Ionic formulae

Ionic compounds contain positive and negative ions.

The number of positive charges must equal the number of negative charges, so that the compound has no overall charge. When the number of charges on the positive ion is the same as the negative ion, they cancel each other out.

When the number of charges is different, a ‘swap and drop’ method can help.

  • write down the symbol and charge of each ion in the compound.
  • ignore the sign and if the number of the charges are the same, then write the formula in a 1:1 ratio.
  • if the number of the charges is different, swap the charges by crossing sides, drop the signs and write the formula.
  • watch out for molecular ions – you’ll need to use brackets if there is more than one of them.

Here are some worked examples of the ‘swap and drop’ method.

Swap and drop table