Formula of Ionic CompoundsFor ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero.
If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral.
The subscript of the cation is numerically equal to the charge on the anion, and the subscript on the anion is numerically equal to the charge on the cation.
For example, in aluminium oxide (Al3O2), the cation is Al3+ and the anion is O2-. The following diagram helps us to determine the subscripts for the compound formed on the cation and the anion: