Magnesium oxide is an ionic compound. Explain why magnesium oxide has a high melting point.

Cambridge International IGCSE Chemistry (0620) — 2.4 Ions and ionic bonds · Explain · 4 marks · View as Markdown

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

Magnesium oxide is used to line the inside of furnaces because it can withstand very high temperatures without melting.

Model answer (4 marks)

Magnesium oxide is an ionic compound made of Mg²⁺ and O²⁻ ions.
The ions are held together by strong electrostatic forces of attraction.
These forces are very strong because the charges are high and the ions are small.
A large amount of energy is required to overcome the forces and break the lattice, giving MgO a high melting point.

Examiner tips

Use the word ‘ionic compound’ and name the ions. Mention ‘electrostatic forces’ and ‘high charge’. Explain that a large amount of energy is needed to break the lattice. Show the lattice structure is a giant array of ions.

Common mistakes

Saying ‘covalent bonds’ instead of ionic. Forgetting to mention the high charge or small size. Using vague terms like ‘strong bonds’ without specifying electrostatic attraction.

Mark scheme (4 marks)

Magnesium oxide consists of oppositely charged ions (Mg²⁺ and O²⁻)
There are strong electrostatic forces of attraction between the oppositely charged ions
A large amount of energy is needed to overcome these forces / break these bonds
The ions are held in a giant ionic lattice / structure

Key terms in this question

ion