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Vol. 26 No. 1
January-February 2004

Atomic Weights and the International Committee —A Historical Review

Just 20 years ago, while completing his chairmanship of the IUPAC Commission on Atomic Weights, Norman Holden prepared and published in Chemistry International (1984, issue No.1) an early historical review of the International Commission on Atomic Weights. Since then, his interest of the topic has not faded, and au contraire, he has now reviewed, extended, and updated the historical review. An excerpt of this article can be found on pages 4-7 of the print version of the January-February 2004 CI.

by Norman E. Holden

Norman E. Holden, when chairman of the commission from 1979-1983.

I. Prehistory

Speculation on the ultimate form of matter has engaged philosophers from the earliest times. In the Ionian culture of Greece 2500 years ago, Leukippos, Demokritos and Epikuros philosophized that all matter was composed of atoms1 (Greek: indivisible). They taught that all matter was composed of four elements: fire, water, air and earth. The 16th century alchemist Paracelsus added to those elements his three principles in matter, i.e., sulfur, salt and mercury.

By the seventeenth century, the Irishman Robert Boyle denied that these seven were the basic elements. He developed chemical analysis—the technique for breaking down substances into their most elementary parts. He defined an element as a material that could be identified by scientific experiment and could not be broken down into still simpler substances. This is the definition that is still in use today.

In the late eighteenth century, the French scientist Antoine Lavoisier revolutionized chemistry by the introduction of accurate weighing. He determined that a given amount of matter has a total mass, which remains the same when it changes in chemical combination, whether in the solid, liquid or gaseous state. The French chemist Joseph Proust's analyses showed that a particular chemical compound always contains the same elements combined in the same mass ratio. However, Greek atomism had taken on the connotation of atheism to the English clergy.2 Newton rarely used the term 'atom', although his work was rooted in an atomistic theory of matter.3

The English school teacher, John Dalton, is usually given credit for the first clear statement of atomic theory and for proceeding to establish it on a thoroughly sound experimental basis, when he tested Proust's law and noted that the same elements combined in different proportions to produce different substances. An inquiry into the relative weights of these ultimate particles of matter was an entirely new subject with Dalton. He assigned weights to atoms and expressed the relations between atoms of elements in precise numerical terms. It is possible to assign relative weights by determining the ratio in elements reacted with each other. Since it had the smallest atomic weight value, Dalton chose hydrogen as his reference scale unit, hydrogen = 1 and he calculated atomic weights by comparing weights of other atoms with that of hydrogen. In an attempt to explain the problem of the variation of gaseous solubilities discovered by William Henry and its effect on his own theory of mixed gases, Dalton compared these relative weights of atoms.4 The originality of Dalton's work was his calculating system to compare the weights of chemical particles.5 The first table of atomic weight values was given by him in a paper entitled, "On the Absorption of Gases by Water and Other Liquids." It was read to the Manchester Literary and Philosophical Society, of which Dalton was secretary, on 21 October 1803. (It might be noted that the Manchester Society was England's oldest continuing scientific society, other than the Royal Society of London). Values given in the table indicate that Dalton grasped the ideas of constant composition in compounds and of multiple proportions.

However, when two elements combine in a compound, it is insufficient to merely determine the percentage of each element present to obtain the correct atomic weight. One must also determine the valence of each element in the compound. Valence is a measure of how many atoms of one element combine with an atom of the other element, e.g., is water HO or H2O or perhaps H2O2? Unfortunately, this is a "catch-22" situation since it can not be ascertained until one knows the atomic weight. Dalton assumed that if only one compound of two elements is known, it contains one atom of each element. The later tables of Dalton's in 1808 and 18106 show a marked improvement in accuracy but the values are still difficult to recognize because of these errors in valency, i.e., some equivalent weights (atomic weight/valence) are quoted rather than atomic weights.

The French physicist, Joseph Gay-Lussac, determined7 that gases form compounds with each other in simple (numerical) volume ratios proving that Dalton's idea of combining gases by weight alone was insufficient.

The Italian physicist Amedeo Avogadro suggested8 that all gases under the same conditions of temperature and pressure contain the same number of molecules and a molecule (Greek: a small mass) may contain more than one atom. He made a distinction between the chemical atom (smallest part of matter that can enter into combination) and physical molecule (smallest particle that can exist in a free state). This could have solved the equivalent weight problem but unfortunately he used the term molecule throughout his discussion with a series of qualifying adjectives: integral, constituent and elementary. The terms atom and molecule were often used interchangeably at that time. Some scientists understood Avogadro to imply that there could be half-atoms. This confusion caused Avogadro to be ignored for half a century.

In an anonymous article in 18159, the English physician, William Prout (Oxford), proposed that elemental atomic weight values were integral multiples of the hydrogen value. The rough estimates then available for atomic weight values lent apparent support for this idea.

Jons Jacob Berzelius, who discovered a number of chemical elements and developed the chemical notation and formula still presently used (with slight modification) provided the first major improvement in measured atomic weight values. Beginning in 181010, Berzelius published a series of articles on atomic weight measurements in which he reported actual weighings along with results from other chemists on an oxygen scale i.e., value of O = 100. Berzelius felt that oxygen was a good reference material because it combined with most elements (in inorganic chemistry) and many molecular and atomic weights could be measured directly. The drawback to hydrogen as a reference was that it combined directly with a limited number of elements only and the atomic weight must be evaluated in comparison with oxygen. As the oxygen to hydrogen ratio could never be determined with absolute accuracy, any errors in this ratio would be carried over into all other values of atomic weights. The results of Berzelius' analyses tended to disprove Prout's hypothesis on integral relations among the elements, e.g., carbon/hydrogen: 76.438/6.25 = 12.23 (where O = 100).

The Belgian chemist from Leuven, Jean Servais Stas, studied under Jean Dumas in Paris. Dumas believed in Prout's hypothesis and doubted Berzelius' results. With Dumas, Stas obtained a value for carbon of 75.02 based on O = 10011. With H = 1, oxygen was approximately 16, and carbon would be 12.0, so that Prout's hypothesis might still have validity within the experimental error.

Stas measured atomic weights for a number of elements which Dumas thought supported Prout's hypothesis, i.e., nitrogen, chlorine, sulfur, sodium, potassium, silver and lead, in a series of experiments finally published in 186012. Stas proved that atomic weight values were not multiples of hydrogen, one-half the hydrogen value or even one quarter of the hydrogen value (as was maintained by Dumas). Following Stas' first paper, he redetermined the atomic weights even more carefully in 186513, after Marignac14 had cast doubt on the law of constant proportion and on the possibility of obtaining chemically pure substances.

In spite of all the atomic weight determinations that were being performed, Dalton's atomic theory did not gain widespread acceptance. The French chemist C.L.Berthollet immediately critically attacked Dalton.15 As late as the 1870s, Scientists used the calculational system, while scorning the underlying theory.16 It might be noted that by mid-century there was a rapid rise of organic chemistry and Dalton's scale was still in use since the disadvantage of hydrogen mentioned above did not apply to organic chemistry.

At the First International Scientific Congress in September 3-5, 1860 at Karlsruhe, about 140 of the leading European chemists met to formulate an area of agreement regarding the nature of atoms and molecules and to reach a mutual satisfactory atom weight scale for organic chemistry. The Italian chemist Stanislao Cannizzaro presented his "Sketch of a Course in Theoretical Chemistry,"17 where he called attention to the value of Avogadro's distinction between atoms and molecules as an organizing device for the interpretation of chemical phenomena. No conclusions were agreed upon at the Conference but Cannizzaro's presentation influenced the thinking of young chemists who attended including J. Lothar Meyer and Dimitri Mendeleev.

Both Meyer and Mendeleev subsequently wrote textbooks18,19 which spread Cannizzaro's solution throughout the scientific community. Scientists corrected equivalent weight values to atomic weights. Elements with similar properties fell one under another when arranged in rows by ascending order of atomic weights and the periodicity of the elements became clearer. Both Meyer in his textbook and Mendeleev20 developed periodic tables of the chemical elements based on the revised atomic weight values.

