Writing formulae of ionic compounds

The formula of an ionic compound shows the relative numbers of the ions it contains. For example:

  • the formula for sodium chloride is NaCl - it shows that for every Na+ ion there is one Cl- ion
  • the formula for magnesium chloride is MgCl2 - it shows that for every Mg2+ ion there are two Cl- ions

Working out an ionic formula

The formula for an ionic compound must contain the same number of positive and negative charges. This is so that the charges are balanced and the compound is neutral overall. Here are some examples.

Example 1

Sodium chloride contains Na+ and Cl- ions:

  • this is one positive charge and one negative charge
  • the charges are balanced (add up to zero)
  • so the formula is NaCl

Example 2

Magnesium oxide contains Mg2+ and O2- ions:

  • this is two positive charges and two negative charges
  • the charges are balanced
  • so the formula is MgO

Example 3

Aluminium oxide contains Al3+ and O2- ions:

  • this is three positive charges and two negative charges
  • to balance we need, two Al3+ ions and three O2- ions
  • so the formula is Al2O3