Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.
The table shows some Ar values:
|Element||Relative atomic mass|
These values tell you that a magnesium atom has twice the mass of a carbon atom, and 24 times more mass than a hydrogen atom. They also tell you that hydrogen atoms have 12 times less mass than a carbon atom. The Ar values also allow you to work out that three oxygen atoms have the same mass as two magnesium atoms.
Chlorine’s Ar of 35.5 is an average of the masses of the different isotopes of chlorine.
The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes – because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.
For example, chlorine has two isotopes: 35Cl and 37Cl. But the relative atomic mass of chlorine is not 36. In any sample of chlorine, 75 per cent of the atoms are 35Cl and the remaining 25 per cent are 37Cl.
The relative atomic mass is worked out using the following formula, illustrated for two isotopes, where the abundances are given in percentage values.
For example, using chlorine: