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The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound.

For example, in iron sulfide every iron atom is joined to one sulfur atom, so its formula is shown as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so its formula is shown as Na₂O. Notice that the ‘2’ is written as a subscript, so writing Na2O would be wrong.

This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is written as CO₂.

You may be required to use a key to draw your own diagram of a molecule, eg:

Using this key, the molecules SH₂ and NH₃ would be represented like this.

Sometimes you see more compound formulae such as Na₂SO₄ and Fe(OH)₃:

• a unit of Na₂SO₄ contains 2 sodium atoms, 1 sulfur atom and 4 oxygen atoms joined together – this results in a total of 7 atoms
• a unit of Fe(OH)₃ contains 1 iron atom, 3 oxygen atoms and 3 hydrogen atoms – the brackets show that the 3 applies to O and H – this results in a total of 7 atoms