Chemists use relative atomic masses and relative formula masses to carry out mole calculations.

Atoms have very little mass, so they’re difficult to measure accurately. Instead, chemists use a **scale**. On this scale the mass of a ** ^{12}C atom is exactly 12**.

For example:

^{24}Mg has a relative mass of 24 – it is twice as heavy as a^{12}C atom.^{1}H has a relative mass of 1 – it is 12 times lighter than a^{12}C 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 ^{37}Cl and 75% are ^{35}Cl. 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 ^{12}C isotope, which has a mass of exactly 12.**

Relative atomic masses **do not have units**.