Meyer's table was based on the element's valences. The elements did occur in increasing order of atomic weight but valence headings predominated. Another table by Meyer, which was similar to Mendeleev's table, was published posthumously.21 Priority claims over the discovery of the Periodic Table continued until Meyer's death in 1895.

Mendeleev left open spaces in his table, when no known element filled that space.22 He predicted properties of these unknown elements. When scandium,23 gallium24 and germanium25 were later discovered and agreed with Mendeleev's predicted chemical properties and atomic weight, the periodic table was established and the usefulness of atomic weights was further enhanced.

II. Atomic Weight Commissions

Frank W. Clarke

In addition to the measurements quoted above, a large number of individuals were contributing results on atomic weight values. Frank W.Clarke (U. Cincinnati, USA), who was chairman of the chemistry section of the American Association for the Advancement of Science (AAAS), presented his first evaluation of atomic weights at the AAAS meeting in August 187926 and in 1882 published his recalculation of the atomic weights.27

Although the atomic weight had taken on the concept of a constant of nature like the speed of light, the lack of agreement on accepted values created difficulties in trade. Quantities measured by chemical analysis were not being translated into weights in the same way by all parties. With so many different values being reported, the American Chemical Society (ACS), in 1892, appointed a permanent committee to report on a standard table of atomic weights for acceptance by the Society.28 Clarke, who was now chief chemist for the U.S. Geological Survey, was appointed a committee of one to provide the report. Clarke, who was later elected ACS President, presented his first report at the 1893 annual meeting29 and published it in January 189430. He continued as a one man committee for the Society until he asked to be relieved of this responsibilty in March 191331 and Gregory P. Baxter (Harvard) was appointed to replace him.

In 1897, the Deutsche Chemische Gesellschaft, following a proposal by Emil Fischer,32 appointed a working committee to report on atomic weights. The committee was chaired by Hans Landolt (Berlin), who had produced the physical-chemical tables with Richard Börnstein. Other members of this committee were Wilhelm Ostwald (Leipzig), who later won the Nobel Prize for chemistry for catalysis, and Karl Seubert (Hannover), who, as previously mentioned, had published the atomic weights of the elements,33 when working for Lothar Meyer at Tübingen. This committee published its first report in 1898.34 In contrast to Clarke, who presented a review of every atomic weight value published during the year along with his recommended values, the German committee merely gave the table with its estimated best value for each element. In addition to presenting their table of values, the committee suggested the desirability of an international committee for atomic weights and they argued for the adoption of the O = 16 scale. Landolt, Ostwald and Seubert issued an invitation on 30 March 1899 to other organizations having an interest in chemistry to appoint delegates to the international body.35 The Committee initially formed had 57 chemists.

This international committee's first report, for 1901, was a table on the O = 16 scale, which appeared as a flyleaf in issue 1 of the Chemische Berichte for 1902.36 All of the committee's conferences were conducted by correspondence. The German committee asked the international committee members to elect a smaller committee of three members to avoid the difficulties and delays associated with correspondence among members of the larger committee.37 They also asked for a vote on an O = 16 scale versus an H = 1 scale or both. Sixteen votes were cast for the smaller committee. The results are shown in table 1.

Table I. Voting for Membership on the International Committee








































The top three names, Clarke, Seubert and Thomas Edward Thorpe (London, UK) were elected.38 Thorpe had been the first Professor of Chemistry at Leeds (then called the Yorkshire College of Science) and he was now the Director of the Government Laboratories and responsible for their design and equipment. This smaller International Committee on Atomic Weights (ICAW) published its first report, for 1902, in January 1903.39 The only Frenchman to receive a vote was Henri Moissan (Sorbonne), who was to win a Nobel Prize for the isolation of fluorine and the development of the high temperature furnace. (It is interesting to note that Moissan constructed his famous electric furnace in an attempt to produce artificial diamonds and thought that he had succeeded.40 This was later shown to be incorrect.41) Moissan was appointed as a French representative on the Committee in 1903.42#42

When Seubert resigned from the Committee in 1906, Ostwald, who was the next highest German in the original vote, was appointed to replace him43. When Moissan died in 1907, George Urbain (Sorbonne), who discovered lutetium in ytterbium samples in 1908,44 replaced him. Urbain would serve as a link with the reorganized atomic weights committee within IUPAC following World War I.

This Committee reported annually to 1921, except for 1918 (due to the World War). The War ended correspondence with Ostwald in 1916,45 which was never resumed46 because of the wartime hostilities. (e.g., on September 9, 1918 Ostwald, who had been unanimously elected an honorary member of the American Chemical Society on June 26, 1900, was formally expelled by a resolution made effective back to August 1, 1914).47

The Germans would later consider the reorganized Atomic Weights Commission within the IUPAC structure as no longer international in scope48 and would form their own atomic weights committee with Ostwald as their initial chairman.49

III. International Affiliations

The first International Chemical Conference was held in Karlsruhe in September 1860 to try to reach agreement on the theory of organic chemistry to allow standardization of nomenclature and the writing of formulae. The first of a series of International Congresses on Applied Chemistry was held in Brussels, Belgium in 1894. In 1910, the President of the French Chemical Society proposed the formation of the International Association of Chemical Societies (IACS) and just seven months after this proposal, the IACS was formed on April 25, 1911 in Paris. At the Berlin meeting in April 1912, Prof. Ostwald, who was the Association President at the time, stated that the International Committee on Atomic Weights wished to join the International Association. The International Committee was asked to provide its organizational statutes for the next meeting and was formally affiliated with the Association at the Brussels meeting on September 19, 1913.50 The ICAW was charged with publishing an updated Table of Atomic Weights every year. The IACS never recovered from the onset of the War and it was formally dissolved in 1919. However, the ICAW continued to publish their annual tables until their final report for the years 1921-1922.51

Toward the end of World War I, a conference of scientists from allied (countries at war with the Central Powers) Scientific Academies met in London in October 1918 and resolved to withdraw from existing International Scientific Associations (IACS) and form a new association. In Paris in November 1918, the Academies agreed to form the International Research Council (IRC). The IRC consisted of the nations at war with the Central Powers but would later accept neutral countries, who agreed not to have any relations with countries from the Central Powers.

While the Scientific Academies were setting up the IRC, industrial chemists decided to form an interallied organization of chemists as a result of discussions by Paul Kestner, who was President of Société de Chimie Industrielle, and Henry Louis, who was President of the Society of Chemical Industry. At a Conference organized in Paris in April 1919, Belgium, England, France, Italy and the United States proposed that the various Chemical Societies in each country form a National Council. The Inter-allied Chemical Conference was held in London, on July 14-18, 1919, where the allied Chemical Societies of Belgium, England, France, Italy and the United States framed a constitution for the International Union of Pure and Applied Chemistry (IUPAC) as the chemical section of the International Research Council (IRC) with autonomous powers. The Conference continued in Brussels on July 22, in connection with the IRC meeting, where the final adoption of the statutes of IUPAC was made.

The first IUPAC Conference took place at the International Chemical Conference, held in Rome on June 21-25, 1920. The IUPAC Council met and established a series of Committees: one on Atomic Weights, one on Tables of Constants, one on Patents and an Institute of Chemical Standards. It is interesting that no Nomenclature Committee was initially set up by IUPAC. The Council requested that the old Committee on Atomic Weights of Clarke, Thorpe and Urbain be asked to continue their work. The provisional membership of the Atomic Weights Committee was approved after the proposed membership of Phillippe Guye's (Geneva) onto the Committee was disallowed because Switzerland (as a neutral country) did not adhere to the IRC statutes. Two proposals were made for consideration at the following year's Conference: atomic weight values should only be revised every 10 years and Dalton's proposal of H = 1 should be re-adopted as the standard for the atomic weight system. As far as the first proposal was concerned, by coincidence as it turned out, only one new Table of Atomic Weight values was published during the remainder of the decade. The history of the second proposal (H = 1) is discussed in section IV below.

