# Why are the atomic masses printed on the Periodic Table not whole numbers?

Magnesium, $Z = 12$, has a number of isotopes. There is ""^24Mg, $\text{79% abundance}$, ""^25Mg, $\text{10% abundance}$, ""^26Mg, $\text{11% abundance}$. The average atomic mass is the weighted average of these isotopes, $\cong$ $\text{24.31 amu}$. Of course each isotope has $12$ nuclear protons (it would not be a magnesium atom otherwise), but the nucleus contains DIFFERENT numbers of neutrons, massive, neutrally charged nuclear particles. You should be able to tell relatively straightforwardly, just how many neutrons there are in each individual isotope. In ""^26Mg, there are $14$ neutrons. Why?