities, he furnished science with valuable results. However, it appears that he never got far enough
along with the method to be able to infer from the results acquired the general conclusion for which he
aimed. Be that as it may concerning this reserve he thought necessary, it can be said that it was he who put
chemists on the path to Avogadro's theory, because he, more than anyone else, was responsible for
introducing the habit of choosing formulae for volatile substances corresponding to the same volume as
that taken up by hydrochloric acid and ammonia.
"The most evident display of this influence of Mr. Dumas's school appears in a paper by one of his
students, Mr. Gaudin. Mr. Gaudin accepted Avogadro's theory without reservation. He established a clearcut distinction between the words atom and molecule, by means of which he was able to reconcile all facts
with theory. This distinction had already been made by Mr. Dumas, who had called the molecule the
physical atom in his lessons on chemical philosophy. It is certainly a mainspring of Gerhardt's system.
"Sticking more closely to Avogadro's theory than Gerhardt did later, and taking advantage of new
experimental data on vapor densities, Mr. Gaudin established that atoms are not always the same fraction
of the molecules of simple bodies--that is to say, these molecules do not always result from the same
number of atoms; while the molecules of oxygen, hydrogen, and other halogens are formed from two
atoms, the molecule of mercury is made of a single atom. He went so far as to compare the composition of
equal volumes of alcohol and ether in order to deduce the relative composition of their molecules. But his
mind did not seize upon all of the results of this comparison, and chemists have forgotten the idea that he
had. And yet this comparison was one of the starting points for Gerhardt's proposed reform.
"Other chemists, Proust among them, also accepted Avogadro's theory and arrived at the same general
conclusions as Mr. Gaudin.
"What did Gerhardt do in this state of the science?
Pág 15 de 26
"He accepted Avogadro's theory and the consequence that atoms of simple bodies are divisible, and he
applied this theory to deduce the relative make-up of the molecules of hydrogen, oxygen, chlorine,
nitrogen, hydrochloric acid, water, and ammonia. If he had stopped there, he would not have gotten ahead
of Avogadro and Mr. Dumas. But he then subjected all of the formulae of organic chemistry to a general
investigation, and he realized that all of these formulae corresponding to equal volumes of hydrochloric
acid and ammonia were confirmed by all reactions and by all chemical analogies. Thus he contemplated
modifying formulae that were the exception to the rule introduced by Mr. Dumas. He tried to show that
the reasons for the violation of the equal volumes rule were unfounded. To reduce the formulae of all
volatile substance of organic chemistry to equal volumes had been the starting point for Gerhardt's
proposed reforms. The modifications of atomic weights of certain simple substances, the discovery of the
relations that the hydrates, whether acidic or basic, have with water, had been the consequence of this first
step. What happened next? The unforgettable experiments of Mr. Williamson on etherification, on mixed
ethers, on acetones, those of Gerhardt on anhydrous acids, those of Mr. Wurtz on alcoholic radicals, etc.,
successively confirmed what Gerhardt had predicted as a consequence of his system. Thus there occurred
in chemistry something analogous to what happened in optics when the undulatory theory was introduced.
This theory predicted with wonderful accuracy the facts that experiments later confirmed. Gerhardt's
system in chemistry was not less fruitful in exact predictions. It is intimately mixed up with and tied to all
of the works of chemistry which had preceded it and to all of the advances that followed it in the history
of the science. It is not an abrupt leap, an isolated event. It is a regular step forward, small in appearance,
but large in results. From now on, this system cannot be effaced from the history of science. It can and
must be discussed and modified. But it is the system that must be taken as the starting point, when it is a
matter of introducing into chemical science a system of formulae in accord with the actual state of our
knowledge. Some chemists will perhaps be tempted to say: the difference between Gerhardt's formulae
and those of Berzelius is very small, because the formula for water, for example, is the same in the two
systems. But we must be careful. The difference is very small in appearance, but it is large at bottom.
Berzelius was under the influence of Dalton's ideas. The idea of a difference between the atom and the
molecule of substances never entered his mind. In all of his arguments he assumed implicitly that atoms
of simple substances, are, vis-à-vis physical forces, units of the same order, compound atoms. For this
reason he began by assuming that equal volumes contain the same number of atoms. Soon he realized that
this rule could only be applied to simple substances, and throughout the whole of his scientific career, he
attributed no value to atoms of compound substances in choosing formulae. He was even forced to restrict
the rule for equal numbers of atoms in equal volumes to a very small number of simple substances--that
is, to those that are permanent gases--thereby introducing into the makeup of gases and vapors a
difference that no physicist was ever able to admit. Berzelius did not assume that molecules of simple
substances could divide in combining. On the contrary, he assumed that two molecules often form the
quantity that enters wholly into the combination. This is what he called double atoms. Thus he assumed
that water and hydrochloric acid contain the same quantity of hydrogen--a quantity equal to two physical
molecules joined together.
"So you see, gentlemen, what a profound difference exists between the ideas of Berzelius and those of
Avogadro, Ampère, Mr. Dumas, and Gerhardt.
"I am surprised that Mr. Kekulé, who said in his book that Gerhardt is the first and only one who
completely understood the atomic theory, has accepted the commission's proposition.
"I believe that I have shown, Mr. Cannizzaro continued, that a discussion of formulae must take as
starting points, the formulas of Gerhardt, but I do not maintain that all of them must be accepted in the
form that he proposed them. Far from it; I tried, some years ago, to introduce certain modifications into
them, in such a way as to avoid the inconsistencies which appeared to me to exist in Gerhardt's system. In
effect, it is strange to see how this chemist renounced Avogadro's theory after having used it as the basis
for his reforms. Here is how he put it himself: 'There are molecules in 1, 2 and 4 volumes, as there are in
1/2, in 1/3, in 1/4 of a volume.' (Comptes rendus des travaux de Chimie, 1851, p. 146). And he continued
as follows (p. 147): 'It is perhaps surprising to see me defend this thesis, when I have recommended and
still recommend every day that a regular notation in organic chemistry be followed, in representing all
volatile substances by the same number of volumes, by 2 or by 4. The chemists who see 2 contradictory
Pág 16 de 26
assertions in that forget that I never acknowledged the preceding principle as a molecular truth, but as a
condition to be satisfied in order to arrive at the knowledge of certain laws or certain relations that would
be allowed to escape the observer's attention in an arbitrary notation, or one suitable for special cases.'
"There certainly were facts that forced Gerhardt to renounce Avogadro's theory, but there were also
unwarranted hypotheses. The facts were the densities of monohydrate sulfuric acid vapor, of sal
ammoniac, and of phosphorus perchloride.
"You already know, gentlemen, that on the occasion of the publication of the paper of Mr. Deville on the
dissociation of certain compounds by heat, I was the first to try to interpret the occurrence of these
abnormal densities, by supposing that the bodies in question are split in two, and that in reality a mixture
of vapors is weighed in the determination of these densities. After me, Mr. H. Kopp proposed the same
interpretation in his own way.
"I will not repeat here the arguments that we invoked in favor of this interpretation. I will only add that
one of the members of this Congress just told me that the boiling point of sulphuric acid is almost
constant at very different pressures--a fact that shows that it is not a matter of a boiling point here, but of a
decomposition point. I am convinced that other facts will confirm the interpretation that we have given to
abnormal densities, and, as a result, will dispel the doubts that some scientists still appear to harbor
concerning Avogadro's theory.
"But independently of the facts that I have just cited, there were also unwarranted hypotheses that led
Gerhardt away from Avogadro's theory. I am going to show that this is the case.
"Gerhardt took as a demonstrated truth that all metallic compounds have formulae analogous to those of
the corresponding hydrogen compounds. From that it follows that the formulae for the mercury chlorides
are HCl, Hg2Cl, in assuming that the free mercury molecule is formed of two atoms, like that of hydrogen.
Let us observe that the vapor densities lead to a different result. In effect, in order to represent the
composition of equal volumes of the following five bodies: hydrogen, hydrochloric acid, mercury,
mercurous chloride, and mercuric chloride, we will have the following formulae:
H2, HCl, Hg2, Hg2Cl, Hg2Cl2
The comparison of these formulae show that in the molecules of
free mercury and of its two chlorides, there exists the same amount of
mercury, expressed by Hg2, and that mercurous chloride is analogous to
hydrochloric acid, while the mercuric chloride contains twice the amount
of chlorine in its molecule.
"As a result, the same reason that directed us to double the carbon atom also commits us to double the
mercury atom. This comes down to saying that the amount of mercury expressed by Hg2 in the preceding
formulae represents a single atom. In this case it is seen that the atom is equal to the molecule of the free
body; and that in mercurous salts this atom is the equivalent of a single hydrogen atom, while in the
mercuric salts it is the equivalent of two hydrogen atoms. In other terms, to employ the language generally
in common use today, the mercury is monoatomic in the mercurous salts, but in the mercuric salts it is
diatomic like the radicals of Mr. Wurtz's glycols.
"It is important to point out now that in doubling the atomic weight of mercury, as was done with the
atomic weight of sulphur, we arrive at numbers that accord with the law of specific heats.
"But if the atomic weight of mercury is doubled, one is led by analogy to double those of copper, zinc,
lead, tin, etc.--in a word, one ends up back in Mr. Regnault's system of atomic weights that agree with
specific heats, with isomorphism, and with chemical analogies.
"That Gerhardt's system conflicted with the law of specific heats, as well as with isomorphism, was the
truly unfortunate thing. This clash has produced two different chemistries--one, which dealt with
inorganic substances, and accorded great value to isomorphism; and the other, which investigated organic
substances, that took no account f this. Therefore, the same substance could not have the same formula in
one chemistry as in the other. The clash that I have just indicated stemmed from the fact that Gerhardt's
system was not entirely consistent; it disappears as soon as the inconsistencies are done away with.
Pág 17 de 26
"Vapor densities provide a means of determining the weight of molecules of substances, whether simple
or compound. Specific heats are used to check the weights of atoms and not those of molecules.
Isomorphism reveals analogies in molecular constitution.
"In support of the modification of atomic weights of certain metals which I have just suggested, I will cite
the following facts: all of the volatile compounds of mercury, zinc, tin, and lead contain amounts of metal
represented in ordinary notation by Hg2, Zn2, Sn2, Pb2. This fact alone is enough to indicate to us that
these quantities represent the true atoms of the metals in question. The fact that there exist three oxalates
of potassium and ammonium (monoatomic radicals), while there exist only two oxalates of barium and
calcium (diatomic radicals), could also be cited. But for the moment, I do not stress this point, and I
cannot deny, on the other hand, that there is one case where the atomic weight deduced from the
comparison of molecular compositions is in conflict with the one deduced from specific heat. This case is
relative to carbon. But it could be that the law of atomic heats remained masks by other causes that
intervene in specific heat.
"In summary, gentlemen, I propose that Gerhardt's system be accepted, taking into consideration the
modifications of the atomic weights of certain metals and the formulae for their salts which I suggest be
brought about.
"And if we are unable to reach a complete agreement upon which to accept the basis for the new system,
let us at least avoid issuing a contrary opinion that would serve no purpose, you can be sure. In effect, we
can only obstruct Gerhardt's system from gaining advocates every day. It is already accepted by the
majority of young chemists today who take the most active part in advances in science.
"In this case let us restrict ourselves to establishing some conventions for avoiding the confusion that
results from using identical symbols that stand for different values. Generalizing already established
custom, it is thus that we can adopt barred letters to represent the doubled atomic weights."
Mr. Strecker offers some clarifications concerning the drafting of the second proposition submitted to the
Congress. This draft originally mentioned Gerhardt's name, but the majority of the committee had wanted
to substitute Berzelius's name. The speaker did not share the opinion of the majority. It did not seem to
him that there was good reason to go back to Berzelius, who could perhaps be criticized for a logical flaw
on the question of atoms and equivalents. The useful and urgent thing is to improve what exists by taking
into account the advances of science since Berzelius. Mr. Strecker adds that the doctrines expressed in
"Gerhardt's system" offer real advantages. As for himself, he will henceforth adopt the new atomic
weights in his papers, but he does not think that the time has come to introduce them into teaching and
into elementary books.
Mr. Kekulé shares all of Mr. Cannizzaro's opinions. It appears useful to him, however, to have
reservations about one point of detail. Mr. Cannizzaro considers mercurous chloride as containing HgCl
(Hg=200). It appears more rational to Mr. Kekulé to envision it as a combination analogous to "Dutch
liquid", that is, as containing Hg2Cl2 (Hg=200) and to assume that at the moment of vaporization the
molecule Hg2Cl2 splits.[11]
Mr. Will does not wish to enter into the details of the questions submitted before the Congress. He
confines himself to pointing out that the Congress must proceed directly to its goal. This goal is to find a
clear, logical notation, incapable of generating confusion in the minds of those uninitiated in the formulae,
and suitable for not only expressing the long accepted facts in science, but those that modern discoveries
add to it each day as well.
Mr. Erdmann suggests that the first two questions be dropped and that discussion be confined to the last
one. It appears difficult to him to reach an agreement concerning questions of principle, and especially to
impose a notation by vote, as it were.
Mr. Wurtz points out that it was not anyone's idea to impose some idea or other. One is faced with two
kinds of questions--those that concern the very root of things, and others that are questions of form. If
there were not yet good grounds for resolving the first by vote, because they were not yet ripe enough,
nothing prevents agreement on, and even voting on, the purely formal questions.
Mr. Hermann Kopp notes that, on many theoretical points, the opinions of chemists are divided. These
differences of opinions are caused in part by misunderstandings and are reflected in the notation itself. A
discussion could be very useful for terminating the misunderstandings.
Pág 18 de 26
Mr. Erlenmeyer suggests that barred symbols always be used to express atomic weights that represent the
old double equivalents.[12]
Mr. L. Meyer points out that this point seemed settled, because no one has raised an objection in this
respect.
A discussion among several members on the suitability of casting a vote began.
Mr. Cannizzaro's opinion is that it is pointless to vote on the third question.
Mr. Boussingault draws attention to the possible difficulty in misunderstanding the meaning of the votes
the Congress can cast concerning the questions submitted before it. Voting is an expression of the wishes
of the Congress and the Congress is not intending to impose the majority opinion on anyone.
Mr. Will aligns himself with the same opinion.
Mr. Normandy points out that the scientists who suggest the introduction of certain reforms concerning
notation into science are those who principally cultivate organic chemistry. Now, it can be noted that the
scientists do not even agree among themselves on some points. Thus it appears premature to apply
principles which are still under discussion to inorganic chemistry.
Mr. Odling speaks about the question of barred symbols. He remembers that Berzelius introduced them
into the science to express double atoms. The bar, he says, is thus the sign of divisibility, and it appears
contrary to logic to bar symbols expressing indivisible atoms of oxygen and carbon.
Agreeing completely that Berzelius's double atoms had a different meaning from the indivisible atoms,
the barring of whose symbols had been proposed, Mr. Kekulé points out that these barred symbols must
express not the divisibility of atoms, but the divisibility of the value represented by these symbols, which
is twice what they had been taken to be in the past.
In reply to the observations made by Messrs. Erdmann and Normandy, Mr. Kekulé adds that it is not
enough to impose a theoretical opinion or a notation by vote, but that a discussion of such subjects is
necessary and useful and will not fail to bear fruit.
The Congress consulted by the chairman expresses the wish that the use of barred symbols, representing
atomic weights twice those that have been assumed in the past, be introduced into science.
Mr. Dumas adjourns the third and last session of the Congress, after paying respects to the Grand Duke of
Baden and thanking him for his hospitality.
[1] I am indebted to my colleague, Prof. Gaufinez in Bonn, for
examining the French text. [Note: I am reproducing all footnotes that
appear in the 1929 publication. It is not clear to me which notes are
Wurtz's and which Anschütz's. Some notes contain the parenthetical
notation (A.); however, others, such as this one, also appear to be
Anschütz's. --CJG]
[2]The circular was sent in German, French, and English. The German text is dated: "Carlsruhe, July 10,
1860"; the English text: "London, July 1, 1860." With the French version of the proceedings of the
Congress, I have incorporated the French text of the circular, which, compared with the German and
English, contains the name "Regnault" amongst the undersigned; the English lacks the name
"Mitscherlich," as well.
[3]The printed list of members, supplemented by handwritten additions, contains 126 names. (A.)
[4]I have arranged the participants by country and by the cities in which they worked at the time. (A.)
[5]Cf. Appendix 9. (A.)
[6]In Kekulé's manuscript, he has added here the following marginal note: "Not always!" (A.)
[7]In Kekulé's manuscript, he has added: "Kekulé and Will declined."
