Question

How can an atomic mass not be a whole number?

Answer 1

Answer:

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  μ, 55.935  μ, 56.935  μ, and 57.933  μ

. If you get their abundances and multiply the corresponding mass to their abundance, you get the average atomic mass, which is 55.846  μ.

Hope this helps :)