Question

How can an atomic mass not be a whole number?

Answer 1

It's an average of all isotopes.

Explanation:

There are different isotopes of every element and the atomic mass is an average mass of all those isotopes.

E.g. Iron has for naturally occurring isotopes with masses: 53.940 `mu`, 55.935 `mu`, 56.935 `mu`, and 57.933 `mu`. If you get their abundances and multiply the corresponding mass to their abundance, you get the average atomic mass, which is 55.846 `mu`.

Hope this helps :)