The second International Conference of Pure and Applied Chemistry met at Brussels during June 25-30, 1921. Following a proposal presented by Phillippe Guye, the old committee on atomic weights was reorganized, enlarged and renamed the Committee on Chemical Elements. This Committee, in addition to providing atomic weight values, was also asked to cover the discovery of isotopes in radioactive and non-radioactive elements. Tables of radioactive elements and their principal constants, a table of isotopes and a table of atomic weights were to be prepared. Clarke and Thorpe, who were both in their late seventies, were appointed honorary presidents and Urbain presided over the new committee. Other members appointed to this new committee included: Francis Aston (Cambridge, UK), who was the Nobel Prize winner for the discovery of a large number of non-radioactive isotopes, Bohuslav Brauner (Charles University, Prague), who became president of the sub-committee on atomic weights, Phillippe Guye, Theodore Richards (Harvard, USA), who was the Nobel Prize winner for his work on atomic weights, Fredrick Soddy (Oxford, UK), who was the Nobel Prize winner for his investigation of the origin and nature of isotopes, and Andre Debierne (Paris, France), who was a student of Marie Curie and was credited as the discoverer of actinium.52 (Debierne's claim to the discovery of actinium appears to be doubtful considering the critical reevaluation by Kirby.53 Debierne's major contribution appears to be the name of the chemical element, actinium.)

Not all members were present at the third IUPAC conference in Lyon, France in 1922, so the Committee on Chemical Elements met later in Paris on July 13, 1922. In consequence of the death of Phillippe Guye, they elected Gregory Baxter and Anatole Leduc (Sorbonne) as additional members. They voted to publish the table of isotopes and of radioactive elements in 192354 and to continue the old committee's table of 1921-1922, if a new general table of atomic weights could not be completed in time. This Committee published a completely revised atomic weight table in 192555 but did not revise the table again before it was reorganized after 1930. Following the death of Prof. Richards in 1928, Prof. H.V.A. Briscoe of Armstrong College, Newcastle upon Tyne (part of the University of Durham, UK), who served on the UK Atomic Weights sub-committee was elected as a replacement and IUPAC nominated Prof. Enrique Moles (Madrid, Spain), from the Spanish Atomic Weights Commission to the Committee.

In 1923, the Committee on Chemical Elements had urged all national members of IUPAC to create national committees to work with this International Committee. As noted above, the Germans had previously done this and were reporting annually and the Swiss, Spanish and English followed suit but they produced only one report each.

The statutes of the International Research Council (IRC), to which IUPAC adhered, effectively barred former Central Powers from membership in any of the various Unions. After a number of attempts by the Unions to remove this political pressure and to change the IRC statutes, these efforts were finally successful and in 1930 the Executive Committee of IRC agreed. The IRC became the International Council of Scientific Unions (ICSU). (The ICSU is probably best known at this time for serving as the umbrella organization for the 67 countries and 14 disciplines of the Third International Polar Year [IPY-3] from July 1, 1957 to December 31, 1958, which had been renamed the International Geophysical Year). IPY-4 is proposed to ICSU to start in 2007.

In 1928, the Committee on Chemical Elements had been criticized for failure to publish an annual Table of Atomic Weights since 1923. In view of possible liaison with the German Committee, Urbain recommended the reorganization of the Committee on Chemical Elements into three separate committees, including one dealing with atomic weights, one with atoms and one with radioactive constants.

Germany agreed to join IUPAC under the condition that the full name of the Union be changed, possibly because IUPAC had previously excluded Germany from participation. IUPAC underwent a name change and it became the International Union of Chemistry (IUC) and Germany was admitted. There was finally a truly international group of chemists.

In 1930, Baxter became the President of the Atomic Weights Committee and was joined by Marie Curie (Paris), the Nobel Prize winner in both physics and chemistry, Paul LeBeau (Paris), who had been a student of Moissan, Richard J. Meyer (Berlin), who had been a member of the German Atomic Weights Committee since 1921, and Otto Hönigschmid (Munich), who was a former student of Richards and was chairman of the German Atomic Weights Committee after Ostwald retired. The national committees were asked to cease publishing reports and the new ICAW produced its first report in 193156 and it then continued to publish annual reports through 1941.

Edward Wichers

Following Curie's death and the retirement of Meyer and LeBeau, Marcel Guichard (Paris) and Robert Whytlaw-Gray (Leeds) were appointed to the Committee in 1938. The ICAW published two additional reports in 1943 and 1947. The ICAW was reorganized once again in 1949, following Baxter's retirement and the death of Hönigschmid. Since the only two remaining members of the old Committee, Whytlaw-Gray, who was retiring to Coventry and Guichard, who in fact never participated again, were both in their seventies, Edward Wichers, who was later the Deputy Director of the US National Bureau of Standards (US-NBS, Washington, D.C.) was asked to assume the presidency and to reactivate the Committee.

In 1949, the IUPAC statutes limited the number of Titular members of Committees (those who were eligible to receive financial support to attend the Conferences, i.e., General Assembly meetings) to a maximum of ten. These members could serve up to two terms of four years each, after which they were not eligible for election for two years. The era of long serving members was coming to a close. The statutes also changed the name Committee to Commission. In 1959, IUPAC Council adopted a proposal for Associate-non-Titular (or just Associate) members, who were not eligible for travel or subsistence expenses. In 1961, Council reduced the maximum number of Titular members to eight.

Alfred Nier

With the emphasis on physical rather than chemical determinations, the modern era with a large committee began. Mass spectrometry experts were added such as Alfred Nier (Minnesota, USA), who singlehandedly revised all of the physical determinations of atomic weight values for the 1955 Atomic Weights Table, Josef Mattauch (Munich, FRG), who convinced the world of physicists to accept the switch of the atomic mass scale from the 16O = 16 standard to the 12C = 12 standard, and Angus E. (Gus) Cameron (Oak Ridge National Lab., Tennesee, USA), who revised all of the physical measurements to the 12C mass standard for the 1961 Atomic Weights Table.

Josef Mattauch

In 1959, Professor Tomas Batuecas (University of Santiago de Compostela, Spain) was elected the Commission President for four years. In 1963, Wichers was re-elected Commission President until 1969. During this time, Professor Norman N. Greenwood (University of Newcastle on Tyne, UK), Professor Etienne Roth (Commissariat a l'Energie Atomique, Saclay, France) and H. Steffen Peiser (Chief, Office of International Relations, US-NBS, Washington, D.C.) were elected to membership on the Commission. They would all assume leadership roles during the modern era of the Commission.

In 1975, the Bylaws stated that the service of Titular members could not exceed ten years, which included service as an Officer. In 1980, the IUPAC Bureau limited Commissions to no more than six Titular members. In 1985, the Bylaws stated that the 10-year limit of service applied to Titular members would include both broken as well as continuous service. There would be no more long serving Presidents or Chairmen on the Commission in the future.

IV. The Atomic Weights Scale

The atomic weights scale of H = 1 was originally used by Dalton and (except for Berzelius' time) had been used for approximnately 100 years when the ACS and the German committees began reporting their tables. Lothar Meyer and Seubert had published on the hydrogen scale,33 but Ostwald and Brauner57 strongly urged the adoption of the O = 16 scale. Clarke reported his table on both scales, while the German committee used the O = 16 scale exclusively and argued for its adoption. In October 1899, the German committee asked the international delegates if O = 16 should be fixed as the future standard. Of the 49 replies, 40 favored oxygen only seven favored hydrogen with two accepting either or both35. As a result, the first international table was published on the O = 16 scale.36 A vigorous protest against the decision was made by university chemistry teachers in Germany and by the committee for the decennial revision of the United States Parmacopoeia among others, who preferred the H = 1 standard. In the third report of the German Atomic Weights Commission,37 the results of another vote indicated 106 chemists in favor of H = 1 and 78 chemists in favor of O = 16. At the time, doubt was expressed as to whether a majority opinion could ever be accepted as final in such theoretical matters. As a result, the smaller ICAW continued publishing the annual tables on both scales until a consensus could be reached. This practice brought a reply in strong opposition from Ostwald58.

