# 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

## 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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