Formulae of ionic compounds

Ionic compounds are made up of oppositely charged ions joined together by ionic bonds. They almost always contain at least one metal element and at least one non-metal element.

The formula of an ionic compound can be deduced from the formulae of its ions. Remember that for the metal elements in groups 1, 2, and 3 the charge on the ion can be deduced by how many outer shell electrons there were in the neutral atom. Similarly, the non-metal elements in groups 6 (IUPAC group 16) and 7 (IUPAC group 17), the ionic charge can be deduced by working out how many electrons must be gained to fill the outer shell.

Positive ionsNegative ions
lithium Li+chloride Cl-
sodium Na+bromide Br-
potassium K+iodide I-
silver 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+

Deducing a formula

Although ionic compounds contain electrically charged ions, they are neutral overall. The formula for an ionic compound must give the same number of positive and negative charges. Here are some examples.

Example 1

Sodium chloride contains Na+ and Cl- ions:

  • this is one positive charge and one negative charge
  • the number of charges are already the same
  • so the formula is NaCl

Example 2

Magnesium oxide contains Mg2+ and O2- ions:

  • this is two positive charges and two negative charges
  • the number of charges are already the same
  • so the formula is MgO

Example 3

Aluminium oxide contains Al3+ and O2- ions:

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

Compound ions

Polyatomic ions are formed from groups of two or more atoms.

Positive ionsNegative ions
ammonium NH4+hydroxide OH-
nitrate NO3-
carbonate CO32-
sulfate SO42-

If the formula of an ionic compound needs more than one polyatomic ion, the formula of this ion is written inside brackets.

Example

Calcium hydroxide contains Ca2+ and OH- ions:

  • this is two positive charges and one negative charge
  • to make the number of charges the same, we need one Ca2+ ion and two OH- ions
  • so the formula is Ca(OH)2
Question

Deduce the formula for lead(II) sulfate.

Lead(II) nitrate contains Pb2+ and SO42- ions:

  • this is two positive charges and two negative charges
  • the number of charges are already the same, and we only need one SO42- ion
  • so the formula is PbSO4
Question

Deduce the formula for ammonium carbonate.

Ammonium carbonate contains NH4+ and CO32- ions:

  • this is one positive charge and two negative charges
  • to make the number of charges the same, we need two NH4+ ions and one CO32- ion
  • so the formula is (NH4)2CO3