The proponents for the H = 1 scale, such as Clarke, argued that it had the advantage of being Dalton's standard and it was the most natural basis for atomic weights because hydrogen is the lightest atom known and it was also the standard for gaseous densities. Teachers also argued that it is easily intelligible to beginners, whereas the oxygen standard was more difficult to explain.

The proponents for the oxygen scale argued that oxygen was the experimental reference standard. Every atomic weight was related to oxygen either directly or indirectly. Hydrogen was only a nominal unit with the actual determination of atomic weight referred to hydrogen through the ratio H/O. Every time that this fundamental base ratio, H/O, would be redetermined (see e.g., William Noyes' contribution59), the entire atomic weights table must be changed. Thus, this apparent historically conservative approach for the standard implied a wide-spread and radical change to all of the data. It thus became a debate between the teacher (theory) and the laboratory chemist (practice).

Beginning with the 1906 report, the ICAW used the O = 16 scale following a new survey of the larger committee. The final count was thirty one votes for O = 16, two votes for H = 1, nine votes for both scales and seventeen abstaining. Thus, the scale was settled for some thirty years, except for a brief discussion in 1920 on going back to the hydrogen scale. Beginning in the 1930s, when the neutron was discovered and the structure of nuclei was accepted to be a combination of protons and neutrons, H = 1 became a near impossible choice as a reference for atomic weights. The atomic number of heavy elements would not represent the number of nuclides in the nucleus in an H = 1 scale.

In 1929, the discovery of the two oxygen isotopes, 17O and 18O by Giauque and Johnston60 led to a situation in which the chemist's scale of O = 16 differed from the physicist's scale of 16O = 16. When Dole61 reported the variation in oxygen's atomic weight value in water versus air, this implied a variation in the isotopic composition of oxygen and the two scales took on a small but a variable difference. The ICAW briefly discussed the atomic weight standard in their 1932 report,62 where they considered 1H = 1, 4He = 4, 16O = 16 and O = 16 before choosing to follow Aston, who argued that the two scales satisfied everyone's requirement.

The variable scale difference was of great concern to Wichers and for a number of years he attempted to have the ICAW fix the difference between the two scales by definition. This would effectively define the isotopic composition of oxygen to be a particular value in nature. Failing with this solution, he solicited proposals for an alternate scale which would be acceptable to both the physics community as well as to the chemists worldwide.

In April 1957 at the bar in the Hotel Krasnapolski in Amsterdam, Nier suggested to Mattauch that the 12C = 12 mass scale be adopted because of carbon's use as a secondary standard in mass spectrometry63. Also, 12C = 12 implied acceptable relative changes in the atomic weight scale, i.e., 42 parts-per-million (ppm) compared to 275 ppm for the 16O = 16 scale (which would not acceptable to chemists). Enthusiastically, Mattauch made a worldwide effort in the late 1950s to publicize the 12C = 12 scale and obtain the physicist's approval, while Wichers obtained the chemist's approval. (Mattauch was so anxious to tell the world of Nier's suggestion of the 12C mass scale as the resolution of the atomic weight dispute between chemists and physicists that he rushed off to the IUPAC General Assembly in Paris without his passport. After phoning Nier at the Max Planck Institute in Mainz, Mattauch had to spend the night at Trier on the German-French border, while he waited until Nier retrieved his passport from the Institute before he could continue on to the Commission meeting and unveil the solution64). Following the approval of the International Union of Pure and Applied Physics (IUPAP) General Assembly at Ottawa, Canada in 1960 and the IUPAC General Assembly at Montreal, Canada in 1961, the atomic weights were officially given on the 12C = 12 scale for the first time in the 1961 report65. Mattauch and his colleagues66 combined data on direct nuclidic mass measurements with data on measured binding energies and beta decay energies derived from the masses to produce a consistent least squares fit of all nuclidic masses. This mass data was combined with the isotopic compositions to provide atomic weight values used in that 1961 Atomic Weight report.

V. Expanded Topics for the Commission

In the years between the (when the mass scale change occurred) and 1969, there were relatively few changes in the atomic weights table. However in 1961, the Commission, through then President Batuecas, recommended that the name of the Commission be changed to the "Commission on Atomic Masses" and that the table of the recommended values be changed to "Table of Relative Atomic Masses". These recommendations were rejected by IUPAC's Inorganic Chemistry Division and by its Commission on Nomenclature of Inorganic Chemistry. In 1963, Edward Wichers was quickly re-elected President (he continued to serve in that position until 1969) and Jules Gueron (Euratom, Brussels, Belgium) was elected as the Secretary.

In the 1969 report67, there was a definition of terms of atomic weight, isotope, nuclide, and normal material. The terms, variations and uncertainties, were also discussed. Footnotes or annotations were added to table entries for the first time. A Table of Radioactive Isotopes with half-life values and a Table of Atomic Masses of Selected Isotopes was included. In the elections at the conclusion of the meeting, Norman N. Greenwood was elected to head the Commission, replacing Ed Wichers, and H. Steffen Peiser was elected as the secretary to replace Jules Gueron.

The introduction of these definitions into the Atomic Weights report led to an interdivisional fight within IUPAC with various terminology committees about these terms, not the least of which was "atomic weights" itself. The various discussions that followed would continue over a decade until the IUPAC General Assembly at Davos, Switzerland in 1979.

The fallout from the terminology wars was discussed in the Commission's meetings at both the 1971 IUPAC Washington, D.C. General Assembly and the 1973 IUPAC Munich, FRG, General Assembly. In the 1971 report68, there was a discussion as well as a graph of the relative precision of the atomic weight values of all elements across the periodic table. The two Tables of half-life values and atomic masses from the 1969 report were combined into a single Table in the 1971 report.

At the 1973 Munich General Assembly meeting, the so-called "Young Turks" of the Commission, Paul de Bievre (Central Bureau of Nuclear Measurements, Geel, Belgium) and Norman E. Holden (General Electric Knolls Atomic Power Laboratory, Schenectady, New York, USA) proposed forming a Working Party to review the data on isotopic abundance measurements and their impact on atomic weight values. The IUPAC Mass Spectrometric Evaluation Group (IMSEG) was created by the Commission and charged to perform an initial review of the data and report back at the 1975 Commission meeting during the IUPAC Madrid, Spain General Assembly.

Also at the Munich meeting69, the Commission pointed out that a large variety of materials in commerce contained elements having an isotopic composition other than the "normal" value. Some users and manufacturers favor introduction of precise statements on labels to minimize misunderstandings and errors in the interpretation of analytical data or the inadvertant use of valuable isotopically enriched materials for common purposes. The Commission included a discussion on the matter in their report.

In 1974, the IMSEG Working Party held an intermediate meeting at the US-NBS to prepare an initial Table of Isotopic Compositions of the Elements as determined by Mass Spectrometry. David Lide (head of the National Standard Reference Data System at US-NBS) agreed to provide financial assistance for the meeting. Etienne Roth and Paul De Bievre flew to Washington from Paris for the meeting on the Russian airline, Aeroflot, which had the most convenient time schedule. This was during the height of the "Cold War" between the USA and USSR and it took almost six months before the US Government would agree to pay the reimbursement for the use of the Russian airline.

At the 1975 IUPAC Madrid, Spain General Assembly, the matter of isotopic compositions was discussed when the IMSEG Working Party reported back to the Commission with the results of their 1974 meeting at the US-NBS. The Commission created the Subcommittee on Assessment of Isotopic Composition (SAIC) under the chairmanship of Paul De Bievre in order to continue the work of IMSEG on a more permanent basis. The initial SAIC Table of Isotopic Compositions was published within the 1975 Atomic Weights report as a result of IMSEG recommendations70.

