How would I calculate the relative mass of an element?

All the atoms of an element may or may not have the same atomic mass. The atoms that have the same atomic number but different atomic mass are known as the isotopes. All the isotopes of an element may not have the same abundance. Once we know the relative abundance of each isotope of an element, we can calculate its relative mass.

The relative mass or relative atomic mass of an element is the average mass of its isotopes weighted according to their relative abundance. So, if we have an element with 3 isotopes that have atomic masses m1, m2 and m3 and relative abundance levels of a1 %, a2 % and a3 % (remember, a1 + a2 + a3 = 100%); its relative mass can be calculated as:

M = (m1a1 + m2a2 + m3a3)/(a1 + a2 + a3)

Oxygen has 3 isotopes, O-16, O-17 and O-18, with relative abundance levels of 99.762%, 0.038% and 0.200%. Thus, its relative mass is

= (99.762 x 16 + 0.038 x 17 + 0.200 x 18) / (100) = 16.008.

(Note that actual relative mass may be slightly less than these calculations, due to conversion of a small amount of mass into nuclear binding energy.)

Hope this helps.