[8]In Kekulé's manuscript, there is the following marginal note: "A molecule is 1, at most 2 atoms." (A.)
[9]The following note is in Kekulé's manuscript: "Striking example: NH4Cl, SO3.OH2." (A.)
[10]H = 1. C = 8. O = 16. Az = 14. az = 14/3.
[11]Cf. Kekulé's Ann. (1857), 104, 132n.
[12]Originally suggested by Williamson. (A.)
Pág 19 de 26
ANEXO III
A OBTENCIÓN DO MENDELEVIO.
Naturalmente, o sistema periódico era o escenario perfecto para poñerse a buscar os elementos preditos por
Mendeleev e aínda moitos máis.
Así, en 1925, os químicos alemáns W. e I. Noddack introducían na Táboa Periódica o último elemento estable: o
Renio (Re, Z=75). En 1937, os italianos Perrier e Segré identificaron o Tecnecio (Tc, Z=43) nun trozo de Mo que
fora bombardeado con deuterones. En 1939 a francesa M.Perey identificó o Francio (Fr, Z= 87), e en 1940, outra
vez Segré identificó o Astato (At, Z=85).
A finais da Segunda Guerra mundial, varios físicos en Oak Ridge (EEUU) identificaron o Prometio1 (Pm, Z=61).
Na década dos corenta, exactamente de 1940 a 1952, oito elementos foron sintetizados en Berkeley (California).
Son os transuránidos: do Neptunio (Np, Z=93) ao Fermio (Fm, Z=100).
O procedemento para a obtención destes transuránidos coñécese como
“Síntese neutrónica”. Trátase de bombardear con neutróns núcleos pesados e
agardar a que a desintegración radiactiva dos núcleos inestables actúe. Era,
pois, preciso bombardear Uranio con neutróns, polo que a partir de 1946
comezou a súa búsqueda sistemática aproveitando o funcionamento de
reactores nucleares -que como sabemos xeran intensos fluxos neutrónicos e
"queiman" Uranio- baixo a dirección de I.V.Kurchátov, en Rusia, e de
G.T.Seaborg en EEUU.
Pero é máis, se a irradiación neutrónica se realiza durante un tempo
bastante prolongado (un ano ou máis), entón non so se acumularán cantidades
grandes de Neptunio, senón dos elementos que o seguen. En efecto, a través do
progresivo aumento de neutróns nos novos núcleos formados e de sucesivos
decaementos Beta comezan a formarse novos elementos:
O intento de continuar co procedemento non daba resultado para os
elementos seguintes ao Californio. A razón estriba en que segundo
avanzamos cara a núcleos maiores aumenta a inestabilidade e polo tanto a
probabilidade de decamentos α e β , e sobre todo a desintegración por
fisión espontánea.
Os elementos 99 e 100 foron descubertos en 1952 e 1953
respectivamente no pó radiactivo resultante das primeiras explosións
nucleares de fusión. Nestas condicións, un fluxo de neutróns decenas de
veces superior o que hai nun reactor de fisión, pasa a través do
revestimento de uranio que ten a bomba, polo que este pode captar máis
dunha ducia de neutróns. Sucesivas decaementos β darán lugar a aparición
do Einstenio e o Fermio:
1
A e xi s t e n c i a d o P r o m e t i o n o n f o r a p r e d i t a p o r Me n d e l e e v , s e n ó n p o r B o h u s l a v B r a u n e r e n 1 9 0 2 ; e s t a
p r e d i c c i o n f o i a p o i a d a p o r H e n r y Mo s e l e y e n 1 9 1 4 , q u e n a t o p o u u n o c o p a r a u n e l e m e n t o
descoñecido con número atómico 61. O Prometio foi producido e illado por primeira vez no Oak
R i d g e N a t i o n a l L a b o r a t o r y ( O R N L ) n o s E E U U e n 1 9 4 5 p o l o s f í s i c o s J a c o b A . Ma r i n s k y , L a wr e n c e E .
Glendenin e Charles D. Coryell por separación e análise dos productos de fisión do uranio no
R e a c t o r d e G r a f i t o ; p o r é n , a c a u s a d a S e g u n d a G u e r r a Mu n d i a l , o d e s c u b r i m e n t o n o n f o i a n u n c i a d o
ata 1947.
Pág 20 de 26
Paralelamente á síntese neutrónica, comezouse a utilizar outra
estratexia: bombardear brancos de elementos pesados con ións acelerados.
Trátase de lanzar contra brancos de transuránidos de vida suficientemente
longa, partículas con carga para que entren nos núcleos dos átomos do
branco, formándose daquela núcleos de maior tamaño. Deben ser partículas
cargadas -núcleos ou ións- para poder seren acelerados e dirixidos mediante
campos eléctricos e magnéticos, xa que se han de posuír elevadas velocidades
para vencer a repulsión culombiana cos núcleos branco. Por outra parte, dado
o escaso tempo de "supervivencia" dos novos elementos obtidos, non é preciso
insistir no enxeño que tiveron que desenvolver os investigadores.
Por razóns obvias, comezouse coas partículas máis fáciles de
acelerar: os núcleos de deuterio ou helio -a “artillería lixeira” como diciá
F l i o r o v ( 1 9 8 5 ) 2. D e s t a f o r m a f o r o n s i n t e t i z a d o s o s e l e m e n t o s 9 4 , 9 6 , 9 7 e 9 8 e
o último en ser obtido foi o 101 - o Mendeleevio- no ano 1955:
Neste experimento, como branco servía unha capa fina duns mil
millóns de átomos de Es–253 -imperceptible para os ollos- aplicada sobre
unha folla finísima de ouro. Esta folla foi irradiada no acelerador de Berkeley
(California) con partículas alfa. Estas partículas movíanse cunha velocidade
de preto de 50000 km/s e expulsaban aos núcleos de Einstenio cos que
colisionaban. Estes núcleos chegaban a unha segunda folla de ouro situada
detrás da primeira e alí “asentábanse” pero xa como Md-256. Foron
recollidos soamente 17 átomos, pero os “artistas” chamados Albert Ghiorso,
Bernard G. Harvey, Gregory R. Choppin, Stanley G. Thompson, and Glenn
T.Seaborg3 foron quen de “identificalos”.
Dado que a súa vida media era de só uns 80 minutos non era posible
esperar cantidades mensurables do novo elemento. Pero no seu decaemento o
Mendelevio-256 captura un electrón para converterse en Fermio-256.
256
101
Md +
0
−1
e→
256
100
Fm
Ainda que este isótopo do Fm tamén ten
unha vida media moi curta (~3 h), “houbo
tempo” para que se poidesen detectar tres
átomos
deste
elemento.
Esa
era
a
proba
inequívoca
de
que
se
tiña
sintetizado
o
Mendelevio.
O isótopo do Mendelevio máis estable é
Md–258 que ten unha vida media de 52 días; a
súa síntese lógrase bombardeando Einsteinio–
255 con partículas alfa. Isto ten permitido saber
o
2
F L I O R O V , G . N . , I L I N O V , A . S . ( 1 9 8 5 ) : “ E n e l c a m i n o h a c i a l o s s u p e r l e m e n t o s ” . E d i t o r i a l MI R .
Mo s c o v a , p . 3 7 .
3
E n r e l a c i ó n a G l e n n T. S e a b o r g , é i n e v i t a b e c o m e n t a r q u e f o i o ú n i c o q u í m i c o q u e t i v o a f o r t u n a d e
v e r o s e u n o m e n a Tá b o a P e r i ó d i c a ( S b , Z = 1 0 6 ) , p o i s o s d e m á i s t i ñ a n m o r t o c a n d o s e l l e p u xo o s e u
nome a algún elemento.
Pág 21 de 26
a l g u n h a s p r o p i e d a d e s 4, c o m o q u e p r e s e n t a u n h a p r i m e i r a E n e r x í a d e
Ionización de 635 kJ/mol y que o seu estado principal de oxidación é o Md3+.
O número de isótopos do Mendelevio é de dezaseis como se indican na
t á b o a s e g u i n t e 5:
Isótopo
245
Md
246
Md
247
Md
248
Md
249
Md
250
Md
251
Md
252
Md
Vida
media
0.35 s
1.0 s
1.12 s
7 s
24 s
52 s
4.0 m
2.3 m
Isótopo
Vida
media
253
Md
Md
255
Md
256
Md
257
Md
258
Md
259
Md
260
Md
254
6 m
10 m
27 m
78.1
5.52
51.5
96 m
31.8
m
h
d
d
Como xa se comentou con anterioridade, Mendeleev foise convertendo
nos seus últimos anos nunha persoa facilmente irritable (digamos que
doadamente desestabilizable desde o punto de vista emocional). Xa que logo,
c o m o b e n t e n i n d i c a d o o e s c r i t o r i n g l é s P a u l S t r a t h e r n 6, s e m e l l a m o i
apropiado que tódolos isótopos do Mendelevio sexan tan inestables.
4
5
6
V e r h t t p : / / e n v i r o n m e n t a l c h e m i s t r y . c o m / y o g i / p e r i o d i c / Md . h t m l ( E n v i r o n m e n t a l , C h e m i s t r y & H a z a r d o u s
Ma t e r i a l s N e ws , C a r e e r s & R e s o u r c e s ) .
T HE I SOT OP E S P ROJ E CT HOME PAGE ( LAW RE NCE B E RKE LE Y NAT I ONAL LAB ORAT ORY) :
HT T P ://I E .LB L.GOV/I P.HT ML.
STRATHERN, P. (2000). “El Sueño De Mendeleiev. De la Alquimia a la Quimica”.
Siglo XXI de España Editores.
Pág 22 de 26
ANEXO IV
Revista Iberoamericana de Polímeros Volumen 5(3), Diciembre de 2004
LA TABLA PERIÓDICA DE LOS ELEMENTOS EN VERSIÓN CUATRILINGÜE DE
EDITORIAL TÉBAR Y LOS ANALES DE QUÍMICA DE LA RSEQ*
Pascual Román Polo
Facultad de Ciencia y Tecnología, Departamento de Química Inorgánica
Universidad del País Vasco (UPV/EHU), Apartado 644, 48080 Bilbao.
E-mail: [email protected]
INTRODUCCIÓN
El 23 de enero de 1903 fue fundada la Sociedad Española de Física y Química en el Decanato de la
Facultad de Ciencias de la Universidad Central situado en la calle de San Bernardo de Madrid, siendo su
primer presidente José Echegaray, y dos meses más tarde veía la luz el primer número de la revista
científica de la Sociedad: Anales de la Sociedad Española de Física y Química. Los fines de la revista
eran difundir entre especialistas los trabajos científicos españoles y en la medida que fuera posible los
extranjeros, y divulgar cuanta información científica estuviera al alcance de la Sociedad para llegar a
quienes no estando dedicados a la investigación se interesaran por la actualidad científica; en definitiva,
contribuir al fomento de la ciencia básica, dar a conocer sus aplicaciones y crear un ambiente favorable
para que la cultura científica arraigara entre la ciudadanía. En el año de aparición de la revista, se
conocían 84 elementos químicos con el descubrimiento del europio en 1901 por el francés Eugene
Demarcay. Desde aquella fecha y a lo largo de un siglo, la tabla periódica ha aumentado en 32 nuevos
elementos con el reciente descubrimiento en Dubna (Rusia) de los elementos de número atómico 113 y
115 por científicos rusos y estadounidenses.
EL CENTENARIO DE LA REVISTA ANALES DE QUÍMICA
Con ocasión de celebrarse en 2003 el centenario de la fundación de la revista Anales de Química de la
Real Sociedad Española de Química (RSEQ), Editorial Tébar ha querido sumarse a este importante
acontecimiento científico para la ciencia española, ofreciendo 3.000 ejemplares de una nueva versión de
la tabla periódica de los elementos a los socios y colaboradores de la RSEQ, a quien desea larga vida y los
mayores éxitos científicos en su 101 aniversario
Es preciso destacar que tanto Meyer como Mendeléiev participaron en el Primer Congreso Internacional
de Químicos que se celebró en la ciudad alemana de Karlsruhe en el mes de septiembre de 1860, donde
brilló con luz propia el químico italiano Stanislao Cannizzaro (1826–1910) que fue rápidamente captada
por Meyer y Mendeléiev. Gracias a las ideas de Cannizzaro y con tan sólo 63 elementos, Mendeléiev en
menos de diez años consiguió establecer el orden en la caótica situación en la ciencia química que
imperaba en aquella época y que marcó su rumbo. Antes que Meyer y Mendeléiev, ilustres científicos
habían realizado intentos de clasificar los elementos químicos, pero unas veces por no contar con un
número suficiente de ellos y otras por utilizar pesos atómicos incorrectos no fueron capaces de hallar la
anhelada clasificación periódica. En 1913, el joven químico inglés Henry Gwyn Jeffreys Moseley (1887–
1915) fue el primero en demostrar experimentalmente que las propiedades de un elemento químico vienen
determinadas por el número atómico (Z) en lugar de por el peso atómico. Ahora sabemos que las
propiedades de los elementos químicos siguen la ley periódica en orden creciente del número atómico,
que coincide con el número total de protones en el núcleo atómico. En aquel año, se conocían 86
elementos con el descubrimiento del lutecio en 1907 por Georges Urbain y Carl Auer von Welsbach y el
protactinio en 1913 por Otto Hahn, Lise Meitner, Frederick Soddy y John Cranston. Hoy en día, existen
Pág 23 de 26
más de mil versiones de la tabla periódica y pueden clasificarse en alguno de los tipos siguientes:
continuas o discontinuas, según el número de grupos –cortas, medias y largas– y por su dimensionalidad –
bidimensionales y tridimensionales–, a su vez, las bidimensionales pueden ser: curvas o matriciales y las
tridimensionales: curvas o helicoidales.
De entre ellas la más extendida es la bidimensional matricial con 18 columnas verticales, llamadas
grupos, separadas en los bloques s (2 grupos), p (6 grupos), d (10 grupos) y f (14 grupos), situándose este
último al pie de la tabla. A las filas horizontales de elementos químicos se las conoce con el nombre de
periodos.
La tabla periódica está permanentemente sujeta a revisión. Así, en febrero de 2004 se ha comunicado en
una revista científica de gran prestigio que científicos rusos y norteamericanos aislaron en Dubna (Rusia)
los elementos 113 y 115, quienes obtuvieron cuatro núcleos del elemento 115 al bombardear Am–243 con
un haz de iones Ca–48 y por ulterior emisión de partículas _ se formaron cuatro isótopos del elemento
113. Por otra parte, en la 42ª Asamblea General celebrada en agosto de 2003 en la ciudad de Ottawa
(Canadá), el Consejo de la Unión Internacional de Química Pura y Aplicada (International Union of Pure
and Applied Chemistry, IUPAC, en sus siglas inglesas) aprobó oficialmente el nombre de darmstadtio
(en honor de la ciudad alemana de Darmstadt) y símbolo Ds para el elemento de número atómico 110. En
mayo de 2004, un Comité de expertos de la División de Química Inorgánica de la IUPAC ha propuesto
para el elemento de número atómico 111 el nombre de roentgenio y de símbolo Rg –para honrar al físico
alemán Wilhelm Conrad Roentgen, que descubrió los rayos X en 1895 y obtuvo por ello el primer premio
Nobel de Física en 1901–.
UNA NUEVA VERSIÓN DE LA TABLA PERIÓDICA
Editorial Tébar ha editado en el año 2004 una nueva tabla periódica de los elementos que, en su anverso,
recoge una versión moderna, actualizada, de 18 grupos, separados en los bloques s, p, d y f, donde se
recogen las últimas aportaciones de la IUPAC y las más recientes incorporaciones de nuevos elementos.
De esta tabla periódica destaca su fondo verde y está dirigida a los alumnos de Bachillerato, a los
estudiantes de los primeros cursos de Universidad y, en general, al público interesado en el estudio de la
Química para que asocien esta ciencia con el desarrollo sostenible. De cada elemento químico se muestra
el número atómico, símbolo y peso atómico en su formato abreviado de cinco dígitos, junto con sus
nombres en castellano, catalán, euskera y gallego. En el reverso, se muestra una tabla con las propiedades
y datos más usuales de los elementos en un formato de diez columnas y doce filas, donde se ubican 120
elementos químicos de los que todavía no se han aislado los de número atómico 117, 118, 119 y 120,
aunque se tienen fundadas esperanzas en que serán descubiertos en un futuro próximo. De cada elemento
se ofrece el número atómico, símbolo, año de su descubrimiento, punto de fusión, punto de ebullición,
electronegatividad de Pauling, radio atómico, radio covalente y configuración electrónica. La realización
de esta nueva versión de la tabla periódica de los elementos ha estado a cargo del profesor Pascual Román
(Universidad del País Vasco).
