Average Atomic Mass

When you look at the atomic weight of an element, it is rarely a whole number as you might expect. Instead, it shows a decimal number. This is due to the fact that that there are various forms of most elements (there are 3 different isotopes for Hydrogen) and they occur at different percentages. The average atomic mass reflects both the different masses for the different isotopes and their different occurence rates.

The atomic weight of Hydrogen is 1.007 even though it has only 1 proton which has a weight of 1. Why the .007? There are three versions of Hydrogen: protium with a weight of 1, deuterium with a weight of 2 and tritium with a weight of three. Protium is by far the most abundant type. When you consider the 3 isotopes and their abundances, you can begin to see why elements don't have whole number atomic weights.
