Question

Why are atomic masses not whole numbers?

Answer 1

Atomic mass is never an integer number for several reasons: The atomic mass reported on a periodic table is the weighted average of all the naturally occuring isotopes. Being an average it would be most unlikely to be a whole number. The mass of an individual atom in atomic mass units is the mass relative to carbon-12.

Answer 2

Answer:

This is because, the atom mass of an atom is the average of the atomic masses of all its isotopes multiplied by their abundance in nature. Therefore, is it highly unlikely to get a whole number as the atomic mass.
Another reason is that, since carbon-12 is the reference atom, the atomic mass is calculated relative to it.