Explain why magnesium oxide (MgO) has a higher melting point than sodium chloride (NaCl).

IB DP Chemistry Standard Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both MgO and NaCl are ionic compounds with lattices of oppositely charged ions.
1. MgO contains Mg²⁺ and O²⁻ ions, while NaCl contains Na⁺ and Cl⁻ ions.
2. The ions in MgO carry a charge of ±2, whereas those in NaCl carry a charge of ±1.
3. The larger charges in MgO give stronger electrostatic attractions between the ions.
4. Consequently, more energy is required to break these forces, so MgO has a higher melting point than NaCl.

Examiner tips

Use the command word ‘explain’ – give a clear cause–effect chain.
Mention the ionic nature, the charge difference, the resulting stronger attraction, and the higher energy needed to melt.
Keep each point concise and use correct terminology (ionic lattice, electrostatic attraction).

Common mistakes

Confusing the charge of the ions (e.g., writing Mg⁺ instead of Mg²⁺).
Failing to mention that both compounds are ionic lattices before comparing charges.
Over‑generalising that all ionic compounds melt at high temperatures without linking to charge magnitude.

Mark scheme (4 marks)

Both compounds are ionic, consisting of a lattice of oppositely charged ions held together by electrostatic attraction.
The ions in MgO carry charges of 2+ and 2−, whereas the ions in NaCl carry charges of only 1+ and 1−.
The greater charge on the ions in MgO results in a stronger electrostatic attraction between the ions.
More energy is therefore required to overcome these stronger electrostatic forces in MgO, so its melting point is higher.

Key terms in this question

melting point