Why is the average atomic mass for an element typically not a whole number?
Because of the existence of isotopes.
An element is defined by it atomic number,
On the other hand, a given nucleus may contain different numbers of neutrons, massive, neutrally charged nuclear charges. If we take
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
Transition metals typically have a range of isotopes, and thus non-integral atomic masses.