what is the difference between gram atomic mass, unified mass and atomic mass unit
Atomic mass of an element expressed in grams is called gram atomic mass.
Atomic mass unit (a.m.u) is the quantity of mass equal to 1/12th of the mass of an atom of carbon (C-12).
Earlier O-16 was used as the basis for the atomic mass unit (a.m.u) but due to the discovery of isotopic oxygen lead to a divergence in atomic weight representation and this resulted in errors in computations. Hence, the reference was changed to C-12 and a new symbol ‘u’ (unified mass) replaced the old symbol ‘a.m.u’.