Names and formulae of ionic compounds

To deduce the formulae of ionic compounds, the formulae of their ions can be used.

The table shows the names and formulae of some common ions.

Cations are positively charged ions, and anions are negatively charged ions.
CationsAnions
Lithium, Li+Chloride, Cl-
Sodium, Na+Bromide, Br-
Potassium, K+Iodide, I-
Silver(I), Ag+Oxide, O2-
Barium, Ba2+
Calcium, Ca2+
Copper(II), Cu2+
Iron(II), Fe2+
Lead(II), Pb2+
Magnesium, Mg2+
Zinc, Zn2+
Aluminium, Al3+
Iron(III), Fe3+

Working out a formula

The formula for an ionic compound must contain the same number of positive and negative charges so that the charges are balanced and it 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
  • 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