Elements contain atoms that differ slightly in atomic mass. This difference, as we encountered earlier, is due to different numbers of neutrons in the nucleus of atoms of the same element. Although all atoms of the same element behave in an identical manner chemically, they differ in mass. For example oxygen is made up of three different isotopes, its isotopic composition is:
- Oxygen with mass 16 makes up 99.76% of the naturally occuring oxygen;
- Oxygen with mass 17 makes up 00.04% of the naturally occuring oxygen;
- Oxygen with mass 18 makes up 00.20% of the naturally occuring oxygen;

So how do we come up with the relative atomic mass of oxygen as being 16? The relative atomic mass of any element is the weighted average of the isotopic masses. The calculations are as follows.

A_r of oxygen = (16 X 99.76 + 17 X 0.04 + 18 X 0.2) / 100

= 16 atomic mass units.

So the formula is
A_r = (% X isotopic mass1 + (100 - %) X isotopic mass2) / 100
We simply multiply all the isotopic masses by their respective relative abundance(%) and then divide by 100.

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