After an Interdivisional open meeting was held at this General Assembly, there was another discussion of terminology in the 1975 Atomic Weights report including a definition of atomic weights. In the report, there was also a continued discussion of well characterized materials and some examples of labelling that might be used. At the close of the meeting, Etienne Roth was elected Chairman to replace Norman Greenwood and Norman E. Holden (now at the Brookhaven National Laboratory, Upton, New York, USA) was elected as the Secretary to replace H. Steffen Peiser. At the conclusion of the meetings and at the request of the outgoing chairman, Norman Greenwood, Steffen Peiser prepared a Technical Policy statement for the use of the Commission members and to help to provide continuity for decisions to be made by future members. Details will be discussed later under Commission policies and guidelines.

At the 1977 IUPAC Warsaw, Poland General Assembly, there was another Open Meeting to discuss terminology. This Interdivisional meeting included the nomenclature commissions from Analytical Chemistry, Inorganic Chemistry, Organic Chemistry, and the committees on Teaching of Chemistry and on Nomenclature and Symbols (IDCNS), as well as Atomic Weights. There was also a discussion at the Commission meeting on the labelling of well characterized materials with examples of suggested labels.

The SAIC subcommittee presented another interim version of the Table of Isotopic Compositions of the Elements as determined by Mass Spectrometry, which was published in the 1977 Atomic Weights report71. The Commission heard a report on atomic weights and isotopic compositions for non-terrestrial data and formed a working party under Walter Johnson (Univ. of Minnesota, USA) as chairman to continue the work. Beginning with the 1977 report, the IUPAC Scientific Publications Secretary, Dass Gujral, insisted that the name of the person preparing the report be added to the publication in Pure and Applied Chemistry. This practice continued for many years, until the Commission decided that its reports should be authorless. In 1995, the Commission reversed itself and again added the name of the person preparing their reports for publication.

VI. A Cultural Aspect to Commission Meetings

With the increased importance of the isotopic abundance measurements in the determination of the atomic weights, the Commission's name was changed at the 1979 IUPAC General Assembly in Davos, Switzerland to "Commission on Atomic Weights and Isotopic Abundances (CAWIA)". A new definition of atomic weight was presented, which indicated that atomic weights could be defined for a sample. Tables of Standard Atomic Weights published by the Commission refer to best knowledge of the elements in natural terrestrial sources. Atomic weight (mean relative atomic mass) of an element from a specified source was defined as "the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C". The Commission presents the most accurate available values for those who need to use them but the concept of accuracy implies the existence of a true value and the new definition fails to recognize the existence of one true value for every element.

The 1979 Atomic Weights report72 included a discussion on labelling of well characterized materials and an updated version of a Table of Isotopic Compositions of the Elements determined by Mass Spectrometry. The Commission heard a report from its Working Party on non-terrestrial isotopic abundances, which illustrated some of the significant variations with normal terrestrial values. The Working Party's intention was to provide a more comprehensive listing in the future. A table of relative atomic masses and radioactive half-lives of selected nuclides was published and would continue to be published along with the Table of Standard Atomic Weights in all future reports. Norman E. Holden was elected as Chairman to replace Etienne Roth and Ray Martin (Monash University, Clayton, Victoria, Australia) was elected as Secretary to replace Norman Holden.

There was also a cultural aspect to the Commission meetings. At the 1975 sessions at the Madrid, Spain General Assembly, the traditional IUPAC English tea and coffee break was replaced by a beer break at ten o'clock in the morning. Some of the European members seemed to adapt to this cultural variation more easily than the other remaining Commission members.

In 1979, the SAIC subcommittee meeting was hosted in Geel, Belgium by the chairman, Paul De Bievre, who also arranged a cultural three day drive to the Davos Switzerland General Assembly Commission meeting. Paul and Robert Hagemann (CEN, CEA, Saclay, France) provided the cars. After overnight visits in Aachen and Heidelberg, Germany, the caravan approached the Swiss border. Passengers had been encouraged to switch cars after meals to allow members to become better acquainted with each other. When the Swiss border guard approached the car window and questioned the nationality of the four occupants, he received replies of Belgium, Canada, France and the USA. At that point, he ordered everyone out of the car. When he asked the travelers to open their suitcases in the trunk of the car, he received the reply that those suitcases belonged to other confederates who happened to be traveling in another car. At this response, he ordered everyone into the police station for interrogation. At this point, we were all hoping that the other car was not ahead of us on the road and already in Switzerland. Fortunately within ten minutes, the other car arrived and the various SAIC members could open their bags and show the scientific material that would justify their escape from the police station.

The 1979 meeting occurred before the advent of the Euro currency. Tom Murphy (US-NBS, Washington, D.C.) had been invited to attend the SAIC and Commission meetings as a potential new member. At work, he announced that he was flying to Brussels for the SAIC meeting in Geel. At the time, Brussels was one of the most expensive cities in Europe. Tom received a travel advance of 75 USD per diem. IUPAC was paying the other members at the per diem rate of 35 USD for these meetings. Since he was being paid more than double everyone else, Tom volunteered to buy the house wine for the dinner table in Heidelberg. First, he attempted to pay the wine bill with a 100 belgium franc note, which he had mistakenly failed to convert before leaving Geel. He was embarrassed by the incident and quickly found a 100 mark note to cover the bill and then waited for the waitress to return with his change. Eventually, the waitress was called over to the table and she explained that the house wine was 49 DM a jug and assumed that the change of 2 DM was her tip. Tom bowed his head and left the restaurant without a further response. To add insult to embarrassment, when Tom returned to work and filed his expense account, he was told that since he never stayed in Brussels during this trip, his travel per diem was 35 USD. Needless to say, Tom remained the butt of various Commission jokes during his entire twelve year tenure period on the Commission.

VII. The Growing Importance of Isotopic Compositions

For the 1981 IUPAC General Assembly in Leuven, Belgium, the Commission decided to publish its report in Pure and Applied Chemistry in two separate parts for the first time, i.e., the Atomic Weights of the Elements, 198173 and the Isotopic Compositions of the Elements, 198174. The latter report was the result of SAIC's work. A Table of Atomic Weights abbreviated to Five Significant Figures was prepared by Norman Greenwood and Steffen Peiser and published in the Atomic Weights report. It was provided in the hope that this Table of the atomic weight values would remain unchanged, at least for a number of years, and it would provide practicing chemists and others with all their needed but not superfluous data.

The Commission reviewed the changes in relative uncertainty of Standard Atomic Weight values between 1969 and 1981 and there were 29 changes for 25 elements during that period of time. There was another discussion on a non-terrestrial data report from the Commission's Non-Terrestrial Data Working Party .

At the 1983 IUPAC General Assembly in Lyngby, Denmark, the Commission continued the practice of publishing its report in two parts75,76, i.e., the 1983 Table of Standard Atomic Weights and the 1983 Table of Isotopic Compositions, respectively. The Subcommittee on Assessment of Isotopic Compositions (SAIC) under chairmanship of Paul De Bievre reviewed the literature and recommended a change in atomic weight for twelve elements and a change in footnotes for nine elements. This Sub-committee was dissolved, after having completed its task of presenting a comprehensive element-by-element review77 of all measurements78 for deriving the isotopic compositions and the best values which are consistent with the Table of Standard Atomic Weights.

During this meeting, the Commission changed its method for expressing uncertainties in atomic weight values. Previously these uncertainties were restricted to one of two values, i.e., either ± 1 or ± 3. Beginning with the 1983 report, these uncertainties could now take on any digit from ± 1 up to ± 9.

Another change with the 1983 report dealt with the treatment of elements with no stable isotopes.
For radioactive elements with no unique naturally occurring isotopic composition from which an atomic weight can be calculated with five or more figure accuracy without prior knowledge of the sample, the concept of a standard atomic weight has little meaning. There is no general agreement on which of the isotopes of these radioactive elements is, or is likely to be "important". No value is listed in the Table of Standard Atomic Weights for these radioactive elements, merely the element name, the atomic number and the chemical symbol.

