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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

The subscript numbers refer to the element before them.

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