Why was oxygen replaced by carbon 12 as reference atom on the atomic scale?

Oxygen atom is replaced by C-12 atom because:

Mass of 1 carbon atom = 1.99 x10-23g

Number atoms in 12 g of C-12 =12g/1.99 x10-23 g =6.023 x1023

which is equal to the number of atoms in 1 mole of any substance and hence C-12 was selected as the reference of atomic mass unit.

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"The atomic weights scale of H = 1 was originally used by Dalton and (except for Berzelius' time) had been used for approximately 100 years when the ACS and the German committees began reporting their tables. Lothar Meyer and Seubert had published on the hydrogen scale, but Ostwald and Brauner 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 both. As a result, the first international table was published on the O = 16 scale. 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, 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 Ostwald. 

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, 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 Johnston led to a situation in which the chemist's scale of O = 16 differed from the physicist's scale of 16O = 16. When Dole 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, 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 spectrometry. 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


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