For the non-terrestrial data, elements with variations in isotopic compositions could be classified into the three categories: mass fractionation, nuclear reactions and solar wind, where nuclear reactions would include: nucleosynthesis, spallation reactions, low energy neutron irradiations and radioactive decay products. A number of variations could be quite large, so scientists dealing with non-terrestrial samples were warned to exercise caution when the isotopic composition or atomic weight of a non-terrestrial sample is required.

The Commission approved a Table of Atomic Weights to four significant figures (prepared by Greenwood and Peiser) for the IUPAC Committee on Teaching of Chemistry. At the close of these proceedings, the Commission elected Ray Martin as Chairman to replace Norman Holden and John de Laeter (Western Australian Institute of Technology, Perth, Western Australia, Australia) as Secretary to replace Ray Martin.

At the 1985 IUPAC General Assembly in Lyon, France, no Table of Isotopic Compositions of the Elements was published79. A Working Party, under I. Lynus Barnes (US-NBS, USA), that had been formed to examine the procedures used to assign uncertainties to atomic weights met at the US-NBS in July 1984 and reported to the Commission that they had prepared a set of Technical Guidelines. These Guidelines were used to help assess the data but emphasized that the collective experience and judgement of Commission members was its most valuable asset and must be applied in each case. In the future, John de Laeter would reproduce the guidelines and other significant information in the Commiossions Technical Booklet (see the section on policies and guidelines). Having completed its task, this Working Party was disbanded.

New publications of atomic mass values and of an element by element review of atomic weights resulted in a re-assessment of atomic weight values and uncertainties and changes for thirty-five elements in the Table of Standard Atomic Weights. Non-terrestrial data were reclassified into five categories as mass fractionation, nuclear reactions, radioactive decay products, solar particle emission and cosmic rays. Maximum isotopic abundance variations in non-terrestrial materials were tabulated for a number of elements. Masako Shima (National Science Museum, Tokyo, Japan) was elected as chairman of the Working Party to replace Walter Johnson, who retired from the Commission.

A Sub-committee for Isotopic Abundance Measurements (SIAM) was established under I. Lynus Barnes as chairman to identify and assess experimental methods leading to isotopic abundances and atomic weights and to evaluate critically any new data uncovered. Two working parties were formed on "Uncertainty in Chemical Measurements" and on "Natural Isotopic Fractionation".

At the 1987 IUPAC General Assembly in Boston, Massachusetts, USA, no new Table of Isotopic Compositions of the Elements was published. In a discussion of mononuclidic elements, the use of a demarcation of 3 x 1010 years led to twenty elements being considered mononuclidic. A result of this decision was the removal of protactinium's atomic weight value from the Table of Standard Atomic Weights80.

For the Table of maximum isotopic variations of non-terrestrial data, information was classified into three processes, i.e., mass fractionation, nuclear reactions and radioactive decay products and into six sources, i.e., solar particles, cosmic rays, cool stars, planets and satellites, comet Halley and interplanetary dust (cosmic dust). The Commission undertook tasks to update the Table of Atomic Weights to Four Significant Figures and to Five Significant figures. At the conclusion of the meeting, John de Laeter was elected chairman to replace Ray Martin and Klaus Heumann (University of Regensburg, Regensburg, FRG) was elected secretary to replace John de Laeter.

At the 1989 IUPAC General Assembly in Lund, Sweden, the Commission once again published its report in two parts81,82, i.e., Atomic Weights of the Elements, 1989 and Isotopic Composition of the Elements, 1989. Due to the nearly constant isotopic composition of protactinium in nature, where 231Pa is the predominant isotope, an atomic weight value was reinserted in the Table of Standard Atomic Weights for protactinium. A Table of Standard Atomic Weights which was abridged to five significant figures was published in the report and a four figure Table prepared by Greenwood and Peiser was published separately.

The Non-terrestrial Data Working Party produced another Table of Maximum Isotopic Variations and a Table of Isotopic Composition and Atomic Weight from different non-terrestrial sources. A working party on Statistical Evaluation of Isotopic Abundances was established.

At the 1991 IUPAC General Assembly in Hamburg, FRG, the Commission decided to publish a single report83 on Atomic Weights of the Elements, 1991. A graph of the changes in the relative uncertainties of the standard atomic weight values from 1969 to 1991 was displayed. The report of the Working Party on Non-terrestrial Data contained a graph and a few tables on variations in extra-terrestrial materials. At the conclusion of the meeting, Klaus Heumann was elected chairman to replace John de Laeter and Tyler Coplen (US Geological Survey, Reston, Virginia, USA) was elected secretary to replace Klaus Heumann.

At the 1993 IUPAC General Assembly in Lisbon, Portugal, the Commission chose to publish a single report84 on Atomic Weights of the Elements, 1993. A Table of Standard Atomic Weight Vaues abridged to five significant figures was published in the report. A report on relative abundance data for stable hydrogen, carbon and oxygen isotopes was presented and would be published separately. An extensive report from the Non-terrestrial Working Party was presented.
With the retirement of Masako Shima after the meeting, Ludolf Schultz (Max Planck Institute, Mainz, FRG) was elected chairman of the Working Party.

At the 1995 IUPAC General Assembly in Guilford, UK, the Commission chose to publish a single report85 on Atomic Weights of the Elements, 1995. A graph of changes in relative uncertainty of recommended atomic weights of the elements between 1969 to 1995 was presented in the report.
The Non-terrestrial Working Party gave an extensive report on the variation of oxygen isotopic ratios in meteorites and in stars relative to oxygen isotopic ratios in Standard Mean Ocean Water.
At the conclusion of the meeting, the Commission elected Ludolf Schultz (Max Planck Institute, Mainz, FRG) as chairman to replace Klaus Heumann and Robert Vocke (USNBS, USA) was elected secretary to replace Tyler Coplen. Mitsuru Ebihara (Tokyo Metropolitan University) was elected chairman of the Non-terrestrial Working Party replacing Schultz, who would now serve as Commission chairman.

At the 1997 IUPAC General Assembly in Geneva Switzerland, the Commission decided to publish its report in two parts86,87, i.e., Atomic Weights of the Elements, 1997 and Isotopic Composition of the Elements, 1997. A graph of changes in relative uncertainty of recommended atomic weights of the elements between 1969 and 1997 was presented in the report. The Working Party on Non-terrestrial Data provided an extensive report on processes for altering isotopic abundances in non-terrestrial samples and used xenon isotope ratios as an example. The Commission created a working party under De Laeter to produce another element-by-element review similar to the 1984 SAIC review.

At the 1999 IUPAC General Assembly in Berlin, Germany, the Commission chose to publish a single report88 on Atomic Weights of the Elements, 1999. The Working Party on Non-terrestrial Data reported on the processes explaining isotopic variations and provided a table of anomalous isotopic compositions in extra-terrestrial materials due to decay of radioisotopes. Robert Loss (Curtin University, Perth, Australia) was elected secretary to replace Robert Vocke.

At the 2001 IUPAC General Assembly in Brisbane, Australia, the Commission chose to publish a single report89 on Atomic Weights of the Elements, 2001. The Commission elected Philip Taylor (Institute for Reference Materials and Measurements, Geel, Belgium) as chairman to replace Ludolf Schultz. To emphasize the great importance of the isotopic abundance values as the sole source (along with the atomic mass values of the stable isotopes) for determining atomic weight values for the elements, the Commission once again changed its name to the Commission on Isotopic Abundances and Atomic Weights (CIAAW).

At the conclusion of the Brisbane General Assembly, changes to the IUPAC bylaws and statutes resulted in all scientific Commissions of IUPAC being terminated and the right of titular members of Commissions for travel expenses to General Assemblies was also terminated. After numerous discussions within IUPAC, the Commission on Isotopic Abundances and Atomic Weights (CIAAW) was made an exception to the rule change on the termination of Commissions and allowed to continue within IUPAC.

