why carbon12 is used to define the atomic mass unit?

Question

why scientist shifted oxygen & hydrogen?

Answer 1

It is practically mono isotopic. That means C12 is hardly 'polluted' by C14 or other isotopes.

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Answer 2

H and O can be used too its just that its simpler using Carbon12.
If you were to use Oxygen or Hydrogen as the atomic mass unit the ratio would come out to be the same

Answer 3

The idea is to make the atomic mass of an isotope of any element be as close to an integer (specifically, the number of protons plus neutrons) as one can. It is also desireable to have the standard easily realizable in the laboratory. Carbon-12 fits both requirements, and does so better than oxygen. Hydrogen does not work well as it is too heavy to give integral results for the heavier atoms.

Answer 4

It's sorta arbitrary.

Prior to 1959 both the IUPAP and IUPAC tended to use used oxygen to define the mole, the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, the physicists using a similar definition but with the oxygen-16 isotope only. The two organizations agreed in 1959/1960 to define the mole as:

"The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol.""
This was adopted by the CIPM (International Committee for Weights and Measures) in 1967, and in 1971 it was adopted by the 14th CGPM (General Conference on Weights and Measures). In 1980 the CIPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state.

Answer 5

It is arbitrary. It was by decision. Before that, there were many standards, oxygen, oxygen-16, carbon, hydrogen.

At some point it was decided for C-12.

Answer 6

It's doesn't matter what else you chose as a standard. The length can be measured in inches and meters etc. It's the same thing.