# Interpreting formulae

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

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

• two sodium atoms
• one sulfur atom
• and four oxygen atoms

The numbers refer to the element before them.

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