Normally in the period between IUPAC General Assemblies, the members of the Commission and the various sub-committees perform the literature search for data from the journal and document sources and an initial assessment of the results and impact on the data base. In the period after the Brisbane General Assembly, it was determined that although IUPAC approved continuation of the Commission, the termination of funding for titular members of the Commission was not changed. It had been concluded that without adequate funding, there would be no Commission meeting in 2003. This thinking continued for more than one and one half years, until the President of the IUPAC Inorganic Chemistry Division, Gerd Rosenblatt, made monies available to bring together all members of the Commission and sub-committees in Ottawa to discuss the future course of the Commission. As a result of the above confusion, no preparatory work for the scientific agenda had been done to analyze the data and recommend updated values for the Table of the Standard Atomic Weights. At the 2003 IUPAC General Assembly in Ottawa, Canada, the Commission chose not to publish a report on Atomic Weights for 2003 (for the first time in almost forty years). The Commission and the sub-committees discussed the future work of these bodies and a mechanism for the funding for continued operation either within IUPAC or outside of the IUPAC framework. Tiping Ding (Chinese Academy of Geological Sciences, Beijing, China) was elected chairman to replace Philip Taylor and Michael Wieser (University of Calgary, Alberta, Canada) was elected secretary to replace Robert Loss at the conclusion of the meeting.

One atomic weights publication that did appear in a pre-print form at the time of the Ottawa General Assembly was another element-by-element review called EXER-200090. It had been the result of the six year effort by the members of the 1997 working party and it was written in a similar manner to the earlier SAIC review77.

VIII. Uncertainties and Annotations

In 1949, when Ed Wichers became head of the reorganized Atomic Weights Commission, he preferred that most recommended atomic weight values carry neither experimental uncertainties nor any indication of variability between terrestrial sources. He held the view that atomic weight values were constants of nature. Mentioning uncertainties might cause users to lose confidence in the reliability of these values. His position was that the Commission reviews the published literature and determines a best current value and then rounds back to an abbreviated value, which is more likely closer to the true atomic weight than a value whose terminal digit would differ by ± 1 from this abbreviated value.

Following Becqueral's discovery of radioactivity 91 , radioactive materials were studied and many substances were found with various atomic weight values. Soddy showed the chemical identity of meso-thorium ( 228 Ra) and radium 92 . In 1913, he concluded that there were chemical elements with different radioactive properties and with different atomic weights but with the same chemical properties and therefore occupying the same position in the Periodic Table. He coined the word “isotope” (Greek: in the same place) to account for these radioactive species 93 .

The study of the natural radioactive decay chains for thorium and uranium led to speculation that these parent isotopes, 232 Th and 238 U would decay into different daughter isotopes of lead, 208 Pb and 206 Pb, respectively. The lead from radioactive minerals should differ in atomic weight according to the proportion of uranium and thorium in the mineral. The atomic weight value for “common” lead (from a non-radioactive source material) was measured to be 207.2 by Baxter 94 . Soddy 95 measured lead from a thorium silicate mineral to have an atomic weight value of 208.4. Richards 96 measured the atomic weight of lead in uranium minerals as low as 206.4. These long known differences in lead samples were considered an exception and attributed to the lead isotopes being products of the natural radioactive decay chains.

However, the reported variation in oxygen's atomic weight value in water and in air by Dole that was mentioned earlier 61 and Nier's measurement 97 on carbon, which showed a 5% variation in the isotopic composition indicated that the atomic weight for an element could vary depending upon the source of the material studied. The Commission continued to think that the variations, if any, were less than the experimental uncertainty in the measurement. The reported atomic weight value would be provided to one less digit, so that any uncertainty or variation would not occur in any of the digits being reported.

Based on a report by Marble 98 , a range was finally added to the atomic weight value of sulfur for the first time in the 1951 report 99 . In the 1961 report 65 , ranges were listed for six elements due to the natural variation in their isotopic composition and experimental uncertainties were added for five other elements following a systematic review by Cameron and Wichers. Minor changes occurred in 1965, when an uncertainty was added to copper and the uncertainties were lowered on bromine and silver. In 1967, an uncertainty was added for neon and the uncertainty for chromium was eliminated. Finally in 1969, this transition from a concept of a constant of nature was completed with the publication of the 1969 Atomic Weights report 67 . The values and the uncertainties were now listed for all elements, using one of two possible uncertainty values, either ± 1 or ± 3 in the final digit quoted for the atomic weight. Thus the uncertainties were considered to be symmetric in the positive and negative sense and they were roughly equal proportional steps in uncertainties by factors of 3.00 and 3.33.

Wichers' successor, Norman Greenwood, argued that the atomic weights were consensus values enunciated by uniquely qualified experts and could not be subject to statistical concepts. For many elements, there was only one “absolute” measurement of its isotopic composition available and the reliability of this measurement could be judged only by expert analysis of the details from the published reference.

At the 1983 meeting, SAIC completed its comprehensive element-by-element review of all of the measurements for deriving isotopic composition and their atomic weights. Using this information, the Commission changed its method for expressing uncertainties in atomic weight values to allow any digit from ± 1 up to ± 9.

Footnotes or annotations were discussed in the 1969 Atomic Weights report 67 , where a) and b) referred to mononuclidic elements and elements with one predominant isotope, respectively. Footnote c) indicated an atomic weight of that element was based upon calibrated measurements. Footnote d) indicated an element for which variations in isotopic abundance in terrestrial samples limited the precision of the atomic weight. Footnote e) cautioned users about the possibility of large variations in atomic weight due to inadvertent or undisclosed artificial isotopic separation in commonly available materials. Footnote f) indicated the most commonly available long-lived nuclide. Finally, footnote g) indicated that in some geological specimens the element has a highly anomalous isotopic composition, corresponding to an atomic weight significantly different from that given in the table.

This set of footnotes was continued to be used in the 1971 and the 1973 reports. In the 1975 report 70 , it was decided to eliminate footnotes a), b) and c) which gave reasons why many atomic weight values could be given to high precision because this information was no longer necessary. The other footnotes d), e), f) and g) were renamed w), y), z) and x) to avoid confusion with the earlier Tables. This renamed set of footnotes continued to be used in the 1977 and the 1979 reports.

In the 1981 report 73 , the same four footnotes were retained but renamed. The geological footnote was again symbolized by g), while m) was used for the artifically modified footnote, r) was used for range to indicate the variation limits on atomic weight values and L) was used to indicate the isotopic mass of longest half-life.

There was another change in footnotes with the 1983 report 75 . The first three footnotes remained the same. An * was added to various element names for elements with no stable isotopes. The Commission removed isotopic masses with long half-lives from the Table along with footnote L). Footnote A) was added to indicate a radioactive element whose more well known isotopes were listed in another table with mass and half-life and footnotes X) and Y) were added to explain thorium and uranium having a meaningful atomic weight value in the table.

In the 1985 report 79 , footnotes X) and Y) were replaced by Z) to explain why meaningful atomic weight values were available in the table for thorium, protactinium and uranium. In the 1987 report 80 , the atomic weight value for protactinium was removed and its footnote changed from Z) to A).

In the 1989 report 81 , footnote A) was removed, along with footnote Z) but the explanation of footnote Z) was incorporated under the * added to the element name. There was no change in the 1991 report.

For the 1993 report 84 , the symbol “_” was added to the atomic weight value of lithium to indicate that commercial values varied between 6.94 and 6.99. There was no change in footnotes for the 1995 report. In the 1997 report 86 , the same lithium footnote now indicated the range was between 6.939 and 6.996. These same footnotes continued in the 1999 report.

During these years, the foremost proponents on the Commission for the footnotes and annotations on Atomic Weight values were Steffen Peiser and Etienne Roth. Following the 1985 meeting in Lyon, France, Roth provided a tour of his laboratory at Saclay to the Australian members of the Commission. To insure that they would make proper connections for their flight from Brussels back to Australia, Roth rushed his colleagues to the train station only to find that there was no Paris to Brussels train. Roth (the footnote expert) had failed to observe the footnote on the train schedule indicating that the Paris to Brussels train did not run on that particular day of the week.

