Question

Why is the average atomic mass for an element typically not a whole number?

Answer 1

Because of the existence of isotopes.

Explanation:

An element is defined by it atomic number, `Z`, the number of massive, positively charge particles (protons): `Z=1, H; Z=2, He; Z=3, Li...........Z=92, U`.

On the other hand, a given nucleus may contain different numbers of neutrons, massive, neutrally charged nuclear charges. If we take `C` as our exemplar, it is a fact that all carbon nuclei have 6 protons; `Z=6` by definition. Most carbon nuclei contain 6 neutrons, to give the `""^12C` isotope. A few carbon nuclei contain 7 neutrons, to give the `""^13C` isotope, which is a very important isotope as it allows `""^13C` `NMR" Spectroscopy"`. A smaller few contain 8 neutrons to give the `""^14C` isotope.

Now all these isotopes have 6 protons, but the different number of neutrons gives rise to different elemental masses. The weighted average of the individual isotopes is the quoted atomic mass, which for `C` is `"12.011 amu"`.

Transition metals typically have a range of isotopes, and thus non-integral atomic masses.