Nuclear masses are measured in terms of atomic mass units with the carbon-12 nucleus defined as having a mass of exactly 12 amu. It is also common practice to quote the rest mass energy E=m₀c² as if it were the mass.

 

a.m.u is also defined to be one twelfth of the mass of an unbound atom of the carbon-12 nuclide, at rest and in its ground state.

 

1 u = 1/N_A gram = 1/ (1000 N_A) kg   (where N_A is Avogadro's number)

 

1 u = 1.660538782(83) × 10^-27 kg = 931.494028(23) MeV/c²

 

Since, for berillium amu is 9.012 > 9, so amu based on Be-9 would be slightly larger than amu based on C-14.