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2.2 Ionic compounds and their formulae

The structure of ionic compounds as giant lattices held by strong electrostatic forces, the strengths and limitations of lattice models, working out formulae from the charges on ions, and the -ide and -ate endings.

AQA Edexcel OCR 2 min read

Giant ionic lattices

Key terms

  • Giant ionic lattice: a regular three-dimensional arrangement of oppositely charged ions, held together by strong electrostatic forces acting in all directions.

An ionic compound is a giant structure of ions. In sodium chloride, each \mathrm{Na^+} ion is surrounded by six \mathrm{Cl^-} ions, and each \mathrm{Cl^-} ion by six \mathrm{Na^+} ions. The pattern repeats in all directions.

The sodium chloride lattice as a ball and stick model and a space filling model, with a table of common positive and negative ions and their charges
The sodium chloride lattice as a ball and stick model and a space filling model, with a table of common positive and negative ions and their charges
Models of an ionic lattice
ModelUseful becauseLimitation
Ball and stickshows the 3D arrangement of the ions clearlythe ions are not joined by sticks, and the gaps are far too big
Space fillingshows the ions touching, with their relative sizeshard to see the ions inside the structure
2D diagramquick to drawdoes not show the 3D arrangement

All of these show only a tiny part of the lattice. A real crystal contains billions of ions.

Working out formulae

An ionic compound has no overall charge: the total positive charge equals the total negative charge.

  1. Write down the ions and their charges.
  2. Use enough of each ion to make the charges cancel.
  3. Put brackets around a compound ion if you need more than one of it.

Examples:

  • sodium oxide: \mathrm{Na^+} and \mathrm{O^{2-}} → two \mathrm{Na^+} for each \mathrm{O^{2-}} → Na2O
  • aluminium oxide: \mathrm{Al^{3+}} and \mathrm{O^{2-}} → 2 × (3+) = 6+ and 3 × (2−) = 6− → Al2O3
  • calcium hydroxide: \mathrm{Ca^{2+}} and \mathrm{OH^-} → Ca(OH)2
  • ammonium sulfate: \mathrm{NH_4^+} and \mathrm{SO_4^{2-}} → (NH4)2SO4

Naming: -ide and -ate

  • -ide: the compound contains just the two elements named, e.g. sodium chloride (NaCl), magnesium oxide (MgO). Hydroxide is an exception.
  • -ate: the negative ion also contains oxygen, e.g. copper sulfate (CuSO4), sodium nitrate (NaNO3), calcium carbonate (CaCO3).

Test yourself on this lesson

6 exam questions · 10 marks · with mark schemes and model answers

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