Agradecimientos. Los textos en catalán, euskera y gallego han sido revisados por los profesores Oriol
Rossell (Universidad de Barcelona), Jacinto Ilurbe (Universidad del País Vasco) y Manuel Bermejo
(Universidad de Santiago de Compostela), respectivamente, a quienes el autor desea agradecer su
inestimable colaboración.
BIBLIOGRAFÍA
1. Corish, J.; Rosenblatt, G. M. Pure Appl. Chem., 2003, 75(10), 1613–1615.
2. http://www.iupac.org/publications/pac/2003/pdf/7510x1613.pdf (“Name and Symbol
of the element with atomic number 110”, visitada el 5 de junio de 2004).
3. http://www.iupac.org/reports/provisional/abstract04/Corish_pr111.pdf (“Name and
Symbol of the element with atomic number 111”, visitada el 5 de junio de 2004).
4. Moreno, A. An. Quím., 2003, 99(2), 244–265.
Pág 24 de 26
ANEXO V
Phys. Rev. C 74, 044602 (2006)
Synthesis of the isotopes of elements 118 and 116 in the 249Cf and
245
Cm+48Ca fusion reactions
Yu. Ts. Oganessian, V. K. Utyonkov, Yu. V. Lobanov, F. Sh. Abdullin, A. N. Polyakov,
R. N. Sagaidak, I. V. Shirokovsky, Yu. S. Tsyganov, A. A. Voinov, G. G. Gulbekian, S.
L. Bogomolov, B. N. Gikal, A. N. Mezentsev, S. Iliev, V. G. Subbotin, A. M. Sukhov,
K. Subotic, V. I. Zagrebaev, G. K. Vostokin, and M. G. Itkis
Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
K. J. Moody, J. B. Patin, D. A. Shaughnessy, M. A. Stoyer, N. J. Stoyer, P. A. Wilk, J.
M. Kenneally, J. H. Landrum, J. F. Wild, and R. W. Lougheed
University of California, Lawrence Livermore National Laboratory, Livermore,
California 94551, USA
(Received 31 January 2006; revised 23 August 2006; published 9 October 2006)
The decay properties of 290116 and 291116, and the dependence of their production cross sections on the
excitation energies of the compound nucleus, 293116, have been measured in the 245Cm (48Ca, xn)293x116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are
produced via the 249Cf+48Ca reaction. We performed the element 118 experiment at two projectile
energies, corresponding to 297118 compound nucleus excitation energies of E*=29.2±2.5 and 34.4±2.3
MeV. During an irradiation with a total beam dose of 4.1×1019 48Ca projectiles, three similar decay
chains consisting of two or three consecutive decays and terminated by a spontaneous fission (SF) with
high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the
even-even isotope 294118 (E =11.65±0.06 MeV, T =0.89 ms) produced in the 3n-evaporation channel of
the 249Cf+48Ca reaction with a maximum cross section of 0.5 pb.
Pág 25 de 26
ANEXO VI
List of Solvay conferences on chemistry
1. 1922 Cinq Questions d'Actualité, Chair: William Pope (Cambridge)
2. 1925 Structure et Activité Chimique, Chair: William Pope (Cambridge)
3. 1928 Questions d'Actualité, Chair: William Pope (Cambridge)
4. 1931 Constitution et Configuration des Molécules Organiques, Chair:
William Pope (Cambridge)
5. 1934 L'Oxygène, ses réactions chimiques et biologiques, Chair:
William Pope (Cambridge)
6. 1937 Les vitamines et les Hormones, Chair: Fred Swarts (Ghent)
7. 1947 Les Isotopes, Chair: Paul Karrer (Zurich)
8. 1950 Le Mécanisme de l'Oxydation, Chair: Paul Karrer (Zurich)
9. 1953 Les Protéines, Chair: Paul Karrer (Zurich) (Image)
10.1956 Quelques Problèmes de Chimie Minérale, Chair: Paul Karrer
(Zurich)
11.1959 Les Nucléoprotéines, Chair: A.R. Ubbelohde (London)
12.1962 Transfert d'Energie dans les Gazs, Chair: A.R. Ubbelohde
(London)
13.1965 Reactivity of the Photoexited Organic Molecule, Chair: A.R.
Ubbelohde (London)
14.1969 Phase Transitions, Chair: A.R. Ubbelohde (London)
15.1970 Electrostatic Interactions and Structure of Water, Chair: A.R.
Ubbelohde (London)
16.1976 Molecular Movements and Chemical Reactivity as conditioned by
Membranes, Enzymes and other Molecules, Chair: A.R. Ubbelohde
(London)
17.1980 Aspects of Chemical Evolution, Chair: A.R. Ubbelohde (London)
18. 1983 Design and Synthesis of Organic Molecules Based on Molecular
Recognition", Chairs: Ephraim Katchalski (Rehovot, Israel) and
Vladimir Prelog (Zurich)
19.1987 Surface Science, Chair: Frederik W. de Wette (Austin)
20.1995 Chemical Reactions and their Control on the Femtosecond Time
Scale, Chair: Pierre Gaspard (Brussels)
21. 2007 From Noncovalent Assemblies to Molecular Machines, Chair:
Jean-Pierre Sauvage (Strasbourg)
Pág 26 de 26
ANEXO VII
A N E X O V III
4
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
D. I. MENDELEEV’S CONCEPT OF
CHEMICAL ELEMENTS AND THE
PRINCIPLES OF CHEMISTRY
Masanori Kaji, Tokyo Institute of Technology
Introduction: Mendeleev’s Textbook, The
Principles of Chemistry
Dmitrii Ivanovich Mendeleev (1834-1907) was primarily a chemist even though he later worked in many
other fields. One of his most important contributions
to chemistry was the discovery in 1869 of the periodic
law of the chemical elements, which is still a fundamental concept in modern chemistry. In 1905, shortly
before his death, he listed what he considered his four
main contributions to science (1): the periodic law, the
elasticity of gas, the understanding of solutions as associations, and The Principles of Chemistry (hereafter
referred to as Principles). Mendeleev himself stated
the close relationship between the first and fourth contributions in his first paper on the discovery of the periodic law, written in early March of 1869 (2,3):
In undertaking to prepare a textbook called ‘Osnovy
khimii’ [Principles], and to reflect on some sort of
system of simple bodies in which their distribution
is guided not by chance, as might be thought instinctively, but by some sort of definite and exact principle.
Few outside Russia, however, have pointed to the direct relationship between Principles and the periodic
law (4). In Russia B. M. Kedrov (1903-1984), who
made a very detailed analysis of Mendeleev’s discovery of the periodic law, has discussed this close relationship. In the late 1940s he found new archival material related to Mendeleev’s first periodic table, and in
the 1950s he published reliable source books on
Mendeleev’s discovery. His work culminated in his
book The Day of a Great Discovery (5) in 1958, a very
detailed analysis of Mendeleev’s process of compiling
his first periodic table. All subsequent works on this
topic have begun from this work (6). From a critical
examination of Kedrov’s works, the author has also
published a book on Mendeleev’s discovery, considering social, as well as scientific, factors (7). All recent
studies have included a consideration of this direct relationship between Principles and the periodic law (8).
However, there are no studies that consider the background of Mendeleev’s writing of Principles and the
changes made in subsequent editions (9). The purpose
of this paper is to analyze the text of the first and later
editions of Principles with its background and show the
role played by Mendeleev’s concept of the chemical
elements in the discovery of the periodic law and its
later development.
Origin of Mendeleev’s Concept of the
Chemical Elements and So-called Indefinite
Compounds
Mendeleev entered the Main Pedagogical Institute at
St. Petersburg in 1850 after graduating from the gymnasium in the Siberian city of Tobol’sk, where he was
born in 1834. While a student, he published his first
scientific papers on the chemical analysis of minerals
from Finland (10). His undergraduate thesis was on
isomorphism and was concerned with the development
of mineral analysis (11). Even this thesis foreshadows
Mendeleev’s future line of research: first, it shows his
talent for compiling and systematizing large amounts
of data; second, it mentions Auguste Laurent (1808-
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
1853) and Charles Gerhardt (1816-1856), the reformers
of chemistry in the 1840s and 1850s (12); and third, its
theme, the relationship between similarities of crystal
form and composition, made Mendeleev seriously consider the problem of the similarity of substances. I think
this was the beginning of his involvement with the problem of classifying substances.
Mendeleev taught briefly at gymnasiums in southern Russia before returning to the capital to receive a
master’s degree and become a lecturer at St. Petersburg
University. His master’s thesis on specific volumes illustrates his later line of thought even more clearly (13).
He adopted the atomic weight system of Gerhardt and
Laurent and Avogadro’s hypothesis
(which Mendeleev called
Gerhardt’s law). This thesis also
shows Mendeleev’s interest in the
natural classification of substances
based on their specific volume.
5
the inconsistency of Gerhardt’s atomic weights of metals and arguing that Cannizzaro corrected them with the
“multiatomicity of metals,” Mendeleev clearly recognized Cannizzaro’s successful system of atomic weights.
In his letter to Voskresenskii, Mendeleev showed that,
for various substances, the atomic heat (i.e., the product
of specific heat and atomic weight) divided by the
substance’s number of atoms results in a constant (about
6-7). Thus, Cannizzaro’s atomic weights were found
to be in accord with the law of Dulong and Petit.
Early in 1861 Mendeleev returned to Russia. That
same year, while teaching at various schools, he completed his first chemistry textbook, Organic Chemistry.
In this he was already seeking
“some sort of definite and exact principle” as a guide, like
that later in Principles, finding
it in what he called “the theory
of limits” (16). This was the
classification of organic comIn April 1859 Mendeleev went
pounds on the basis of their
to Western Europe to study. Durdegree of saturation and their
ing his two-year stay in Europe he
substitution reactions. Alstudied the “cohesion” of various
though this theory would soon
substances (the forces holding their
be forgotten because of the
molecules together), especially of
advent of the structural theory
organic compounds, through capof organic compounds,
illary phenomena. He tried to find
Mendeleev’s textbook was
a universal formula to explain the
well received in Russia. In
relationship of cohesion expressed
1862 the St. Petersburg Acadin terms of surface tension with
emy of Sciences awarded him
composition, density, or molecular
the Demidov Prize for the outweight. The instruments that
standing book written in RusMendeleev purchased in Heidelsian during the previous year.
berg, Bonn, and Paris enabled him
In this textbook Mendeleev
to measure the properties of subfollowed Cannizzaro’s prinstances with very good precision.
ciple for determining atomic
In September 1860 he attended the
weights and defined them as
International Congress of Chemists
Mendeleev, 1878
“the minimum quantity of an
in Karlsruhe, which considered sigelement in the compound molecules of the element” (17).
nificant contemporary issues in chemistry, especially
He also explicitly distinguished between “bodies” and
atomic weights. Along with everyone else in attendance,
“radicals,” terming the former “something divisible
Mendeleev received a copy of the famous paper on the
(molecule)” and the latter “the theoretical notion” and
new atomic-weight system by Stanislao Cannizzaro
“indivisible whole (atom)” (18).
(1826-1910), who distributed it at the meeting (14).
Immediately after reading the paper, Mendeleev wrote
After completing his textbook of organic chemisto his teacher A. A. Voskresenskii (1808-1883) in St.
try, Mendeleev intended to write a textbook on inorganic
Petersburg with an informative report on both the Conand theoretical chemistry. He tried to extend the idea of
gress and the content of Cannizzaro’s paper. His letter
saturation (his “theory of limits”) to inorganic comwas published in a St. Petersburg newspaper and in a
pounds, but with little success (19). He also left an 1864
Moscow journal that same year (15). In pointing out
lecture notebook on theoretical chemistry (20).
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
6
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
pounds, the so-called indefinite compounds, have shown
I believe that Mendeleev made one more change in
evidence, which is directly against the theory.
his line of thought on atomic weights during 1860s. Even
before his acquaintance with Cannizzaro’s paper,
Almost the same passage appears in the first part
Mendeleev had been especially concerned with deviaof the first edition of Principles (26):
tions from the law of definite proportions. In his 1856
[C]ompounds with indefinite compositions . . . speak
habilitation dissertation, he discussed the structure of
against the atomic doctrine as much as definite chemisilicate compounds (21), arguing that such compounds
cal compounds speak in
must be a kind of “alloy” of
its support.
oxides, because, like alloys, “to
It is important to note that
some extent they can vary their
Mendeleev paid very little
composition (and formula)
attention to atomic
without changing their forms
weights in the first part of
and main properties” (22). He
this new textbook. He
developed this line of research,
mentioned the atomic
calling substances that had conweights of only some 22
stant physical properties, but
of the most familiar elevaried composition—such as
ments (27). It is true that
solutions, alloys, isomorphous
a table of the 63 elements
mixtures, and silicate comthen known appears in the
pounds—“indefinite comsecond chapter of the first
pounds.” Such compounds had
part, but the elements are
been studied very little, and
arranged alphabetically
Mendeleev himself could not
with no mention of their
explain their formation in any
atomic weights (28). It
proper way. However, he emseems likely that the existphasized the following points:
ence of indefinite comthey are not simply physical
pounds made Mendeleev
mixtures; some chemical
accept the limitation of the
power must be involved in their
atomic theory and the narformation; and they show some
row scope of atomic
properties that are similar to
weights (29).
Russian Chemists in Heidelberg in 1859-1860: (left to
those of definite compounds
right) N. Yitinskii, A. P. Borodin, Mendeleev, V. I.
(23). His doctoral thesis “On
Even as Mendeleev
Olevinskii
Compounds of Alcohol with
regarded atomic theory
Water,” submitted in 1865, can
with caution because of exceptions to the law of defibe regarded as a study of solutions that arose from his
nite proportions, he insisted on the existence of distinct
interest in so-called indefinite compounds (24).
chemical elements, which were clearly distinguished
Underlying this interest was Mendeleev’s concern
that the formation or composition of indefinite compounds was difficult to explain in terms of the atomic
theory, which was based on the concept of definite proportions. Even though no previous writers have emphasized the idea that Mendeleev was moving away from
a belief in the atomic theory in this period (1864-1868),
Mendeleev himself made this point clear in a lecture on
theoretical chemistry published in 1864 (25):
In fact, although on the one hand, the law of definite chemical compounds has persuasively proven the
atomic theory, on the other hand, a whole group of com-
from simple bodies. He argued this point in his first
series of lectures at St. Petersburg University in the fall
of 1867 (30):
[I]t is necessary to distinguish the concept of a simple
body from that of an element. A simple body substance, as we already know, is a substance, which
taken individually, cannot be altered chemically by
any means produced up until now or be formed
through the transformation of any other kinds of bodies. An element, on the other hand, is an abstract
concept; it is the material that is contained in a simple
body and that can, without any change in weight, be
converted into all the bodies that can be obtained from
this simple body.
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
A similar definition of element and the same argument
for the need to distinguish clearly between element and
simple body were later presented in the first part of Principles (31).
Thus, this distinction between “simple bodies” and
“elements” is essentially the same as that between “bodies” and “radicals” in Mendeleev’s 1861 organic chemistry textbook, but without any mention of atoms or
molecules. Paradoxically, then, it appears that
Mendeleev was led to the weight of elements as an invariable characteristic and hence to his periodic system,
not by adherence to the concept of chemical atoms, but
by seeking freedom from it, as the failures of the law of
definite proportions seemed to demand. It is reasonable to suppose that he refined the concept of the elements to bear an attribute of an individual chemical entity without employing the notion of atoms because of
the supposed limitations of the atomic theory.
During the 1860s the theory of valence enjoyed
great success, helping in the development of a new theory
of organic chemistry, i.e., a structural theory of organic
compounds. After Mendeleev wrote his textbook of
organic chemistry based on a pre-structural theory, his
“theory of limits,” it seems that he tentatively took the
valences of the elements as a basic principle in writing
7
The Social Background of Mendeleev’s
Writing of The Principles of Chemistry
Before analyzing the relationship between Principles and
the discovery of the periodic law, let us briefly examine
the social background of the writing of Principles. Published between 1868 and 1871, Principles grew out of
Mendeleev’s need for a suitable textbook on chemistry
in Russian, which was lacking when he began teaching
at St. Petersburg University in the fall of 1867 as the
Professor of General Chemistry (32):
I began to write [Principles] when I started to lecture
on inorganic chemistry at the university after [the
departure of] Voskresenskii and when, having looked
through all the books, I did not find anything to recommend to students.
