Formulae

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 we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the '2' is written as a subscript, so 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.

A carbon atom joined to two oxygen atoms.Carbon dioxide units contain one carbon atom and two oxygen atoms

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

  • a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
  • a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H