# How can an atomic mass not be a whole number?

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 $\mu$.