Mendeleev had obtained the position of a permanent
lecturer at St. Petersburg University in 1864. He became an extraordinary professor of technical chemistry
the following year and was promoted to full professor
at the end of the same year. In the fall of 1867 Mendeleev
was transferred to the professorship of general chemistry to succeed Voskresenskii, his own teacher, who left
the university that year.
Mendeleev’s research career in chemistry, which
began in 1854, reached its first zenith with the discov-
Table 1. BOOKS PUBLISHED WITH MENDELEEV AS AUTHOR OR EDITOR
1861
1862
1863
1864
1866
1867
1868
Organic Chemistry, 1st edition
Cahours’ Textbook for Elementary General Chemistry, second pt. (translation)
Wagner’s Technology (1862-1869), 8 Vol. (translation and compilation)
Organic Chemistry, 2nd edition
Gerhardt and Chancel’s Analytical Chemistry, Qualitative Analysis (translation)
Analytical Chemistry, second pt., Vol. 1-3 (1866-1869)
Today’s Development of Some Chemical Productions—From the Point of View for
the Application to Russia (Report of International Exposition at Paris in 1867)
The Principles of Chemistry, first pt., first vol.
his inorganic chemistry textbook, Principles, at the end
of the 1860s, because of the success of valence theory
in organic chemistry. But without the assumption of
atoms, valence was incomprehensible. Hence
Mendeleev had to look further for “some sort of definite and exact principle.” He had to find a fundamental
property of the elements. Out of this exigency, weight—
which we think of as “atomic,” but Mendeleev thought of
as “elementary”—took on a new and increased importance.
ery of the periodic law in 1869. This discovery can also
be considered the culmination of his social activity during this period. Those years, beginning in the middle of
the 1850s after the Crimean War and running their course
by the 1860s with the emancipation of the serfs in 1861,
constituted a period of great change and reform in Russia. This was the second attempt at social and economic
change after the social and political reforms of Peter the
Great in the early 18th century; it has been called by
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
8
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
Table 2. CHRONOLOGY OF THE PUBLICATION OF THE FIRST EDITION OF PRINCIPLES AND
DISCOVERY OF THE PERIODIC
DATE
PUBLICATION
May-June 1868
Principles, first volume (part 1, chapters 1-11)
February 17, 1869
“An Attempt at a System of the Elements Based on Their Atomic Weight and
Chemical Affinity” (the first periodic table)
March 6, 1869
“The Correlation of the Properties and Atomic Weights of the elements” (the
first paper on the periodic law, Paper I)
March 1869
Principles, second volume (part 1, chapters 12-22).
February-March 1870
Principles, 3rd volume (part 2, chapters 1-8).
February 1871
Principles, 4th & 5th volumes (part 2, chapters 9-23).
July 1871
“The Periodic Law of the Chemical Elements” (in Annalen der Chemie und
Pharmacie, Paper II)
some historians “the Great Reforms Era.” It was also a
time of change in chemistry: the dispute over the merits
of different atomic weight systems had finally been
settled after the Karlsruhe Congress; and classical organic structural theory had appeared.
The emergence of a new generation of chemists in
Russia, eager to engage in original laboratory work and
pursue a European trend in chemistry, was the important background to Mendeleev’s activities in this period.
The educational system, especially at the higher levels,
was also reorganized during this time. Because of the
large numbers of Russian chemists moving into posts at
academic institutions, the Russian Chemical Society was
organized in 1868, Mendeleev being one of the founding members.
Let us consider the objectives that Russian chemists, including Mendeleev, were expected to achieve
during this period. They consisted of the practical and
the theoretical. The practical objective was to educate
qualified professionals for the new capitalistic production that Russia required. The theoretical objective was
to deal with current theoretical and experimental problems in chemistry to meet the needs of the time, as the
classical foundations of chemistry were being established. Mendeleev was aware of these objectives. In
his Principles he answered not only the theoretical requirements, but also the practical ones.
This point is illustrated by a listing of the books
Mendeleev published during the 1860s after his return
from Europe (Table 1). The contents of these books
indicate that they all met the practical demands of Russian society. Wagner’s Technology, for example, was
initially the translation of German encyclopedic manuals on technology. As the editor, Mendeleev proposed
to translate the pertinent sections needed in Russia, i.e.,
the parts on agricultural products and processing. Later
on, he added the translations from other related books
and also asked appropriate specialists to write original
texts. They were all issued by the same publisher,
“Obshchestvennaia pol’za” [“Social Benefit”], a company that produced books and pamphlets on science and
technology for the “social benefit and enlightenment of
the people” (33). Principles, offering an advanced
method for systematizing inorganic chemistry, was the
new textbook for higher education urgently needed by
Russian society. Mendeleev’s famous textbook was the
culmination of his work to help satisfy his country’s
needs during that period.
The Principles of Chemistry and the
Discovery of the Periodic Law
First, let us consider the chronology of the publications
of the first edition of Principles and the discovery of the
periodic law (Table 2). In May or June 1868, Mendeleev
published the first volume (Chapters 1-11). On February 17, 1869 (34), he compiled the first periodic table,
titled “An Attempt at a System of the Elements Based
on Their Atomic Weight and Chemical Affinity” (35).
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
On March 6, N. A. Menshutkin (1842-1907), the secretary of the recently established Russian Chemical Society, read Mendeleev’s first paper on his discovery,
“The Correlation of the Properties and Atomic Weights
of the Elements” (Paper I) (36) at a meeting of the society. At almost the same time, Mendeleev published
the second volume of Principles, Chapters 12-22. At
the end of February or early in March 1870, the third
volume, which comprises Chapters 1-8 of Part 2, appeared. Finally, the last volumes (the fourth and fifth),
9
Pharmacie (Paper II) (37). This chronology (Table 2)
makes it clear that Mendeleev discovered the periodic
law in the middle of writing Principles. As Kedrov has
pointed out, a careful reading of this text reveals exactly when he discovered that law (38).
Let us examine Mendeleev’s first paper on the periodic law (Paper I) and the early chapters of the second
part of his textbook, which must have been written
around the same time. He organized the first part of
Principles on the basis of the principle of valence: first
Table 3. DIFFERENCES BETWEEN THE THIRD/FOURTH AND FIFTH EDITIONS
OF PRINCIPLES
Third/Fourth Editions
Fifth Edition
chapters
chapters and elements [group number]
1&2
3&4
5
6&7
9 & 10
11
12 & 13
14
15 & 16 & 19
17 & 18
20
21 & 22
23
24
25 & 26
27
28
29 & 30 & 31
32 & 33
34 & 35
36 & 37 & 38
39
40 & 41
43
42 & 44
Introduction
1: H2O
2: H2O, H [I]
3: O [II]
4: O3, H2O2
5: N [III]
6: N with H &O
7: Molecules and Atoms
8: C & Hydrocarbons [IV]
9: C with O & N
10: NaCl, HCl [VII]
11: Cl, Br, I, F [VII]
12: Na [I]
13: K, Rb, Cs, Li [I]
14: Mg, Ca, Sr, Ba, Be [II]
15: “The Similarity of the Elements and the Periodic Law”
16: Zn, Cd, Hg [II]
17: B, Al,Ga, In, Tl [III], the rare earths
18: Si, Ge, Sn, Pb [IV]
19: P, As, Sb, Bi, V, Nb, Ta [V]
20: S, Se, Te [VI]
21: Cr, Mo, W, U [VI], Mn [VII]
22 Fe, Co, Ni [VIII]
23 Or, Ir, Pt, Pd, Rh, Ru [VIII]
24: Cu, Ag, Au [I]
which include Chapters 9-23, were published in February 1871. In July of that year, his most comprehensive paper on the periodic law, “The Periodic Law of
the Chemical Elements,” was published in a supplemental volume of the Annalen der Chemie und
he discussed univalent hydrogen, then divalent oxygen,
trivalent nitrogen, and tetravalent carbon (39). After his
treatment of the univalent halogens, which concludes
the first part of the textbook, Mendeleev began the second part with a description of the univalent alkali met-
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
10
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
als. At the end of the chapter on heat capacity, which
follows the alkali metals, he explained that he would
next treat the alkaline-earth metals, which are divalent
and not analogs of copper, which awkwardly exhibits
both univalence and divalence (40). Although he had
followed the principle of valence to this point in the textbook, he abruptly began the
next chapter from a different
perspective: a comparison of
the alkaline-earth metals with
the alkali metals on the basis
of their atomic weights. In this
connection, it should be noted
that toward the end of Paper I,
Mendeleev stressed that (41):
[T]he purpose of my paper
would be entirely attained if
I succeed in turning the attention of investigators to the relationships in the size of the
atomic weights of nonsimilar
elements, which have, as far
as I know, been almost entirely neglected until now.
He emphasized the word
“nonsimilar” with italics. Alkali metals and alkaline-earth
metals were obviously such
nonsimilar groups of elements.
atomic weight, by the very essence of matter, is common to the simple body and all its compounds.
Atomic weight belongs not to coal or diamond, but
to carbon.
This “something,” italicized in the quotation above, corresponds exactly to Mendeleev’s definition of element.
In other words, atomic weights belong to elements!
As a result of this
reconceptualization or discovery, Mendeleev realized that he
should use atomic weights, not
valence, as the guiding principle
for the remainder of his textbook. This was the moment
when he started to write the
chapter on alkaline-earth metals. However, since he defined
the concept of element without
the notion of atoms, he considered atomic weights to be the
fundamental property of the elements. They were not necessarily based on atomic theory,
which was still somewhat
speculative. Thus, the scope of
atomic weights would have to
be broader than that of definite
proportions on which the atomic
theory was thought to be based.
Mendeleev even once suggested
the use of the word “elementary
weight” instead of “atomic
If Kedrov’s analysis in
Mendeleev in St. Petersburg, Nov. 19, 1861
The Day of a Great Discovery
(42) of Mendeleev’s process is
followed, then Mendeleev noticed this comparison
weight” (45).
of nonsimilar groups of elements in the middle of February 1869; and he first compiled the central part of the
Changes in Later Editions of
table on the basis of this principle. With the help of
The Principles of Chemistry
cards of the chemical elements, which he made for this
occasion, Mendeleev finally succeeded in organizing a
Contrary to many statements in the existing literature
table of all the known elements on the basis of their
on the periodic law—that Mendeleev kept the original
atomic weights. He completed this on February 17, 1869
version of Principles unchanged through subsequent
(43). Clearly, at that moment, Mendeleev had conceived
edition—(46), he actually revised the structure of the
the idea that atomic weight might be the fundamental
textbook significantly with each new edition. Much
numerical property of the elements.
confusion has resulted from this misunderstanding. In
In Paper I Mendeleev wrote (44): No matter how
all, eight editions were published during Mendeleev’s
properties of simple bodies may change in the free
lifetime. Let us look briefly at some of the changes in
state, something remains constant, and when the eleensuing editions of Principles.
ment forms compounds, this something is material
existence and establishes the characteristics of the
compounds, which include the given element. In this
respect we know only one constant peculiar to an element, namely the atomic weight. The size of the
There were two type fonts in the text of the first
four editions: sections in a larger font for beginning
students and those in a smaller font for advanced learners. In the second edition, published in 1872-1873, just
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
one year after the completion of the first, there were
only minor changes in the text. Mendeleev moved indium and uranium to the appropriate chapters because
of the improved values of their atomic weights. He also
changed the positions of the rare earths, which remained
problematic throughout his life (Fig. 1).
The third edition, which appeared in 1877, underwent substantial change; and the chapters were completely reorganized in accord with the periodic law. The
11
The fourth edition in 1881-1882 was the same as
the third in organization but slightly larger, increasing
in size from 18 x 11 cm to 20 x 12 cm. Mendeleev first
mentioned the discovery of scandium in this edition.
The fifth edition of 1889 underwent the second
major change after the third edition. It was considerably larger, and for the first time the text was printed in
double columns rather than in single columns. Therefore, the whole work became much shorter, reduced from
Table 4. MENDELEEV’S AND BRAUNER’S ARRANGEMENTS OF THE
ELEMENTS (both from the 7TH Russian Edition, 1902
Mendeleev’s arrangement [copy from the 7th Russian edition]
Brauner’s arrangement, 1902 [copy from the 7th Russian edition]
textbook was divided into two parts, as were the first
two editions, but the chapters were now numbered successively throughout. Only small changes were needed
in the first part, which was introductory and devoted to
the elements frequently encountered in daily life.
Mendeleev placed the chapter on the periodic law, entitled “Similarity of Elements and Their System,” in the
second part, immediately after the description of the alkali and alkaline-earth metals. After these chapters he
described the elements in order of their position in the
periodic table: from the second group to the sixth group,
ending with the eighth group, iron and platinum analogs. The final chapters were devoted to the noble metals. The third edition also included gallium, the first of
the elements to be discovered after Mendeleev had predicted their existence.
1176 pages
in the fourth edition to 789 pages in the fifth. Some of
the material from previous editions was moved into the
footnotes in smaller font. There were no longer two
parts, only one, bound as a single volume, a format retained in all subsequent editions. The chapters were also
completely reordered. Many of them were combined,
and the 44 chapters in the fourth edition became only
24 chapters in the fifth (see Table 3). The chapter on
the periodic law was expanded to include the history of
its discovery and the problem of priority (47). This fifth
edition was translated into English, German, and French
(48).
The sixth edition of 1895 was essentially unchanged
in format from the fifth, but Mendeleev revised many
of the footnotes. He added notes on argon, the newly
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
12
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
discovered gas from the air, at the end of the textbook,
and he argued for the possibility that argon might be
N3 .
As shown in his textbook, Mendeleev’s concept of
the chemical elements demonstrates his firm and persistent belief in their conceptual priority. His clear understanding of the elements is evident from the very
first edition. In his concept of an element, Mendeleev
clearly departed from Lavoisier, who had offered a negative definition of an element as an undecomposed substance. For Mendeleev, the concept was defined positively as something abstracted from the diverse properties of simple bodies and their compounds. Therefore,
elements were strictly distinguished from simple bodies.
By the seventh edition of 1902-1903 Mendeleev
had abandoned N3 and fully accepted the noble gases,
which he incorporated into the chapter on nitrogen and
air. Mendeleev asked the Czech chemist Bohuslav
Brauner (1855-1935) to write the section on the rare
earths for the seventh and eighth editions, even though
they had somewhat different opinions on the positions
of these elements within the periodic system. They
agreed to place scandium, yttrium, and lanthanum in
the third group and
Beginning
tantalum in the fifth.
with the first ediHowever, while
tion of Principles,
Mendeleev believed
Mendeleev carethat future research
fully denied the
would reveal suffispeculative concient numbers of
notations of the
rare earth elements
atomic hypothwith different propesis. Although it
erties, so they could
is tempting to say
be placed in different
that his “element”
groups to fit neatly
is a substitute for
into his periodic
“ a t o m , ”
table, Brauner proMendeleev reposed that the rare
sisted the use of
earths should be all
the hypothetical
placed together in
atom. He was also
group IV, which was
opposed to any
formerly occupied
suggestion that
Members of the Chemistry Section of the First Congress of Russian
by cerium alone
Naturalists (front row, 5th from left, A. A. Voskresenskii; back row, 2nd
served to reduce
from right, Mendeleev; 6th from right, N. A. Menshutkin
(Table 4). Effecsimple substances
tively, this demonto a single substrates Mendeleev’s admission of the difficulties in
stance or a few substances called “primary matter” (50).
placement of the rare earths, so many in number and so
This attitude was in sharp contrast to those of other insimilar in properties, within his periodic system. He
dividuals who also sought a system of the elements duralso mentioned the discovery of radium in this edition,
ing the 1860s (51).
but denied the possibility of the transformation of the
elements. He suggested other possible explanations of
Lothar Meyer’s Approach to the
radioactivity, such as a “state” like a magnetic propClassification of the Elements
erty or an absorbency and the projection of the “ether”
in the vicinity of the radioactive atom.
Let us briefly consider the case of Lothar Meyer (1830The eighth edition in 1906 was the last published
before Mendeleev’s death. All the notes were separated from the main text and placed in the second half
of the book. He argued for the possibility of a “chemical ether” as an extremely light element in the noble
gas group, which he thought could explain radioactivity (49).
1895) as an example of the “reductionist” tendency (52).
