Atomic Weights Just 20 years ago, while completing his chairmanship of Manchester Literacy and Philosophical Society, and the IUPAC Commission on Atomic Weights, Norman than published in 1805.) Since hydrogen had the Holden prepared and published in Chemistry smallest atomic weight value, Dalton chose that ele- International (1984, issue No.1) an early historical review ment as his reference scale unit, hydrogen = 1, and he of the International Commission on Atomic Weights. calculated atomic weights by comparing weights of Since then, his interest of the topic has not faded, and other atoms with that of hydrogen. Values given in au contraire, he has now reviewed, extended, and the table indicate that Dalton grasped the ideas of updated the historical review to a length far beyond the constant composition in compounds and of multiple space available here. The excerpts below are extracted proportions. However, he did not account for the from the historical review, which is available online at valence of each element in the compound. Dalton’s <www.iupac.org/publications/ci/2004/2601/ later tables, published in 1808 and 1810, show a 1_holden.html>. The full text is equivalent to about 20 marked improvement in accuracy but the values are pages and includes more than 100 references. still difficult to recognize because of these errors in valence (i.e., some equivalent weights [atomic Atomic Weights and the weight/valence] are quoted rather than atomic International Committee: weights). By the end of the nineteenth century, atomic A Brief Historical Review weight had taken on the concept of a constant of nature like the speed of light, but the lack of agree- ment on accepted values created difficulties in trade. by Norman E. Holden All parties were not translating quantities measured by chemical analysis into weights in the same way. he International Committee With so many different values being reported, the on Atomic Weights (ICAW) American Chemical Society (ACS), in 1892, appointed Thas a long and colorful his- a permanent committee to report on a standard table tory dating back for over a cen- of atomic weights for acceptance by the Society. tury. Initially, the task was to Frank W. Clarke, who had been appointed a commit- provide the chemical community tee of one, presented his first report at the 1893 and trade and commerce with the annual meeting. most accurate atomic weight val- In 1897, the Deutsche Chemische Gesellschaft ues for the chemical elements. For appointed a working committee to report on atomic over the past half century, the iso- weights. The committee, chaired by Hans Landolt topic composition of the stable (or (Berlin), published its first report in 1898. In contrast Norman E. Holden, when very long-lived) isotopes of those to Clarke, who presented a review of every atomic chairman of the commis- elements has taken on a larger weight value published during the year along with his sion from 1979-1983. role, until today the atomic weight recommended values, the German committee merely values are determined by mass gave the table with its estimated best value for each weighting the isotopic abundance values. There was element. The committee argued for the adoption of much interest in the atomic weight values when they the O = 16 scale and invited other chemistry organiza- were considered constants of nature and the building tions to appoint delegates to an international body. blocks of the periodic table and even more now that The resulting International Committee on Atomic they are known to be variable. Weights (ICAW) began with 57 chemists. The com- mittee’s first report, published in 1901, was a table on Background the O = 16 scale, which appeared as a flyleaf in issue 1 of the Chemische Berichte in 1902. Just two hundred years ago, the English school- The ICAW soon decided to elect a smaller commit- teacher, John Dalton, presented the first table of tee of three members to avoid the difficulties and atomic weight values in a paper entitled On the delays of corresponding among a large group. The Absorption of Gases by Water and Other Liquids. (The top three vote getters, Clarke, Karl Seuber, and table was first read on 21 October 1803 to the Thomas Edward Thorpe, were elected. This commit- 4 CHEMISTRY International January-February 2004 Go online for full text and references. tee reported annually (except for 1918) until 1921. In The Atomic Weights Scale 1913, the committee became formally affiliated with The atomic weights scale of H = 1 was originally used the International Association of Chemical Societies by Dalton and (except for Berzelius’ time) had been (IACS), which had been formed two years earlier. The used for approximately 100 years when the ACS and ICAW was charged with publishing an updated Table the German committees began reporting their tables. of Atomic Weights every