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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.

For example:

• 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 A_r.

The term weighted mean mass is used to take into account the contribution made by each isotope to the overall mass of an element.

For example:

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.