His paper, “The Nature of the Chemical Elements as a
Function of their Atomic Weights,” appeared early in
1870 (53). He began with speculation related to Prout’s
hypothesis (54). On some points he went further than
Mendeleev did in 1869 in his Paper I. Meyer succeeded
in vividly conveying the periodic dependence of the
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
properties of the elements on their atomic weights by
plotting the solid-state atomic volumes of the elements
(simple bodies) against their atomic weights (55). Although he admitted in the paper that his table was essentially the same as Mendeleev’s, his table of elements
was more refined than Mendeleev’s first attempt, especially in clearly showing the so-called transition metals. Meyer also had the correct weight of indium, to
which Mendeleev had attributed an incorrect weight in
his first paper. However, the conclusion of Meyer’s
paper was very tentative, even timid (56):
It would be hasty to undertake to alter on such uncertain bases the previously accepted atomic weights.
On the whole, one may not attribute any very great
weight to arguments of the sort here given, nor expect from them so certain a decision [regarding atomic
weight] as is given by determination of the specific
heat or the vapor density. They may however serve
even now to turn our attention upon doubtful and
uncertain assumptions and to challenge us to a renewed testing of them. And again, conversely, this
testing will help to clarify and extend the meager
beginnings of our knowledge of atoms.
Meyer’s conclusion lacks the confidence expressed by
Mendeleev in his first paper. In 1869 there was a noticeable difference between these two men in their attitudes toward the concept of the atom. Whereas
Mendeleev discarded the atom and relied solely on the
refined concept of a chemical element, Meyer embraced
the atom and even supported the speculation of Prout’s
hypothesis of a primordial matter (hydrogen) as the
building block of the elements. This prompted Meyer
to underestimate his findings and prevented his having
full confidence in his discovery of 1869. In 1873, however, Meyer published another paper (57), in which he
fully applied the periodic law, citing Mendeleev’s comprehensive 1872 paper on the subject (Paper II in Table
2) as the evidence for the validity of his own work.
Conclusion: Mendeleev’s Concept of the
Chemical Elements and 19th-Century
Chemistry
Mendeleev’s concept of the chemical elements as a
stable, intermediate level of matter, not necessarily based
on the speculative concept of the atom, corresponded to
the state of chemistry in the mid-19th century. Ironically, it helped him discover the periodic law. This deep
insight, which assured him of the validity of his discovery, allowed him to apply it fully to the chemistry of his
time, without being bothered by a seeming regularity in
13
numbers on the one hand, or being misled by a speculative primordial matter on the other. As a result of his
discovery, the concept of an element gained another
positive characteristic in its definition: an element occupies a specific place in the periodic system (58). Later
Mendeleev’s concept of chemical elements developed
into “chemical individuals,” his further attempt to avoid
the speculative connotations of the atomic theory (59).
Even though the formats of Mendeleev’s textbook
changed substantially with each edition, his firm belief
in the validity of the concept of the chemical elements
remained unchanged from the 1860s.
In the course of revising his textbook, Mendeleev
developed his concepts further. Eventually, however,
he encountered insurmountable difficulties, including the
placement of the rare earths in his system (60), abnormalities in the order of atomic weights, and new phenomena, such as radioactivity. These were the predicaments that could be solved only by a new concept of the
elements, which was beyond Mendeleev’s understanding and that of 19th-century chemistry in general.
ACKNOWLEDGMENTS
I wish to express my gratitude to those who generously
supported my work and provided assistance in improving this paper: Richard Rice, Nathan Books, David
Lewis, Seymour Mauskopf, William Brock, Paul
Forman, Igor S. Dmitriev, and an anonymous referee.
All photographs (including front cover) courtesy of
Mendeleev Museum Archive at St. Petersburg State
University.
REFERENCES AND NOTES
1.
2.
3.
Arkhiv D. I. Mendeleeva, tom 1, Avtobiograficheskie
materialy, sbornik dokumentov [Archive of D. I.
Mendeleev, Vol. 1, Autobiographical Materials, Collection of Documents], Izd. Leningradskogo
Gosudarstvennogo Universiteta, Leningrad, 1951, 34.
This date is in the Julian calendar, used in Russia until
January 1918. The Julian calendar lagged 12 days behind the Gregorian calendar in the 19th century and 13
days in the 20th century. In this paper I am using the
Julian calendar for events in Russia and the Gregorian
dates for events outside Russia.
D. Mendeleev, “Sootnoshenie svoistv s atomnym vesom
elementov” [“The Correlation of the Properties and
Atomic Weights of the Elements”], Zh. Russ. Khim.
Obshch., 1869, 1, No. 2/3, 60-77 (65). I have used the
English translation of this paper, with some modification, from H. M. Leicester and H. S. Klickstein,
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
14
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
Sourcebook in Chemistry 1400-1900, Harvard University Press, Cambridge, MA, 1952, 439-444.
4. Leicester was one of the few historians in the West who
clearly pointed out this connection before Kedrov. See,
e.g., H. M. Leicester, “Factors which Led Mendeleev to
the Periodic Law,” Chymia, 1948, 1, 67-74 (71); also H.
M. Leicester, The Historical Background of Chemistry,
1956, reprinted Dover Publications, New York, 1971,
193.
5. B. M. Kedrov, Den’ odnogo velikogo otkrytiia [The Day
of a Great Discovery], Izd. Sotsial’no-ekonomicheskoi
Literatury, Moscow, 1958. His main points are also available in English: B. M. Kedrov, “Dmitry Ivanovich
Mendeleev,” Dictionary of Scientific Biography, C. C.
Gillispie, Ed., Charles Scribner’s Sons, New York, 1974,
Vol. 9, 286-295 (288).
6. For example, I S. Dmitriev’s recent paper in Russian on
the discovery of the periodic law starts from criticism of
Kedrov’s analysis and tries to present an alternative version: I. S. Dmitriev, “Nauchnoe otkrytie in statu
nascendi: periodicheskii zakon D. I. Mendeleeva” [“Scientific Discovery in statu nascendi: The Periodic Law
of D. I. Mendeleev”], Vopr. Istor. Estestvozn. Tekh., 2001,
No.1, 31-82.
7. M. Kaji, Mendeleev’s Discovery of the Periodic Law of
the Chemical Elements—The Scientific and Social Context of His Discovery [in Japanese], Hokkaido University Press, Sapporo, Japan, 1997.
8. For example B. Bensaude-Vincent, “Mendeleev’s Periodic System of Chemical Elements,” Br. J. Hist. Sci.,
1986, 19, 3-17.
9. Loren R. Graham has emphasized the need to study the
evolution of Mendeleev’s views in later editions of Principles more thoroughly. See L. R. Graham, Science in
Russia and the Soviet Union: A Short History, Cambridge
University Press, Cambridge, 1993, 266, note 34. There
is a recent study of these aspects by N. M. Brooks,
“Dmitrii Mendeleev’s Principles of Chemistry and the
Periodic Law of the Elements,” in A. Lundgren and B.
Bensaude-Vincent, Ed., Communicating Chemistry:
Textbooks and Their Audiences, 1789-1939, Science
History Publications, Canton, MA, 2000, 295-309. However, Brooks’ conclusions are different from mine, especially on the organization of later editions of Principles.
10. Reprinted in D. Mendeleev, Sochineniia [Collected
Works], Izd. Akademii Nauk SSSR, Leningrad, 1949,
Vol. 15, 16-19, 20-23. Hereafter referred to as Works.
11. “Izomorfizm v sviazi s otnosheniiami kristallicheskoi
formy k sostavu” [“Isomorphism in the Relationship
Between Crystal Form and Composition”], reprinted in
Works, Vol. 1, 7-137.
12. On the role of Laurent and Gerhardt in the reform of the
atomic weight-molecular formula problem, see A. J
Rocke, Chemical Atomism in the Nineteenth Century:
From Dalton to Cannizzaro, Ohio State University Press,
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
Columbus, OH, 1984, 192-214. For their influence in
Russia, including that on Mendeleev, see M. G.
Faershtein, Istoriia ucheniia o molekule v khimii (do 1860
g.) [The History of Molecular Theory in Chemistry (up
to 1860)], Izd. Akademii Nauk SSSR, Moscow, 1961,
290-321.
“Udel’nye ob”emy” [“Specific Volumes”], reprinted in
Works, Vol. 1, 139-323; Vol.25, 112-228.
S. Cannizzaro, “Sunto di un corso di filosofia chimica,”
Nuovo Cim., 1858, 7, 321-366 (1858). For an English
translation, see Sketch of a Course of Chemical Philosophy by Stanislao Cannizzaro (1858), Alembic Club Reprint No. 18, Alembic Club, Edinburgh, 1910. This translation is also reprinted in M. J. Nye, The Question of the
Atom: From the Karlsruhe Congress to the First Solvay
Conference, 1860-1911, Tomash Publishers, Los Angeles, CA, 1984, 31-87.
Mendeleev’s letter to Voskresenskii is reprinted in D. I.
Mendeleev, Periodicheskii zakon [The Periodic Law],
B. M. Kedrov, Ed., Izd. Akademii Nauk SSSR, Moscow, 1958, 660-669, and also in N. A Figurovskii,
Dmitrii Ivanovich Mendeleev, 1834-1907, 2nd ed.,
Nauka, Moscow, 1983, 274-280.
D. Mendeleev, “Essai d’une théorie sur les limites des
combinaisons organiques,” Bull. Acad. Imp. Sci. St.Pétersbourg, 1862, 4, 245-250; reprinted in Works, Vol.
8, 22-27.
D. Mendeleev, Organicheskaia khimiia [Organic Chemistry], 1st ed., St. Petersburg, 1861, v.
Ref. 17, note on p 36.
A. A. Makarenia, D. I. Mendeleev i fiziko-khimicheskie
nauki—Opyt nauchnoi biografii D. I. Mendeleeva [D.
I. Mendeleev and the Physico-Chemical Sciences—An
Attempt at a Scientific Biography of DI. Mendeleev], 2nd
ed., Energoizdat, Moscow, 1982, 92-100.
“Fragmenty iz lektsii D. I. Mendeleeva po teoreticheskoi
khimii” [“Fragments from D. I. Mendeleev’s Lectures
on Theoretical Chemistry”], in D. I. Mendeleev,
Izbrannye lektsii po khimii [Selected Lectures on Chemistry], Izd. Vysshaia Shkola, Moscow, 1968.
“O stroenii kremnezemistykh soedinenii” [“On the Structure of Silicate Compounds”], a dissertation pro venia
legendi, 1856, reprinted in Works, Vol. 25, 108-228.
Ref. 21, p 220.
For his arguments during the 1860s, see Ref. 20, pp 969, especially pp 11-14, 26-59.
“O soedinenii spirta s vodoi” [“On Compounds of Alcohol with Water”], St. Petersburg, 1865, reprinted in
Works, Vol. 4, 1-52.
Ref. 24, p 24.
D. Mendeleev, Osnovy Khimii [The Principles of Chemistry], 1st ed., Part 1, Ch.10, reprinted in Works, Vol. 13,
337.
Ref. 26, 1st ed., Part 1, Ch. 10, reprinted in Works, Vol.
13, 342.
Ref. 26, 1st ed., Part 1, Ch. 2, reprinted in Works, Vol.
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
13, 77-82. The third edition has the same table, but with
atomic weights.
Ref. 26, 1st ed., Part 1, Ch. 10, reprinted in Works, Vol.
13, 340-341.
Lektsii po obshchei khimii 1867/68 g. [Lectures on General Chemistry in 1867/68], Lecture V, St. Petersburg,
reprinted in Works, Vol. 15, 381-382. A lithographic
edition of these lecture notes was found in the library of
the former Bestuzhev women’s courses, one of the most
important institutions of higher education for women in
pre-revolutionary Russia. Consisting of sixteen lectures,
these notes are similar to the first half of part 1 of Principles written in 1868. In the fifth lecture, there is a
table of 63 elements, ordered alphabetically by their Latin
names. The atomic weights of 12 of these elements were
incorrect, which could not have been the case after the
discovery of the periodic law. All this evidence shows
that these notes are a record of Mendeleev’s lectures on
general chemistry given at St. Petersburg University in
the fall of 1867.
Ref. 26, 1st ed., Part 1, Ch. 2, reprinted in Works, Vol.
13, 73-74 and also Ch. 15, reprinted in Works, Vol. 13,
488-490.
Ref. 1, pp 52-53.
Tridtsatipiatiletie vysochaishe utverzhdennogo
Tovarishchestva “Obshchestvennaia pol’za” [The
Thirty-Fifth Anniversary of the Founding of the “Social
Benefit” Company], Obshchestvennaia Pol’za, St. Petersburg, 1895, 5.
See the clarification of dates in Ref. 2.
D. Mendeleev, “Opyt sistemy elementov, osnovannoi na
ikh atomnom vese i khimicheskom skhodstve,” [“An
Attempt at a System of the Elements Based on Their
Atomic Weight and Chemical Affinity”], in Ref. 15, p 9.
Ref. 3. Mendeleev was not present for this meeting because he had left St. Petersburg on March 1 for a consulting trip with farmers in cheese-making communities. He was sent by the Free Economic Society (Vol’noe
ekonomicheskoe obshchestvo), one of the oldest scientific societies in Russia.
D. Mendelejeff, “Die periodische Gesetzmässigkeit der
chemischen Elemente,” Ann. Chem. Pharm., 1871,
Supplementband, 8, 133-229. Mendeleev’s manuscript
was translated into German by Felix Wreden, a friend of
Mendeleev in St.Petersburg. The original Russian text
was first published in D. I. Mendeleev, Novye materialy
po istorii otkrytiia periodicheskogo zakona [New Materials on the History of the Discovery of the Periodic
Law], Izd. Akademii Nauk SSSR, Moscow, 1950, 1982; reprinted in Ref. 15, pp 102-176. The Russian versions published earlier were translations from the German, the first by B. N. Menshutkin in D. I. Mendeleev,
Periodicheskii zakon [The Periodic Law], Leningrad,
1926, 70-133; and the second by V. Ia. Kurbatov in
Works, Vol. 25, 239-305.
Ref. 5, Kedrov, 1958, pp 32, 138-145.
15
39. Ref. 26, 1st ed., Part 1, Ch.19, reprinted in Works, Vol.
13, 650-652. Mendeleev argued that their compounds
could be types for all the other compounds. Obviously,
Gerhardt’s “type theory” could be seen as influential here
since Mendeleev was familiar from his student days.
However, he did not mention Gerhardt and went directly
to the concept of valence, for which he used the word
atomnost’ (atomicity).
40. Ref. 26, 1st ed., Part 2, Ch.3, reprinted in Works, Vol.
14, 120-121.
41. Ref. 3, p 77.
42. Ref. 5, pp 39-91. Also see my recent analysis of the
process of the discovery, Ref. 7, pp 183-199.
43. D. N. Trifonov has criticized Kedrov’s version on several minor points: “Versiya-2 (K istorii otkrytiia
periodicheskogo zakona D. I. Mendeleevym)” [“Version
2 (Toward a History of the Discovery of the Periodic
Law by D. I. Mendeleev)”], Vopr. Istor. Estestvozn.
Tekh., 1990, No.2, 25-36; No. 3, 20-32. I. S. Dmitriev
has recently offered an alternative version of
Mendeleev’s discovery; see Ref. 6.
44. Ref. 3, p 66.
45. Ref. 37, D. Mendelejeff, p 136, note. This is Paper II in
Table 2.
46. It is often said that Mendeleev kept the text of Principles unchanged through all the subsequent editions,
but with the addition of footnotes that became longer
and longer. As I show in this paper, this interpretation is
a misunderstanding or at least inaccurate. This may originate partly from the fact that most Western literature refers to the translations of later editions of Principles and
partly from the rather vague description of the textbook
by Leicester, Ref. 4, 1948, p 71. See, for example,
Bensaude-Vincent, Ref. 8, p 8. Brooks has also written
recently that Mendeleev made no substantial change in
the organization of the book for these eight editions (Ref.
9, Brooks, p 307).
47. D. Mendeleev, Osnovy Khimii [The Principles of Chemistry], 5th ed., Ch. 15, 448-472.
48. The format of the fifth and subsequent editions was completely different from that of the preceding editions; i.e.,
this and subsequent editions were bound as a single volume, but English and French translations were issued in
multiple volumes. This has given rise to the incorrect
ideas about the formats of Mendeleev’s textbook. The
English translation appeared in two volumes: The Principles of Chemistry by D. Mendeléeff. Translated from
the Russian (fifth edition) by George Kamensky, edited
by A. J. Greenway in two volumes, Longmans, Green &
Co., London and New York, 1891; Vol. I, xvi + 611 pp.
