Atoms have very little mass, so they’re difficult to measure accurately. Instead, chemists use a scale. On this scale the mass of a 12C atom is exactly 12.
- 24Mg has a relative mass of 24 – it is twice as heavy as a 12C atom.
- 1H has a relative mass of 1 – it is 12 times lighter than a 12C atom.
These masses are called relative atomic masses. They use the symbol Ar.
The term weighted mean mass is used to take into account the contribution made by each isotope to the overall mass of an element.
25% of chlorine atoms are 37Cl and 75% are 35Cl. The weighted mean mass of chlorine is 35.5.
The relative atomic mass of an element is a weighted mean mass of the isotopes of an element compared with that of the 12C isotope, which has a mass of exactly 12.
Relative atomic masses do not have units.