The requirement that the uncertainty values be listed for the atomic weight values has plagued the Commission over many years. This partially results from a difficulty in determining how to treat the random and systematic uncertainties and the natural variations of the stable isotopes. The variation of the low side of the estimated best value is often different from the variation on the high side. In order to obtain a symmetric uncertainty, the best value is shifted to a mid point between the low and the high side variations. It may result in an estimated best value for the atomic weight, which does not correspond to any known sample of that element.

IX. Naming of Natural and Synthetic Elements

The names of the chemical elements arise from many sources, including names given to an element whose origin is in antiquity, mythological concepts or characters, places, areas or countries, properties of the element or its compounds such as color, smell or inability to combine and the names of scientists.

The basis for claim of discovery of an element has varied over the centuries. The method of discovery of the chemical elements in the late eighteenth and the early nineteenth centuries used the properties of the new substances, their separability, the colors of their compounds, the shapes of their crystals and their reactivity to determine the existence of new elements. There were many claims such as the discovery of a mineral ore, from which an element was later extracted. The honor of discovery has often been accorded not to the person who first isolated the element but to the person who first discovered the original mineral itself, even when the ore was impure and that ore actually contained many elements. The reason for this is that in the case of the rare earth elements, the “earth” now refers to oxides of a metal not to the metal itself. This fact was not realized at the time of discovery, until the English chemist, Humphry Davy, showed that earths were compounds of oxygen and metals in 1808.

There are cases where the honor of discovery has not been given to the first person who actually discovered the element but to the first person to claim the discovery in print. If a publication was delayed, the discoverer has often historically been beaten into print and denied the honor of the discovery.

By the mid nineteenth century, the appearance of the Periodic Table helped to resolve some of these problems with the elements of the main table. However, the rare earths presented a special problem, since they did not fit into the main table. Fortunately, when Mendeleev first proposed the Table, many of the rare earth elements had not yet been discovered and those that were known to exist did not have accurately determined atomic weights or properties. The similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate and led to situations where many mixtures of elements were being mistaken for elemental species 100 .

When a chemical element is excited, either by heating or with an electric spark, it emits radiation characteristic of the element, which can be recorded by a spectrometer. Several elements have been discovered by use of the spectrometer. At the 4th International Congress on Applied Chemistry at Paris in 1900, it was agreed that no new element should be accepted until its spark spectrum was identified and shown to be different from all other elements. The requirement for the discovery of an element became the chemical separation of enough material of that element to detemine the atomic weight and the spectrum analysis of that element.This was the situation at the time that the International Commission on Atomic Weights was formed.

Until the twentieth century, fractional crystallization was a major method for purification of an element. In most cases, this procedure required thousands of recrystallizations involving months of work. Although the ICAW did not set internationally approved names, an element with an atomic weight value in their table lent support for the acceptance of that element by the chemical community. There was never a discussion about the names of the elements in the Commission's report. The members took the names of the elements that were favored in their own country. Dual names for some elements were listed in the International Table. When the table was reproduced in various countries, the name and symbol in each Table was the one accepted by the journal for that country. Niobium and columbium, beryllium and glucinium and wolfram and tungsten were a few examples of the practice of dual names. This practice continued until the mid-twentieth century and still persists today in the cases of both aluminium and aluminum and of sulfur and sulphur.

In July 1947, at the 14th IUPAC Conference (General Assembly) in London, the IUPAC Council approved a resolution that the conferring of names for new chemical elements was referred to the Committees on Atoms (Nuclear Chemistry) and Atomic Weights and the committees on chemical nomenclature. However, the Committee on Atoms dissolved itself at the Conference. The ICAW outgoing President, Paul Baxter, advised the incoming President, Ed Wichers, to leave the matter of the names of the elements to the Nomenclature Commission and avoid trouble.

By the 15th IUPAC Conference (GA) in Amsterdam in September 1949, the IUPAC Committees had been renamed Commissions. During the Conference, the Commissions on Atomic Weights and Inorganic Nomenclature met jointly and adopted names for new elements and some old elements. New elements included technetium, promethium, astatine, francium, neptunium, plutonium, americium and curium. The renamed old elements included beryllium, niobium, lutetium, hafnium, wolfram and protactinium. In the future, the Commission on Inorganic Nomenclature would adopt new names for the chemical elements without consulting with the ICAW. When the name wolfram was determined not to be accepted by the chemical community, the element name was changed to the other alternative, tungsten. However, the symbol W was retained in lieu of the alternative symbol, Tu.

In the 1949 Report, the ICAW added these new elements to the Atomic Weights table. These elements are not discovered in nature but are artifically produced in the laboratory. As a result, the atomic weight values of these artifical products will depend upon the production method and will vary. Since atomic weight is a property of an element as it occurs in nature, no atomic weight value would be assigned to that element in the Table. The ICAW decided to provide only the mass number of the most stable (longest-lived) known isotope.

In 1957, The Commission decided to omit the mass number for radioactive elements on the grounds that the mass number is inconsistent with the purpose of the Atomic Weights Table, i.e., to provide accurate values for use in chemical calculations. This is true even for the Table that is restricted to five significant figures. An auxiliary table of Radioactive Elements was included in the report, where the longest known half-life of each element was listed. In the 1973 report, the ICAW reintroduced the mass number in the Atomic Weights Table because users were dissatisfied with their omission from the Table.

In the 1983 Table of Standard Atomic Weights there were 24 radioactive elements listed but these elements had neither a mass number nor an atomic weight value. Since that time, these elements have always been listed with no adjoining data in this Table. Data for these elements have only appeared in the auxiliary Table with the mass number of its longest lived nuclide. However, that mass number has often varied over time as better half-life values or new nuclides with longer half-lives were measured for a given element.

When the elements numbered 104, 105 and 106 were first being synthesized at the Lawrence Berkeley Laboratory in the USA and at the Joint Institute for Nuclear Research in Dubna, USSR, there were arguments over the priority claim for the first valid synthesis for these elements and the right to name them. These arguments continued for a quarter of a century, until a joint IUPAC and IUPAP (International Union of Pure and Applied Physics) task group was able to resolve this dispute 101,102 and provide agreed upon names for the elements numbered 104, 105, 106, 107, 108 and 109. The IUPAC-IUPAP joint task group concept has been used since that time to review the measured data before a decision has been determined on who should have the right to name a new chemical element, e.g., element 110 103 . The latest chemical element to fulfil the requirements is element 111 104 .

X. Conclusions

The International Committee on Atomic Weights has a long and colorful history dating back for over a century. Initially, the task was to provide the chemical community and trade and commerce with the most accurate atomic weight values for the chemical elements. For over the past half century, the isotopic composition of the stable (or very long-lived) isotopes of those elements has taken on a larger role, until today the atomic weight values are determined by mass weighting the isotopic abundance values. There was much interest in the Atomic Weight values when they were considered constants of nature and the building blocks of the Periodic Table and even more now that they are known to be variable 105 .

XI. Acknowledgements

I would like to thank many members of the Atomic Weights Commission, who took time to provide comments on this review including, Tyler Coplen, Paul De Bievre, John De Laeter, Norman Greenwood, Steffen Peiser and Etienne Roth. They helped to greatly improve this document but any additional errors still present are entirely my own. Roger Fennell's “History of IUPAC, 1919-1987" was very useful background information about the early years of IUPAC and IUC.

This research was carried out under the auspices of the US Department of Energy, Contract No. DE-AC02-98CH10886.

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Norman Holden <> has been involved with IUPAC for over 30 years and is today a titular member on the Inorganic Chemistry Division. Since 1974, he has been at the National Nuclear Data Center of the Brookhaven National Laboratory, in Upton, New York, USA.


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