& Vol. II, vi + 487 pp. Later, the sixth and seventh Russian editions were also translated into English and published in 1901 and 1905, respectively, as the second and
third English editions. Each of these English editions
also appeared in two volumes. The German translation
was issued in one volume like the Russian edition:
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
16
49.
50.
51.
52.
Bull. Hist. Chem., VOLUME 27, Number 1 (2002)
Grundlagen der Chemie. Aus dem russischen übersetzt
von L Jawein und A. Thillot, Verlag von C. Ricker, St.Petersburg, [1890]-1891, [4], 1127 S. Both the fifth and
the sixth editions were used for the French translation
since the sixth edition appeared while the French edition was being prepared. The French translation should
have been published in three volumes, but the third volume never appeared for some unknown reason: Principes
de chimie par M. Dimitri Mendéléeff le professeur de
chimie à l’Université impériale de Saint-Pétersbourg.
Traduit du russe par E. Achkinasi [et] H. Carrion, avec
préface de M. le professeur Armand Gautier. Vol. 1-2,
Éditeur B. Tignol, Paris, 1895-1896; Vol. I, [4], iv + 585
pp. & Vol. II, [4], 499 pp.
D. Mendeleev, Popytka khimicheskogo ponimaniia
mirovogo efira [An Attempt at a Chemical Understanding of the Universal Ether], St. Petersburg, 1903. An
English translation appeared as D. Mendeléeff, An Attempt Toward a Chemical Conception of the Ether, trans.
G. Kamensky, Longmans, Green & Co., New York, 1904.
See B. Bensaude-Vincent, “L’éther, élement chimique:
un essai malheureux de Mendéléev?” Br. J. Hist. Sci.,
1982, 15, 183-188.
Ref. 26, 1st ed., Part 2, Ch.6, reprinted in Works, Vol.
14, 247.
Bensaude-Vincent has pointed out that the logical consequence of Lavoisier’s definition is the hypothesis of a
primordial matter. See Ref. 8, p 12.
In 1969 van Spronsen claimed that there were six independent discoverers of the periodic law: A. E. B. de
Chancourtois, J. A. R. Newlands, W. Odling, G. D.
Hinrichs, J. L. Meyer, and D. I. Mendeleev. J. W. van
Spronsen, The Periodic System of Chemical Elements:
A History of the First Hundred Years, Elsevier,
Amsterdam, 1969. On the other hand, in the 1860s, these
six individuals had classified almost all the elements
already discovered on the bas of the atomic weights proposed by Cannizzaro and on some relationships between
different groups of elements. However, as I have argued elsewhere, there were significant differences in their
scientific contents, as well as the social contexts, for the
acceptance of their discoveries. See Ref. 7, pp 101-141,
239-260.
53. L. Meyer, “Die Natur der chemischen Elemente als Function ihrer Atomgewichte,” Ann. Chem. Pharm., 1870,
Supplementband, 7, 354-364. Also reprinted in K.
Seubert, Ed., Das natürliche System der chemischen
Elemente, Ostwald’s Klassiker No. 68, W. Engelmann,
Leipzig, 1895, 9-17. For a partial translation of this paper into English, see Ref. 3, Leicester and Klickstein,
pp 434-438.
54. Ref. 53, Meyer, pp 354-355.
55. Note that Meyer did not strictly and explicitly distinguish elements from simple bodies.
56. Ref. 53, Meyer, p 364.
57. L. Meyer, “Zur Systematik der anorganischen Chemie,”
Ber. Dtsch. Chem. Ges., 1873, 6, 101-106.
58. See J. R. Smith, Persistence and Periodicity: A Study of
Mendeleev’s Contribution to the Foundation of Chemistry, Ph.D. Thesis, University of London, 1976, 516;
also Ref. 8, Bensaude-Vincent, p 15.
59. “Refarat soobshcheniia ‘O edinstve veshchestva v sviazi
s periodicheskim zakonom’” [“Abstract of the Report
‘On the Unity of Substance in Connection with the Periodic Law’”], Zh. Russ. Khim. Obshch., 1886, 18, No. 1,
sect. 1, 66-67, reprinted in Ref. 15, Kedrov, pp 438-439;
D. Mendeléeff, “The Periodic Law of the Chemical Elements” (Faraday Lecture Delivered before the Fellows
of the Chemical Society in the Theatre of the Royal Institution on Tuesday, June 4, 1889), J. Chem. Soc., 1889,
55, 634-656, also in Appendix II of D. Mendeléeff, Principles, 3rd English ed., 1905, reprinted 1969, 494.
60. Ref. 52, van Spronsen, p 260. Van Spronsen has made
the point that the rare earths were such an insurmountable difficulty for the periodic system that it could have
been constructed only during the 1860s when few of
them were known.
ABOUT THE AUTHOR
Masanori Kaji is Associate Professor, Graduate School
of Decision Science and Technology, Tokyo Institute of
Technology. Address: 2-12-1 Ookayama, Meguro-ku,
Tokyo, 152-8852 JAPAN; [email protected].
He teaches the history of science and studies the history
of chemistry in 19th- and 20th-century Russia and Japan.
Copyright © 2006 by Division of History of Chemistry of the American Chemical Society. All rights reserved.
ANEXO IX
El n¶umero de Avogadro.
N0 = 6:023 £ 102 3
Jo s ¶e L u is C¶o r d o va F.
D e p t o . d e Qu ¶ ³m ic a . U A M{ I
Te lfa x. 7 2 4 4 6 { 0 6 , e { m a il: c t s @xa n u m .u a m .m x
Re s u me
Se pre se nta n lo s a nte c e de nte s te ¶o ric o s y e x pe rime nta
de A v o g a dro a s¶ ³ c o mo e l pa pe l q ue jug ¶o e l a n¶a lisis de C
de D a lto n. El a rt¶ ³c ulo c o nc luy e c o n la de sc ripc i¶o n de div e
n¶ume ro de A v o g a dro .
1
n
le s q ue pe rmitie ro n la fo rmula c i¶o n de la hip¶o te sis
a nniz z a ro pa ra la a c e pta c i¶o n de la te o r¶ ³a a t¶o mic a
rso s m¶e to do s e mple a do s pa ra la de te rmina c i¶o n de l
I ntr oducci¶
on
La igualdad que encabeza este art¶³culo aparece frecuentemente en libros de qu¶³mica y ¯sica pues est¶
a relacionada
con las masas at¶omicas, la constante de Boltzmann y la constante R de los gases.
Puesto que conocer la evoluci¶on de un concepto ayuda a su comprensi¶
on, el presente trabajo tiene como ¯n
presentar la historia del n¶
umero de Avogadro y de la hip¶
otesis que lo introdujo.
Anticipemos algunas curiosidades hist¶oricas:
² La hip¶otesis de Avogadro no la enunci¶
o Avogadro.
² El n¶
umero de Avogadro (conocido como n¶
umero de Loschmidt en algunos pa¶³ses europeos) no lo calcul¶o
Avogadro.
² El n¶
umero de Loschmidt tampoco lo calcul¶
o Loschmidt.
² El juego de palabras, com¶
un entre los estudiantes, \Avogadro{Abogado" algo tiene que ver con el apellido
del ilustre turinense; era abogado en derecho can¶
onico.
N0 = 6:023 £ 1023 part/mol es resultado de una convenci¶
on, como lo es que el metro sea igual a 1650763.73
longitudes de onda de la transici¶on 2p10 ¡ 5d5 del Kr86 en el vac¶³o y sin perturbaciones. Esta convenci¶on es
tan arbitraria como que \una docena" sea igual a doce unidades, y \una gruesa" sean doce docenas.
Pero, as¶³ como las de¯niciones de \metro", \docena", \gruesa", etc. tienen un origen hist¶
orico1 el n¶
umero de
Carlo Lorenzo Romano Amadeo Avogadro Conde de Quaregna y de Ceretto (en lo sucesivo, simplemente \N0 ")
tiene un desarrollo hist¶orico. Lo presentaremos a continuaci¶
on advirtiendo que el camino es sinuoso y con tramos
resbaladizos.
2
Antecedentes hist¶
or icos
2.1
La teor ¶³a at¶
omica de Dalton
La hip¶
otesis de Avogadro se basa en la teor¶³a at¶
omica de Dalton de forma que conviene analizar sus enunciados
b¶
asicos. La primera de las leyes de Dalton (1801) dice:
1E
l m etr o se d e¯ n i¶
o a p ar tir d e las m ed icion es d el cu ad r an te d el m er id ian o ter r estr e. La d ocen a se or igin ¶
o, p r ob ab lem en te, en
el sistem a sex ag¶
esim al d e los b ab ilon ios, em p lead o en las ob ser v acion es astr on ¶
om icas.
1
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
2
En c ua lq uie r c o mpue sto q u¶ ³mic o lo s e le me nto s se c o mbina n sie mpre e n la misma pro po rc i¶o n, sin impo rta r
e l o rig e n o mo do de pre pa ra c i¶o n.
La a¯rmaci¶
on anterior ya hab¶³a sido hecha por J. L. Proust e implica que:
² Los compuestos est¶an formados por peque~
nas unidades caracter¶³sticas: las mol¶eculas.
² Las mol¶eculas est¶an formadas por n¶
umeros de¯nidos de ¶
atomos de elementos espec¶³¯cos.
La segunda ley de Dalton (1804) dice:
Si do s e le me nto s fo rma n m¶a s de un c o mpue sto , lo s dife re nte s pe so s de uno e n c o mbina c i¶o n c o n e l mismo
pe so de l o tro e st¶a n e n re la c i¶o n de pe q ue n
~ o s n¶ume ro s e nte ro s.
No es raro que la a¯rmaci¶on anterior (conocida como \Ley de las Proporciones M¶
ultiples) haya sido ignorada
hasta 1804. Si bien los an¶alisis de Lavoisier (1780) para el \aire ¯jado" (hoy d¶³a CO2 ) fueron:
CO2 aire ¯jado
C
O
28% en peso
72% en peso
Los an¶
alisis de Charles Bernard Desormes y Nicolas Clement para el \aire in°amable" (hoy d¶³a CO) fueron:
CO aire in°amable
C
O
44% en peso
56% en peso
La relaci¶
on de peque~
nos n¶
umeros enteros, como puede verse, no es evidente. Pero basta ¯jar un mismo peso
para un elemento, por ejemplo 1 g de C, para que los pesos correspondientes del otro elemento, el ox¶³geno,
resulten en una relaci¶on 2 : 1
De hecho, las dos leyes de Dalton pueden resumirse en una simple ecuaci¶
on, de la cual s¶
olo se conoc¶³an los
t¶erminos de la izquierda:
pA
nA MA
=
(1)
pB
nB MB
donde
pA;B
nA;B
MA;B
= peso de A ¶
o B en el compuesto
= n¶
umero de ¶
atomos de A ¶
oB
en el compuesto
= peso de un ¶
atomo de A ¶
oB
De lo anterior se entiende que puede desarrollarse un sistema de pesos at¶
omicos relativos. En efecto, si se
conocen los pesos de los elementos que intervienen y sus f¶
ormulas, esto es nA;B puede determinarse la relaci¶on
MA : MB . Sin embargo, los qu¶³micos de 1830 desconoc¶³an ambas cosas: las f¶
ormulas y los pesos at¶omicos.
Con todo, Dalton public¶o su teor¶³a at¶omica con una tabla de pesos at¶
omicos relativos; ve¶
amos c¶
omo los calcul¶o.
Dalton sab¶³a que 1 g de hidr¶ogeno se combinaba con 8 g de ox¶³geno para formar agua. Adoptando la regla
de m¶
axima simplicidad (adem¶as no ten¶³a otra alternativa) supuso que la f¶
ormula del agua era HO (notaci¶on
actual). De aqu¶³ se conclu¶³a directamente que un ¶
atomo de ox¶³geno pesaba 8 veces m¶
as que uno de hidr¶ogeno.
Pero no pas¶o mucho tiempo para descubrir inconsistencias. Por ejemplo, en el amoniaco se combinan 3 g de
hidr¶
ogeno con 14 g de nitr¶ogeno, en el ¶oxido nitroso se combinan 16 g de ox¶³geno con 14 g de nitr¶ogeno. Con
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
3
la regla de m¶axima simplicidad llegamos a:
Mnitr¶o g e no
Mhidr¶o g e no
=
14
= 4:666 f¶
ormula N H
3
M nitr¶o g e no
Mo x ¶ ³g e no
=
14
= 0:8750
16
f¶
ormula NO
y, de las igualdades anteriores:
Mo x ¶ ³g e no
Mhidr¶o g e no
=
Mo x ¶ ³g e no
Mnitr¶o g e no
Mhidr¶o g e no
Mnitr¶o g e no
=
4:666
= 5:333
0:875
1
= 0:870
1
4:666
diferente al valor de 8 propuesto originalmente.
La soluci¶
on a esta contradicci¶on se encontr¶
o ¯nalmente en el estudio de los gases, en particular en las relaciones
de los vol¶
umenes con que reaccionan entre s¶³. Alexander von Humboldt y Jos¶e Luis Gay{Lussac investigaron
lo anterior con una precisi¶on asombrosa; sus resultados di¯eren solamente en un 0.19% de los actuales.
2.2
I nvestigaciones de Gay{Lussac
Los estudios de Humboldt y Gay{Lussac se refer¶³an a la formaci¶
on de vapor de agua a aprtir de mezclas de
hidr¶
ogeno y ox¶³geno. Observaron que el volumen de hidr¶
ogeno consumido era el doble del volumen requerido de
ox¶³geno. Podr¶a notarse que hay una \ley de proporciones de¯nidas" en volumen, de forma que pod¶³a pensarse
(como lo hizo Gay{Lussac) que vol¶
umenes iguales de gases (a las mismas condiciones de temperatura y presi¶on)
conten¶³an el mismo n¶
umero de part¶³culas; o bien que hab¶³a una relaci¶
on sencilla entre el n¶
umero de part¶³culas
para ambos vol¶
umenes. De esta manera se preparaba el terreno para la hip¶
otesis de Avogadro.
Curiosamente, Dalton fue uno de los principales adversarios a la proposici¶
on de Gay{Lussac. En efecto, Dalton
(como Franklin y Reamur) estaba cautivado por el poder explicativo de \la esponjosidad de la materia". Seg¶
un
esta propiedad, los gases estaban formados por part¶³culas como esponjas, en contacto mutuo. De aqu¶³ que
los gases fueran compresibles. La \esponjosidad" permiti¶
o explicar fen¶
omenos como la conducci¶
on de calor, el
calentamiento de una barra, la electricidad est¶
atica, la fosforescencia y otros m¶
as; por tanto, era obvio pensar
que la compresibilidad de los gases resultara de su \esponjosidad".
Seg¶
un lo anterior el volumen del gas no pod¶³a depender exclusivamente del n¶
umero de part¶³culas, depend¶³a
tambi¶en del volumen de ¶estas; en consecuencia, el volumen del vapor de agua deb¶³a ser mayor que el de
hidr¶
ogeno2 a¶
un siendo el mismo n¶
umero de part¶³culas pues el volumen de cada part¶³cula de agua era mayor
que el de cada part¶³cula de hidr¶ogeno. N¶otese que, para Dalton, la part¶³cula de agua es HO y la de hidrogeno
es H. En otras palabras, el volumen de un compuesto qu¶³mico gaseoso deb¶³a ser mayor que el de un elemento
qu¶³mico gaseoso pues ¶este, por de¯nici¶on, est¶
a formado por ¶
atomos de un solo tipo.
La conciliaci¶on de los resultados de Humboldt{Gay Lussac y Dalton la hizo un f¶³sico y abogado italiano en 1811
¶ era Carlo Lorenzo Romano Amadeo Avogadro,
en un trabajo que pas¶o desapercibido durante casi 50 a~
nos. El
conde de Quaregna y Ceretto.
2A
igu ales con d icion es d e T y p, se en tien d e.
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
2.3
4
E l modelo de Avogadr o
En el modelo de gas propuesto por Avogadro las part¶³culas de gas no est¶
an en contacto mutuo; no son esponjas
sino part¶³culas individuales cuyo volumen es insigni¯cante comparado al volumen total del gas. De aqu¶³ se
desprende la conocida hip¶otesis de Avogadro:
\Vol¶
umenes iguales de gases, a las mismas condiciones de temperatura y presi¶
on, tienen igual n¶
umero
de part¶³culas."
aunque ¶el, modestamente, la presenta como una simple y obvia consecuencia de los experimentos de Gay{Lussac.
Con todo, aceptar la hip¶otesis de Avogadro lleva a otras di¯cultades.
