Interpreting formulae

A formula gives information about the type and the number of each atom present in a compound.

The formula for sodium sulfate is Na2SO4. It tells you that sodium sulfate contains:

  • two sodium atoms
  • one sulfur atom
  • and four oxygen atoms
Large illustration of chemical formula Na2SO4, labelled to show that there are two sodium (Na) atoms, one sulfur (S) atom, and four oxygen (O) atoms

The subscript numbers refer to the element before them.

curriculum-key-fact
If there is just one atom in the formula a 1 is not written.

Sometimes a formula includes brackets. The atoms inside the brackets are multiplied by the number outside the brackets.

Aluminium sulfate’s formula is Al2(SO4)3. It contains:

  • 2 atoms of aluminium
  • 3 atoms of sulfur
  • 12 atoms of oxygen