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The Carbon-12 atom having an atomic mass of exactly 12u is just a consequence of the definition of the atomic mass unit. Nothing special about it. There are several actual reasons why Carbon-12 is taken as the standard of atomic mass measurement.
- The standard to be chosen should not be very small or very large compared to the atoms that are to be measured. Carbon is one of the qualifying contenders.
- The standard must be easily and uniformly available globally for prompt and consistent measurement. Carbon is again a qualifying contender here.
- The isotopic uncertainty must be minimum. C-12 has almost 98.9% redundancy among all of its isotopes. Even though O-16 has a higher abundancy among its isotopes found in air, it loses in the consistency race when its presence in water and oxides (compounds of oxygen) are considered. C-12 retains a great consistency in all of its naturally occurring forms.
- Last but not least, If O-16 is defined as the standard and to have a mass of exactly 16u, it will be strictly an even number which will mostly leave fractions when odd numbers are divided by it. C-12, on the other hand, is more suitable since it has both 2 and 3 as its factors.Considering the above reasons, C-12 was finally chosen by both physicists and chemists to be the standard of relative atomic mass scale instead of O-16.
- Just to finish it off, there is also the physicists’ ego at play here. Much before the Unified Atomic Mass Unit was formally settled by IUPAC in 1961 and then further adopted by BIPM in 1971, physicists were already using carbon 12 as their standard for mass spectroscopy. Thus they managed to convince the Chemists, who were using oxygen 16 as the standard for atomic mass, to switch to C-12.