En efecto, seg¶
un las f¶ormulas de Dalton y la navaja de Ockam3 se pod¶³a escribir la ecuaci¶
on:
H + O ¡! HO
(2)
y, por la hip¶otesis de Avogadro, el volumen de agua formado ser¶³a igual al inicial de hidr¶
ogeno (e igual al de
ox¶³geno). Pero los resultados experimentales indicaban que dos vol¶
umenes de hidr¶
ogeno se combinaban con
s¶
olo un volumen de ox¶³geno, y se produc¶³an dos vol¶
umenes de agua.
Para sortear esta di¯cultad Avogadro propuso que las part¶³culas elementales del hidr¶
ogeno y ox¶³geno no eran
¶
¶tomos sino grupos de ¶atomos: las mol¶eculas. Esta
a
es la segunda y m¶
as importante de las contribuciones de
Avogadro. De acuerdo a lo anterior pod¶³a escribirse
2H2 + O2 ¡! 2H2 O
(3)
con lo que se elimina la inconsistencia con los resultados experimentales.
Pero. . . >por qu¶e no escribir alguna de las siguientes reacciones?
2H + O2
2H + O4
2H3 + O6
¡! 2HO
¡! 2HO2
¡! 2H3 O3
>C¶
omo pod¶³a determinarse si las part¶³culas (mol¶eculas) eran O2 , O4 , O6 , H, H3 , etc.?
La soluci¶
on la puso Avogadro, como antes Dalton, en el principio de m¶
axima simplicidad.
2.4
Las investigaciones de Cannizzar o
M¶
as tarde, Stanislao Cannizzaro aplic¶o sistem¶
aticamente la hip¶
otesis de Avogadro para determinar las f¶ormulas
moleculares de compuestos gaseosos y las masas at¶
omicas relativas de sus elementos.
Examinando la tabla 1 se puede entender c¶
omo calcul¶
o las masas at¶
omicas; as¶³mismo pueden deducirse las
f¶
ormulas moleculares. Tomemos como ejemplo el ox¶³geno: 1 litro pesa 1.43 g y un n¶
umero igual de part¶³culas
de agua pesa 0.803 g, con 0.713 g de ox¶³geno. Puesto que 1.43 es casi el doble de 0.713
1:43 ¼ 2 £ 0:713
la conclusi¶
on es: el ox¶³geno gaseoso contiene el doble de ¶
atomos de ox¶³geno que igual n¶
umero de part¶³culas de
agua.
Ahora bien, puesto que no se ha encontrado (hasta ahora) ning¶
un compuesto que tenga una cantidad menor
que 0.713 g (con el mismo n¶
umero de part¶³culas considerado) es razonable suponer que en los compuestos de
la tabla 1, donde aparecen 0.713 g de ox¶³geno tendremos la menor cantidad de ¶
atomos de ox¶³geno, esto es, un
s¶
olo ¶
atomo. La conclusi¶on es que el ox¶³geno gaseoso tiene la f¶
ormula O2 .
3T
am b i¶
en con ocid o com o \p r in cip io d e m ¶
ax im a sim p licid ad " o \p r in cip io d e p ar sim on ia". Fu e p r op u esto p or el fr an ciscan o
Gu iller m o d e Ock am en 1340.
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
5
Tabla 1
Ga s
D e nsida d
a
C .N .
g / L
H idr¶o g e no
Ox ¶ ³g e no
A z ufre (< 1 0 0 0 ± )
A z ufre (> 1 0 0 0 ± )
F ¶o sfo ro
C lo ro
N itr¶o g e no
M e rc urio
A g ua
C lo ruro de
hidr¶o g e no
0 .0 9 0
1 .4 3 0
8 .5 9
2 .8 6
5 .5 3
3 .1 6
1 .2 5
8 .9 6
0 .8 0 3
1 .6 3
0 .0 9 0
A mo nia c o
F o s¯na
Sulfuro de
hidr¶o g e no
0 .7 6 0
1 .5 2
1 .5 2
0 .1 3 5
0 .1 3 5
0 .0 9 0
C ia nuro de
hidr¶o g e no
1 .2 0
0 .0 4 5
C lo ruro de
f¶o sfo ro
6 .1 3
C a lo me l
Sublima do
c o rro siv o
1 0 .5 4
1 2 .1 2
Ox ido
nitro so
1 .9 6
0 .7 1 3
1 .2 5
Ox ido
n¶ ³tric o
1 .3 4
0 .7 1 3
0 .6 2 5
Ox ido
c a rb¶o nic o
1 .2 5
0 .7 1 3
0 .5 4
A c ido
c a rb¶o nic o
1 .9 6
1 .4 3
0 .5 3
Ox ido de a z ufre
Etile no
A lc o ho l
2 .8 6
1 .2 5
2 .0 5
g / L
H
O
de c a da e le me nto c o nstituy e nte
S
P
Cl
N
Hg
C
F o¶ rmula
mo le c ula r
y ma sa
mo le c ula r
H2 , 2 .0 2
1 .4 3 0
8 .5 9
2 .8 6
P4 , 1 2 4
5 .5 3
3 .1 6
1 .2 5
8 .9 6
0 .0 9 0
0 .0 4 5
0 .7 1 3
1 .5 8
1 .4 3
0 .6 2 5
1 .3 8
1 .4 3
0 .7 1
0 .5 3
4 .7 4
1 .5 8
3 .1 6
0 .1 8 0
0 .2 7 0
N H3 , 1 7
0 .6 2 5
1 .3 8
8 .9 6
8 .9 6
1 .4 3
1 .0 7
1 .0 7
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
6
Tabla 2
Porcentaje de abundancia de los is¶
otopos
C y O (variable, seg¶
un la procedencia)
3
Is¶
otopos
%
C 12
C 13
C 14
98.89
1.11
trazas
O16
O17
O18
99.76
0.04
0.20
De¯nici¶
on de N0
Durante mucho tiempo los qu¶³micos dudaron acerca de qu¶e elemento convendr¶³a tomar como base de los pesos
at¶
omicos relativos; algunos propon¶³an al hidr¶
ogeno con peso igual a 1; otros al ox¶³geno con peso 16; Berzelius
propon¶³a al ox¶³geno pero con peso 100; Dulong y Petit propon¶³an en cambio al ox¶³geno con peso 1.4
No fue sino hasta 1893 que apareci¶o la primera tabla de pesos at¶
omicos o¯ciales de la American Chemical
Society. Pero. . . toda soluci¶on abre nuevos problemas y ¶este era. . . >cu¶
antos ¶
atomos hay en un peso igual al
peso at¶
omico de un elemento?
Hoy d¶³a tenemos varias respuestas, totalmente equivalentes:
² un mol,
² un n¶
umero de Avogadro,
² 6:023 £ 1023
² el mismo n¶
umero que hay en 22.4 L de gas ideal a S.T.P.
As¶³ como el metro se ha de¯nido de diferentes maneras (cada una con ventajas en ciertas circunstancias), el
n¶
umero de Avogadro tambi¶en ha tenido distintas de¯niciones.
El punto com¶
un a todas las de¯niciones es el siguiente:
N0 es el n¶
umero de part¶³culas presentes en una cantidad de¯nida de una sustancia de¯nida.
La forma de aplicarlo ha cambiado en la historia; ha sido: 1 g de hidr¶
ogeno, 16 g de ox¶³geno, 16 g de O16 y,
12
actualmente, 12 g de C . Se ha empleado, incluso, una sustancia inexistente: el gas ideal (22.4 L a S.T.P.).
N¶
otese la imprecisi¶on del t¶ermino \part¶³culas" empleado en la de¯nici¶
on de N0 . No es lo mismo hablar de 1 mol
de ¶
atomos de hidr¶ogeno que hablar de 1 mol de mol¶eculas de hidr¶
ogeno. En el primer caso se tiene 1 g de
hidr¶
ogeno, en el segundo 2. De aqu¶³ que hablar de \1 mol de hidr¶
ogeno" sea completamente ambiguo pues no
se especi¯ca cu¶al es la part¶³cula en consideraci¶
on, el ¶
atomo o la mol¶ecula de hidr¶
ogeno.
Puesto que tanto el ox¶³geno, como el carbono, empleados en la de¯nici¶
on de N0 tienen is¶
otopos5 no es su¯ciente
para de¯nir el n¶
umero de Avogadro decir el peso de elemento base, se requiere precisar de qu¶e is¶
otopo se trata.
Por supuesto, antes de descubrir la existencia de los is¶
otopos hab¶³a errores inevitables pues la abundancia
relativa de los del ox¶³geno, por ejemplo, in°u¶³a en el n¶
umero de ¶
atomos considerado, v¶eanse tablas 2 y 3.
4 E s clar o q u e la d e¯ n ici¶
on d e p eso at¶
om ico es el p eso d e u n n u ¶ m er o igu al d e ¶
atom os al con ten id o en el p eso b ase d el elem en to
q u ¶³m ico tom ad o com o b ase
5 S on elem en tos q u ¶
³m icos con igu al n u ¶ m er o at¶
om ico, p er o d ifer en te m asa at¶
om ica.
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
7
Tabla 3
De¯nici¶
on de pesos at¶
omicos
4
Escala
f¶³sica
Escala
qu¶³mica
Escala
uni¯cada
O16
O
16.0000
15.99560
16.00000
15.99491
15.9994
C 12
C
12.00382
12.00052
12.011
12.0000
12.0115
I nter medio
Antes de presentar algunos de los m¶etodos empleados para calcular el n¶
umero de Avogadro es preciso mencionar
que durante casi cincuenta a~
nos su hip¶otesis pas¶
o inadvertida para los cient¶³¯cos; posiblemente por lo siguiente:
² La ambigÄ
uedad del lenguaje; Avogadro no distingu¶³a claramente entre ¶
atomo y mol¶ecula.
² La falta de resultados experimentales. Avogadro no tuvo nunca la fama de buen experimentador. Enrique
R. Regnault6 opinaba: \No es un te¶
orico brillante, m¶
as bien es un cient¶³¯co experimental descuidado".
² La generalizaci¶on de la hip¶otesis al estado s¶
olido a partir de especulaciones y analog¶³as.
² La atenci¶on de los cient¶³¯cos a la naciente qu¶³mica org¶
anica. La s¶³ntesis de compuestos org¶anicos, la
importancia industrial de las anilinas.
² La oposici¶on entre la teor¶³a de Berzelius y la existencia de mol¶eculas elementales diat¶
omicas, como H2 ,
O2 , etc.
² La lejan¶³a de Avogadro del ambiente cient¶³¯co.
5
P r imer as estimaciones de N0
5.1
Daniel B er noulli
Las primeras estimaciones del N¶
umero de Avogadro (N0 ) fueron hechas en la segunda mitad del siglo XVIII a
partir de la Teor¶³a Cin¶etica de los Gases. Los cimientos de esta teor¶³a se remontan a 1738 con Daniel Bernoulli,
miembro de la c¶elebre familia de matem¶aticos franceses, quien relacion¶
o la presi¶
on de un gas con el movimiento
molecular para llegar a:
1
pV = nmu2
3
donde
p
V
n
m
u2
=
=
=
=
=
presi¶
on del gas
volumen del gas
n¶
umero de part¶³culas
masa de cada part¶³cula
velocidad cuadr¶
atica promedio
Sin embargo, D. Bernoulli no avanz¶o m¶as en sus investigaciones debido a las limitaciones experimentales de su
¶epoca en el manejo de gases. Al parecer quien hizo el primer c¶
alculo de N0 fue Josef Loschmidt en 1865, nueve
a~
nos despu¶es de la muerte de Avogadro. Sin embargo, como se~
nala Hawthorne,7 hay razones para dudarlo.
6E
7H
n cu y o r econ ocim ien to se r ep r esen ta con R la con stan te u n iv er sal d e los gases.
aw th or n e, J r . R. J . C hem. E d. 1970, 47, 11
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
5.2
8
Josef Loschmidt
Es una curiosidad hist¶orica que el art¶³culo donde Loschmidt present¶
o el m¶etodo de c¶
alculo del tama~
no de las
part¶³culas de un gas no incluya el resultado num¶erico, mucho menos el llamado \n¶
umero de Loschmidt"(NL ),
de¯nido como el n¶
umero de part¶³culas en 1 cm3 a 0±C y 1 at.
El primer art¶³culo donde se da el valor num¶erico de NL es un resumen del trabajo al que hacemos referencia.
Publicado bajo el nombre de J. Loschmidt reporta el valor de NL = 8:66 £ 1017 part/cm3 . Sin embargo, este
n¶
umero no se puede obtener de las consideraciones y datos de este art¶³culo, cuyo estilo de redacci¶
on es, adem¶as,
completamente distinto al de Loschmidt.
Loschmidt comenz¶o el planteo a partir de la expresi¶
on de trayectoria libre promedio derivada por Maxwell y
modi¯cada por Clausius:
1
16 ¼`s2
(4)
=
NL
3 4
donde
NL = n¶
umero de Loschmidt
` = trayectoria libre promedio
s = di¶
ametro de la part¶³cula
Loschmidt multiplic¶o por s los dos miembros de la ecuaci¶
on 4 para llegar a:
·
¸
NL ¼s3
s=8
3
(5)
Puesto que ¼s3 =6 es el volumen de una mol¶ecula, NL ¼s3 =6 es el volumen efectivo de NL mol¶eculas, esto es, sin
considerar los espacios intersticiales. Cuando estos espacios se toman en cuenta el volumen que ocupan es, por
de¯nici¶
on, 1 cm3 .
En una primera aproximaci¶on se puede considerar que
Vm
Vl
=
="
Vg
Vg
(6)
donde
Vm
Vg
Vl
"
=
=
=
=
volumen real de NL part¶³culas
volumen de ¶estas en fase gas = 1cm3
volumen de ¶estas en fase l¶³quida
coe¯ciente de condensaci¶
on (experimental)
Obviamente Vm y Vl no son iguales, pues hay intersticios que dependen de la geometr¶³a del empaquetamiento
en el l¶³quido; " var¶³a de 1.17 a 1.91 llegando a tenerse mayores desviaciones para mol¶eculas no esf¶ericas. Con
todo, el orden de magnitud de los resultados obtenidos con las consideraciones anteriores es aceptable.
Se puede, por tanto, escribir:
s = 8"`
(7)
De esta manera, a partir del coe¯ciente de condensaci¶
on y de la trayectoria libre promedio, Loschmidt pudo
haber estimado s y, sustituyendo en la ecuaci¶
on 1, calcular NL . Pero no lo hizo.
Para la u
¶nica sustancia que era conocida `, el aire, se desconoc¶³a el coe¯ciente de condensaci¶
on pues a¶
un no
se hab¶³a logrado licuar el aire. Esto se logr¶
o casi 15 a~
nos despu¶es de la publicaci¶
on del trabajo original de
Loschmidt. Con todo, mediante otras consideraciones pudo estimar "a ire = 0:00086 pero, repetimos, no hizo el
c¶
alculo que lo habr¶³a llevado a
NL
por tanto, N0
= 1:83 £ 1018
= 4:09 £ 1022
part/cm
3
part/mol
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
5.3
9
William T homson
En 1870, William Thomson, Lord Kelvin, present¶
o en la revista Nature cuatro m¶etodos para determinar
di¶
ametros moleculares. Uno de ellos era b¶
asicamente igual al m¶etodo de Loschmidt. Los dem¶
as empleaban
consideraciones muy ingeniosas para estimar el di¶
ametro molecular. Por ejemplo, a partir de la existencia de
s¶
olidos y l¶³quidos transparentes Kelvin concluy¶
o que la distancia entre los centros de las mol¶eculas deb¶³a ser del
mismo orden de magnitud que la longitud de onda de la luz que los atraviesa. En ese tiempo ya exist¶³an m¶etodos
para determinar longitudes de onda por interferencia, de forma que Lord Kelvin concluy¶
o que las distancias
entre los centros de las mol¶eculas era del orden de 0.5 º
A.
Otro m¶etodo propuesto por Lord Kelvin se bas¶
o en el espesor m¶³nimo de la pel¶³cula de una burbuja de jab¶on.
Calcul¶
o el trabajo necesario para aumentar el tama~
no de la burbuja, lo que signi¯caba disminuir el espesor de
la pel¶³cula. Consider¶o que este trabajo no pod¶³a ser mayor que la energ¶³a de vaporizaci¶
on para esa cantidad
de agua, si lo era era porque hab¶³a estallado la burbuja. En el caso de que el trabajo empleado para in°ar la
burbuja fuera casi igual a la energ¶³a de vaporizaci¶
on de ¶esta se tendr¶³a el espesor l¶³mite de la pel¶³cula (deb¶³a
estar a punto de transformarse en vapor), es decir, era monomolecular. A partir de lo anterior, Kelvin estim¶o
un di¶
ametro molecular de, aproximadamente, 0.5 º
A.
