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mass of a atom of c-12

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C-12 is used as the standard from which the atomic masses of all other nuclides are measured. The atomic mass of C-12 is 12.000000 u (u = atomic mass units). The atomic mass unit is defined as 1/12th the mass of a neutral C-12 atom. Since C-12 is used as the reference mass, no other nuclides have whole number masses. Converting between atomic mass units and grams (microscopic to macroscopic mass) requires the use of Avogadro's constant (6.022*10^23 mol-1) which is the number of atoms/molecules in one mole of a substance. Avogadro's number, 6.022*10^23, is the number of atoms in 12 g of C-12. Converting between u and grams for one atom of any substance is completed by the conversion factor:

1 u = Mu/NA = molar mass constant/avogadro's constant = (1g/mol)/6.022*10^23 mol-1

Notice that mols cancels out and leaves you with grams for the units. 


To clarify, the mass of an atom of C-12 is exactly 12 u (not 12.000000 u, which has 8 sig figs, and implies a certain amount of error).