Explain why magnesium oxide has a significantly higher melting point than sodium chloride, despite both compounds adopting the same crystal structure.

IB DP Chemistry Higher 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 crystallise in a rock‑salt lattice, so the crystal geometry is the same.

Mg²⁺ and O²⁻ carry a charge of ±2, whereas Na⁺ and Cl⁻ carry ±1.

The Mg²⁺ and O²⁻ ions are smaller than Na⁺ and Cl⁻ because the higher nuclear charge pulls the electrons closer and they have fewer electron shells.

The larger charge and the shorter inter‑ionic distance give MgO a much greater lattice enthalpy; therefore more energy is required to break the lattice and melt MgO than NaCl.

Examiner tips

  • State the common crystal structure first to show you understand the premise. Mention both charge and size to cover all key factors. Explain how these factors increase lattice enthalpy and thus the melting point.
  • Common mistakes: 1. Confusing ionic size with atomic size. 2. Forgetting that both charge and distance affect lattice enthalpy. 3. Saying “MgO is harder” without linking to lattice enthalpy.

Mark scheme (4 marks)

  1. Both MgO and NaCl adopt a face-centred cubic (rock salt) lattice structure, so the difference in melting point is not due to crystal geometry.
  2. Mg²⁺ and O²⁻ carry a greater ionic charge (2+ and 2−) compared to Na⁺ and Cl⁻ (1+ and 1−).
  3. Mg²⁺ and O²⁻ have smaller ionic radii than Na⁺ and Cl⁻ respectively, because of greater nuclear charge and fewer electron shells.
  4. Higher charge and shorter interionic distance produce a much greater electrostatic attraction (lattice enthalpy) in MgO, so more energy is required to overcome the lattice and melt the compound.

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