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

1 Answer
May 27, 2016

Answer:

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# #NMR" 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 #"12.011 amu"#.

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