Magnesium oxide is an ionic compound with a giant lattice structure. Explain why magnesium oxide has a high melting point and does not conduct electricity when solid, but does conduct electricity when molten.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.6 Ionic compounds · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Magnesium oxide consists of oppositely charged ions held together by strong electrostatic forces.
The large lattice energy means a large amount of energy is required to overcome these forces, giving a high melting point.
In the solid state the ions are fixed in the lattice and cannot move, so it cannot conduct electricity.
When molten the ions are free to move and can carry charge, so it conducts electricity.

Examiner tips

  • Use the term "lattice energy" or "electrostatic forces" to show understanding of the high melting point.
  • Explain that in the solid the ions are fixed, in the melt they are mobile – this directly answers the conductivity part.

Common mistakes

  • Confusing the melting point with the boiling point.
  • Saying the solid conducts electricity – the ions are not mobile in the lattice.
  • Using vague terms like "strong bonds" without linking to lattice energy.

Mark scheme (4 marks)

  1. Magnesium oxide consists of oppositely charged ions held together by strong electrostatic forces
  2. A large amount of energy is needed to overcome these strong electrostatic forces, so the melting point is high
  3. In the solid state the ions are fixed in position in the lattice and cannot move, so it cannot conduct electricity
  4. When molten, the ions are free to move and can carry charge / conduct electricity

Key terms in this question

ionic compound · lattice · ion

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