# Why is the average atomic mass for an element typically not a whole number?

May 27, 2016

Because of the existence of isotopes.

#### Explanation:

An element is defined by it atomic number, $Z$, the number of massive, positively charge particles (protons): Z=1, H; Z=2, He; Z=3, Li...........Z=92, U.

On the other hand, a given nucleus may contain different numbers of neutrons, massive, neutrally charged nuclear charges. If we take $C$ as our exemplar, it is a fact that all carbon nuclei have 6 protons; $Z = 6$ by definition. Most carbon nuclei contain 6 neutrons, to give the ""^12C isotope. A few carbon nuclei contain 7 neutrons, to give the ""^13C isotope, which is a very important isotope as it allows ""^13C $N M R \text{ Spectroscopy}$. A smaller few contain 8 neutrons to give the ""^14C isotope.

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 $C$ is $\text{12.011 amu}$.

Transition metals typically have a range of isotopes, and thus non-integral atomic masses.