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
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. to both the physics community as well as to the The fallout from the terminology wars was dis- chemists worldwide. cussed in the commis- In April 1957, Alfred sion’s meetings at both Nier suggested to Josef the 1971 IUPAC Mattauch (both were Washington, D.C., members of ICAW) that General Assembly and the 12C = 12 mass scale the 1973 IUPAC be adopted because of Munich, FRG, General carbon’s use as a sec- Assembly. In the 1971 ondary standard in report, there was a dis- mass spectrometry. Edward Wichers Alfred Nier Josef Mattauch cussion as well as a Also, 12C = 12 implied graph of the relative acceptable relative changes in the atomic weight precision of the atomic weight values of all elements scale, (i.e., 42 parts-per-million [ppm] compared to across the periodic table. The two tables of half-life 275 ppm for the 16O = 16 scale [which would not be values and atomic masses from the 1969 report were acceptable to chemists]). Enthusiastically, Mattauch combined into a single table in the 1971 report. made a worldwide effort in the late 1950s to publicize With the increased importance of the isotopic the 12C = 12 scale and obtain the physicist’s approval, abundance measurements in the determination of the while Wichers obtained the chemist’s approval. atomic weights, the commission’s name was changed Following the approval of the International Union at the 1979 IUPAC General Assembly in Davos, of Pure and Applied Physics General Assembly in Switzerland, to Commission on Atomic Weights and Ottawa, Canada, in 1960 and the IUPAC General Isotopic Abundances. A new definition of atomic Assembly at Montreal, Canada, in 1961, the atomic weight was presented, which indicated that atomic weights were officially given on the 12C = 12 scale for weights could be defined for a sample. Tables of stan- the first time in the 1961 report. Mattauch and his col- dard atomic weights published by the commission leagues combined data on direct nuclidic mass meas- referred to best knowledge of the elements in natural urements with data on measured binding energies terrestrial sources (this is still the case today). Atomic and beta decay energies derived from the masses to weight (mean relative atomic mass) of an element produce a consistent least squares fit of all nuclidic from a specified source was defined as “the ratio of 1 masses. This mass data was combined with the iso- the average mass per atom of the element to /12 of topic compositions to provide atomic weight values the mass of an atom of 12C.” From this point on, the used in that 1961 Atomic Weights report. commission presented the most accurate available values for those who needed to use them, but the Expanded Topics for the concept of accuracy implies the existence of a true Commission value and the definition doesn’t recognize the exis- tence of one true value for every element. In the years between when the mass scale change occurred and 1969, there were relatively few changes The Growing Importance of in the atomic weights table. In the 1969 report, a table Isotopic Compositions of radioactive isotopes with half-life values and a table of atomic masses of selected isotopes were For the 1981 IUPAC General Assembly in Leuven, included, and definitions of terms of atomic weight, Belgium, the Commission decided to publish its report isotope, nuclide, and normal material were intro- in Pure and Applied Chemistry in two separate parts duced. These definitions led to an interdivisional fight for the first time (i.e., the Atomic Weights of the
6 CHEMISTRY International January-February 2004 A Brief Historical Review
Elements, 1981 and the Isotopic Compositions of the for more than one and one half years, until the presi- Elements, 1981). dent of the IUPAC Inorganic Chemistry Division, Gerd At the 1983 IUPAC General Assembly in Lyngby, Rosenblatt, made funds available to bring together all Denmark, the Commission changed its method for members of the commission and subcommittees in expressing uncertainties in atomic weight values. Ottawa to discuss the future course of the commission. Previously these uncertainties were restricted to one As a result of the above confusion, no preparatory of two values, (i.e., either ± 1 or ± 3). Beginning with work for the scientific agenda had been done to ana- the 1983 report, these uncertainties could now take on lyze the data and recommend updated values for the any digit from ± 1 up to ± 9. Another change with the Table of the Standard Atomic Weights. At the 2003 1983 report dealt with the treatment of elements with IUPAC General Assembly in Ottawa, Canada, the com- no stable isotopes. For radioactive elements with no mission chose not to publish a report on Atomic unique naturally occurring isotopic composition from Weights for 2003 (for the first time in almost 40 which an atomic weight could be calculated with five years). The commission and the subcommittees dis- or more figure accuracy without prior knowledge of cussed the future work of these bodies and a mecha- the sample, the concept of a standard atomic weight nism for the funding for continued operation either has little meaning. So scientists dealing with non-ter- within IUPAC or outside of the IUPAC framework. restrial samples were warned to exercise caution Tiping Ding (Chinese Academy of Geological when the isotopic composition or atomic weight of a Sciences, Beijing, China) was elected chairman to non-terrestrial sample was required. replace Philip Taylor and Michael Wieser (University At the 1999 IUPAC General Assembly in Berlin, of Calgary, Alberta, Canada) was elected secretary to Germany, the Working Party on Non-terrestrial Data replace Robert Loss at the conclusion of the meeting. reported on the processes explaining isotopic varia- One atomic weights publication that did appear in a tions and provided a table of anomalous isotopic pre-print form at the time of the Ottawa General compositions in extra-terrestrial materials due to Assembly was another element-by-element review decay of radioisotopes. called EXER-2000. This 115-page document had been More recently, at the 2001 IUPAC General the result of the six-year effort by the members of the Assembly in Brisbane, Australia, the commission 1997 working party and it was written in a similar man- emphasized the great importance of the isotopic ner to the earlier Subcommittee on the Assessment of abundance values as the sole source (along with the Isotopic Composition review. atomic mass values of the stable isotopes) for deter- mining atomic weight values for the elements. The Norman Holden
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