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Atoms have very little mass so instead of using their actual masses in kilograms, their relative atomic masses are used. The relative atomic mass of an element, symbol A_r, is defined as the relative mass of its atoms compared to the mass of a particular carbon atom (carbon-12). The A_r values for elements are often stated in the periodic table. Note, as A_r is a measure of the relative masses of atoms, it has no units.

The relative atomic masses of elements are proportional measures. For example, the A_r for carbon is 12, and the A_r for magnesium is 24. This means that magnesium atoms are twice the mass of carbon atoms.

Calculating relative formula mass

Relative formula mass has the symbol, M_r. To calculate M_r for a substance:

1. work out how many atoms of each element are in the chemical formula
2. add together the A_r values for all the atoms of each element

For example, the formula for carbon dioxide is CO₂. It has one carbon atom (A_r = 12) and two oxygen atoms (A_r = 16):

M_r of CO₂ = 12 + 16 + 16 = 44

It could also be calculated this way:

N_r of CO₂ = (1 × 12) + (2 × 16) = 12 + 32 = 44

Like A_r values, M_r values also have no units.

Relative formula masses of ionic compounds

Ionic compounds such as sodium chloride do not exist as molecules. However, their relative formula masses are calculated in the same way. The formulae used are their empirical formulae.