6
C¶
alculos de N0 en el siglo XX
t Las primeras estimaciones de N0 hechas a ¯nes del siglo XIX no empleaban propiedades individuales de un
¶tomo o mol¶ecula, sino que relacionaban propiedades macrosc¶
a
opicas con las microsc¶
opicas mediante deducciones
y conjeturas.
Con el re¯namiento de los instrumentos de laboratorio en el siglo XX fue posible determinar experimentalmente
propiedades microsc¶opicas, por ejemplo, el espaciamiento entre los planos de un cristal o la carga del electr¶on.
De tal suerte pudo calcularse N0 a partir de datos m¶
as precisos.
Sin embargo, la precisi¶on experimental no es la u
¶nica diferencia en los c¶
alculos de N0 del siglo XIX y del XX.
El art¶³culo de Loschmidt, por ejemplo, contiene especulaciones acerca de la naturaleza del ¶eter y las diferencias
entre la materia viviente y la no viviente para concluir relacionando tales cuestiones con el tama~
no de las
mol¶eculas. Boltzmann, por su parte, desarroll¶
o su teor¶³a cin¶etica molecular oponi¶endose abiertamente a la
¯losof¶³a de Schopenhauer y de Ostwald quien, a su vez, rechazaba el materialismo estricto y pertenec¶³a a un
grupo espiritualista.
Los trabajos del siglo XX, en cambio, no contienen especulaciones acerca de las implicaciones ¯los¶
o¯cas de tal
investigaci¶
on (mucho menos mencionan las posibles consecuencias sociales). Lo anterior es, en parte, resultado
de una cada vez mayor especializaci¶on de la actividad humana en general, y de la investigaci¶
on cient¶³¯ca, en
particular.
6.1
Deter minaci¶
on de N0 a par tir de la T eor ¶³a Cin¶
etica de los Gases
En 1884 Lord Kelvin present¶o un nuevo m¶etodo para la determinaci¶
on de N0 . Se bas¶
o en el tratamiento cin¶etico
de Maxwell de la difusi¶on molecular y de las viscosidades de los gases que lleva al coe¯ciente de difusi¶on de un
gas:
1
u
D= p
(8)
s
2 3¼ NL 2
donde
D
u
NL
s
=
=
=
=
coe¯ciente de difusi¶
on del gas
r.m.s. de la velocidad
n¶
umero de Loschmidt
di¶
ametro de las mol¶eculas
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
10
Ahora bien, de acuerdo al trabajo de Maxwell:
D=k
¹
½
(9)
donde
¹ = viscosidad del gas
½ = densidad del gas
por lo que
1 u½
(10)
NL s2 = p
2 3¼ ¹
En aquella ¶epoca hab¶³a su¯cientes valores experimentales para las variables u; ½; ¹ pero, desafortunadamente
Kelvin no ten¶³a valores con¯ables de s, de manera que hizo las mismas consideraciones de Loschmidt para el
coe¯ciente de condensaci¶on; con ello obtuvo
N0 = 1:21 £ 1020
6.2
Deter minaci¶
on de N0 a par tir del espesor l¶³mite de una pel¶³cula
Trabajando independientemente en 1890, J. William Rayleigh y William C. Rontgen llegaron a estimar las
dimensiones moleculares a partir del espesor l¶³mite de una pel¶³cula de aceite en agua. Encontraron que la
tensi¶
on super¯cial del agua no se modi¯ca para pel¶³culas de aceite de espesor menor que 6 º
A. Concluyeron
que este valor es el espesor de una pel¶³cula monomolecular pues, si disminuye m¶
as, no se modi¯ca la tensi¶on
super¯cial del agua. Este argumento los llev¶
o a resultados sorprendentemente exactos para las dimensiones de
las mol¶eculas. Pero no calcularon N0 .
6.3
Deter minaci¶
on de N0 a par tir de la ecuaci¶
on de Van der Waals
A ¯nes del siglo XIX Jean Perrin emple¶o el t¶ermino b de la ecuaci¶
on de Van der Waals para determinar N0 de
la siguiente manera:
µ
¶
n2 a
p + 2 (V ¡ nb) = nRT
(11)
V
donde
R = constante de los gases
p = presi¶
on del gas
a = t¶ermino de correcci¶
on por atracci¶
on intermolecular
b = t¶ermino de correcci¶
on por volumen real de las part¶³culas
V = volumen del gas
T = temperatura absoluta
n0
n =
= n¶
umero de moles de part¶³culas
N0
En la ecuaci¶on 11 el par¶ametro b puede calcularse de los valores cr¶³ticos del gas, pues:
b=
RTc
8pc
(12)
por otro lado, dicho t¶ermino es igual al volumen de N0 esferas de di¶
ametro s, esto es:
1
b = ¼N0 s3
6
(13)
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
11
Puesto que s aparece en la ecuaci¶on 7 y N0 = 22400cm3 £ NL (por de¯nici¶
on), se pueden combinar estas
ecuaciones para calcular N0 .
Jean Perrin obtuvo a partir de los datos del vapor de mercurio:
N0 = 2:8 £ 1019 part/mol
6.4
(14)
Deter minaci¶
on de N0 a par tir del movimiento br owniano
El mismo Jean Perrin, en 1909, determin¶
o N0 a partir de distintas consideraciones acerca del movimiento
browniano de las part¶³culas coloidales y de su distribuci¶
on con la altura, resultante de la fuerza de gravedad.
Albert Einstein hab¶³a mostrado que, para una part¶³cula movi¶endose completamente al azar, la media cuadr¶atica
a relacionada con el coe¯ciente de difusi¶on
de su desplazamiento en una direcci¶on (x2 ) en cierto tiempo (t) est¶
(D) por:
x2 = 2Dt
(15)
Si la part¶³cula sigue la ley de Stokes (lo cual supone un medio continuo), se tiene que:
D=
RT
6¼´aN0
(16)
donde
´ = viscosidad del °uido
a = radio de la part¶³cula
Perrin midi¶o los desplazamientos de las part¶³culas con un microscopio dotado de un ocular reticular que serv¶³a
como sistema de coordenadas. A partir de los desplazamientos y los tiempos empleados Perrin pudo calcular
los valores cuadr¶aticos promedio de los desplazamientos y, conocido a, determinar N0 .
6.5
Deter minaci¶
on de N0 a par tir de la distr ibuci¶
on de B oltzmann
Otro procedimiento, tambi¶en desarrollado por J. Perrin, considera la ley de distribuci¶
on de Boltzmann. Seg¶
un
¶esta la relaci¶on entre los n¶
umeros de particulas n1 y n2 con energ¶³as E1 y E2 , respectivamente, est¶
a dada por:
·
¸
n1
(E2 ¡ E1 )N0
= exp
(17)
n2
RT
En una suspensi¶on coloidal la energ¶³as potencial de las part¶³culas a alturas h1 y h2 est¶
a dada por:
E1
E2
= W h1
= W h2
donde W es el peso efectivo de las part¶³culas. Este peso es diferente del peso real debido a la °otabilidad de
¶estas en el °uido; se calcula a partir de su radio y densidad, lleg¶
andose a:
· ¸
RT
n2
N0 =
ln
(18)
W (h1 ¡ h2 )
n1
Perrin logr¶o producir part¶³culas de tama~
no uniforme por centrifugaci¶
on y, con un microscopio de muy peque~
na
profundidad de campo, midi¶o el n¶
umero de part¶³culas a diferentes alturas. Con ello obtuvo el valor
N0 = 7:2 £ 1023 part/mol
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
1 .
2 .
3 .
e s
4 .
12
T ubo c o n Hg pa ra me dir e l v o lume n de He fo rma do a pre si¶o n c o nsta nte .
C ¶a ma ra a l v a c ¶ ³o , do nde se a c umula r¶a e l He
C ¶a ma ra de v idrio de pa re de s de lg a da s. Pe rmite la sa lida de pa rt¶ ³c ula s ® pe ro no
pe ne tra do po r e l He
M ue stra de ma te ria l ra dio a c tiv o .
F ig ura 1
6.6
Deter minaci¶
on de N0 a par tir de la r adioactividad
En 1910, Thomas Royds y Ernest Rutherford emplearon un equipo con una c¶
amara dentro de otra, ¯gura 1;
la m¶
as interior conten¶³a material radioactivo productor de part¶³culas ®, la exterior estaba conectaba a un
man¶
ometro de mercurio que permit¶³a conocer la cantidad de He formado por las part¶³culas ®. Las paredes
de la c¶
amara interior eran de vidrio delgado permeable a las part¶³culas ® producidos en esta c¶
amara, pero
impermeable al gas He producido en la segunda c¶
amara.
Una muestra de radio en la c¶amara interior produc¶³a las part¶³culas ® las cuales, a su vez, produc¶³an ¶atomos de
helio seg¶
un la ecuaci¶on
® + 2e¡ ¡! He0
Poco antes, en 1908, Hans Geiger y Rutherford hab¶³an inventado un contador de part¶³culas ®, de aqu¶³, y por la
reacci¶
on anterior, pod¶³an calcular el n¶
umero de ¶
atomos de He producidos: es igual al n¶
umero de part¶³culas ®.
Royds y Rutherford encontraron que en un a~
no se formaban 0.0430 cm3 de He0 (a S.T.P.) y que el n¶
umero de
part¶³culas ® emitidas en ese lapso era 11:6 £ 1016 . De aqu¶³ calcularon
N0 = 6:043 £ 1023
6.7
Deter minaci¶
on de N0 a par tir de la car ga del electr ¶
on
En 1917 Robert A. Millikan determin¶o N0 a partir de la carga del electr¶
on utilizando un aparato como el
representado en la ¯gura 2. En el experimento se produc¶³an unas gotitas de aceite con un \atomizador"en una
c¶
amara al vac¶³o. Estas gotas se cargaban el¶ectricamente al chocar con los iones formados por la acci¶on de rayos
X sobre el aire remanente. Millikan supo que la gota ten¶³a carga el¶ectrica por su comportamiento entre las
placas M, N cuyo voltaje pod¶³a regular.
Modi¯cando el voltaje entre las placas pod¶³a hacer que las gotas de aceite ascendieran a velocidad constante y
de ah¶³ calcular la carga el¶ectrica de la gota.
Cuando se eliminaba el campo el¶ectrico la gota estaba sometida exclusivamente a la fuerza de la gravedad
y, debido a la resistencia del aire, la gota no caia aceleradamente sino que alcanzaba una velocidad terminal
constante dada por:
mg
v=
(19)
6¼´r
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
13
A . a to miz a do r. B . c a mpo e l¶e c tric o de 8 0 0 0 V . C . c ¶a ma ra libre de a ire y po lv o . N ,
M . pla c a s de bro nc e de de 2 2 c m de di¶a me tro se pa ra da s 1 6 mm. S . inte rrupto r.
F ig ura 2 . D ia g ra ma de l e q uipo de M illik a n.
donde
m
r
g
´
=
=
=
=
masa de la gota
radio de la gota
aceleraci¶
on gravitacional
viscosidad del aire
Esta ecuaci¶on, combinada con la expresi¶
on de la densidad del aceite:
m
½= 4 3
3 ¼r
(20)
permiti¶
o calcular m y r a partir de la velocidad terminal y la densidad del aceite.
Ahora bien, si la gota tiene una carga q y est¶
a bajo un campo el¶ectrico E, la fuerza que act¶
ua sobre ella es qE.
Puesto que, adem¶as est¶a bajo la acci¶on de la gravedad, la fuerza neta sobre la gota es qE ¡ mg, si ¶esta sube.
En este caso su velocidad es:
qE ¡ mg
v=
(21)
6¼´r
Puesto que v y E pueden medirse en el laboratorio, y m, g, ´ y r pueden calcularse, tambi¶en q puede obtenerse.
Millikan obtuvo valores como los siguientes:
3:2 £ 10¡19
4:8 £ 10¡19
8:0 £ 10¡19
11:2 £ 10¡19
C
C
C
C
Todos estos valores son m¶
ultiplos de 1:6 £ 10¡19 C, de forma que Millikan concluy¶
o que ¶esta era la carga del
electr¶
on. N¶otese la semejanza con el m¶etodo de Cannizzaro empleado para calcular las masas at¶
omicas relativos.
Por otro lado, ya eran bien conocidas las relaciones entre cantidad de electricidad y peso de sustancia depositada
en una electr¶olisis.8 La tabla 2 presenta el peso de diversas sustancias liberado en una electr¶
olisis empleando la
misma cantidad de electricidad (1 ampere durante 1 hora)
8S
on las ley es d e Far ad ay (1834), au n q u e tam b i¶
en fu er on p r op u estas p or el italian o C ar los Matteu cci el m ism o a~
n o. Gr acias a
estas ley es, Far ad ay p u d o an ticip ar la ex isten cia d e \¶
atom os d e electr icid ad ", p oster ior m en te con ocid os com o \electr on es"
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
14
Tabla 2
sustancia
Ox¶³geno
Cloro
Iodo
Fluor
Hidr¶ogeno
Potasio
Sodio
Litio
Bario
Estroncio
g/amp£hora
0.2983
1.3220
4.7303
0.7085
0.0376
1.4584
0.8596
0.2622
2.5619
1.6333
peso
equivalente
8
35.5
126
18.7
1
39.2
23.3
10
68.7
43.8
masa
at¶
omica
16.0
35.5
126.9
19.0
1.008
39.1
23.0
6.9
137.4
87.6
valencia
2
1
1
1
1
1
1
1
2
2
La columna de peso equivalente se obtiene dividiendo el peso de la sustancia depositada en la electr¶olisis entre
el peso correspondiente al hidr¶ogeno. As¶³, por ejemplo, el peso equivalente del ox¶³geno es:
0:2983
= 7:936 ¼ 8
0:03759
Puesto que 1 ampere es igual 1 C/s se pudo calcular la cantidad de electrones (cuya carga hab¶³a determinado
Millikan) para depositar una masa de sustancia igual a su masa at¶
omica.
De esta manera Millikan encontr¶o
6.8
N0 = 6:07 £ 1023 part/mol
Deter minaci¶
on de N0 empleando r ayos X
El valor m¶
as exacto de N0 se obtiene midiendo el espaciamiento de una ret¶³cula cristalina con rayos X.
Tanto la longitud de onda (¸) de los rayos X, como el espaciamiento de la ret¶³cula del cristal(D) se determinan
con la ecuaci¶on de Bragg (1913):
a¸ = 2Dsen µ
(22)
donde
¸
D
a
µ
=
=
=
=
longitud de onda de rayos X
espaciamiento reticular
n¶
umero entero
angulo de difracci¶
¶
on
Ahora bien, el volumen ocupado por una mol¶ecula est¶
a dado por
v=Á
D3
n
donde
D
= espaciamiento reticular
Á = factor geom¶etrico
n = n¶
umero de mol¶eculas/celda unitaria
Por otro lado, como
N0 =
masa molar
M
=
masa de una mol¶ecula
m
(23)
El n¶
umero de Avogadro. Jos¶e Luis C¶
ordova Frunz
se puede escribir
N0 =
15
M
M
nM
=
=
m
½v
½ÁD3
(24)
Para estos c¶alculos la densidad ½ del cristal debe medirse muy exactamente. R. T. Birge encontr¶
o el valor
N0 = (6:02283 § 0:00011) £ 1023
usando cristales de calcita (CaCO3 )
Por su parte T. Batuecas emple¶o diamante, a ¯n de evitar la incertidumbre debida a los is¶
otopos del calcio,9 y
obtuvo
N0 = (6:0236 § 0:00007) £ 1023
Las mediciones m¶as recientes del n¶
umero de Avogadro con rayos X han dado
N0 = 6:02316 £ 1023
En 1963, la Comisi¶on de la Academia Nacional de Ciencias y del Consejo Nacional de Investigaci¶
on de Estados
Unidos recomend¶o adoptar el siguiente valor
N0 = (6:02252 § 0:00028) £ 1023 part/mol
Con todo, el valor recomendado en 1986 en el CODATA Bulletin No. 63, nov. 1986 para la constante de
Avogadro es:
N0 = 6:0221367(36) £ 1023 mol¡1
donde los d¶³gitos entre par¶entesis son la incertidumbre para una desviaci¶
on est¶
andar.
***
9E
l Ca tien e 6 is¶
otop os estab les y el O tien e 3; el C s¶
olo tien e 2 is¶
otop os estab les.