year. Although the IACS was Lothar Meyer (one of the first developers of the peri- formally dissolved in 1919, the ICAW continued to odic table) and Seubert had published on the hydro- publish its annual tables until 1922. gen scale, but Wilhelm Ostwald (a member of the In 1918, a conference of scientists from allied (coun- German committee who later won the Nobel Prize for tries at war with the Central Powers) scientific acade- chemistry for catalysis) and Bohuslav Brauner (mem- mies withdrew from the IACS and formed the ber of the Committee on Chemical Elements) strongly International Research Council (IRC). A year later, at urged the adoption of the O = 16 scale. Clarke the Inter-allied Chemical Conference in London, the reported his table on both scales, while the German allied chemical societies of Belgium, England, France, committee used the O = 16 scale exclusively and Italy, and the United States formed the International argued for its adoption. As a result of a vote within the Union of Pure and Applied Chemistry (IUPAC), which German Committee in 1899, the first international would function—with autonomous powers—as the table was published on the O = 16 scale. However, chemical section of the IRC. after vigorous protests from certain parties, doubt At the first IUPAC conference, held in 1920 in was expressed as to whether a majority opinion could Rome, the IUPAC Council met and established a series ever be accepted as final in such theoretical matters. of committees: one on atomic weights, one on tables As a result, the smaller ICAW continued publishing the of constants, and one on patents, as well as an annual tables on both scales until a consensus could Institute of Chemical Standards. The council be reached. requested that the old ICAW be asked to continue its Beginning with the 1906 report, however, the ICAW work. In 1921, the committee on atomic weights was used the O = 16 scale following a new survey of the reorganized, enlarged, and renamed the Committee larger committee. Thus, the scale was settled for some on Chemical Elements. This committee, in addition to 30 years, except for a brief discussion in 1920 on providing atomic weight values, was also asked to going back to the hydrogen scale. Beginning in the cover the discovery of isotopes in radioactive and 1930s, when the neutron was discovered and the non-radioactive elements. Tables of radioactive ele- structure of nuclei was accepted to be a combination ments and their principal constants, a table of iso- of protons and neutrons, H = 1 became a near impos- topes, and a table of atomic weights were to be sible choice as a reference for atomic weights. The prepared. atomic number of heavy elements would not repre- At a meeting in Paris in 1922, the Committee on sent the number of nuclides in the nucleus in an H = 1 Chemical Elements voted to publish the table of iso- scale. topes and of radioactive elements In 1929, the discovery of the two oxygen isotopes, the following year and to continue 17O and 18O by Giauque and Johnston led to a situation the old committee’s table of in which the chemist’s scale of O = 16 differed from the 1921–1922 if a new general table of physicist’s scale of 16O = 16. When Dole reported the atomic weights could not be com- variation in oxygen’s atomic weight value in water pleted in time. The committee pub- versus air, this implied a variation in the isotopic com- lished a completely revised atomic position of oxygen and the two scales took on a small weight table in 1925, but did not but a variable difference. The ICAW briefly discussed revise the table again until after 1930. the atomic weight standard in their 1932 report, where In 1928, after being criticized for fail- they considered 1H = 1, 4He = 4, 16O = 16 and O = 16 ing to publish an annual table of before choosing to follow Aston, who argued that the Frank W. Clarke atomic weights for many years, the two scales satisfied everyone’s requirement. committee was reorganized into three separate com- The variable scale difference was of great concern mittees: one dealing with atomic weights, one with to Edward Wichers (president of ICAW beginning in atoms, and one with radioactive constants. 1949) and for a number of years he attempted to have CHEMISTRY International January-February 2004 5 Atomic Weights and the International Commitee the ICAW fix the difference between the two scales within IUPAC with various terminology committees by definition. This would effectively define the iso- about these terms, not the least of which was “atomic topic composition of oxygen to be a particular value weights” itself. The various discussions that followed in nature. Failing with this solution, he solicited pro- would continue over a decade until the IUPAC posals for an alternate scale that would be acceptable General Assembly at Davos, Switzerland, in 1979.