International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Why does magnesium oxide have a higher melting point than sodium chloride?

1. Because magnesium forms a gaseous compound

2. Due to stronger electrostatic forces between ions

3. Because sodium is less reactive than magnesium

4. As sodium ions are larger than magnesium ions

The correct answer is: Due to stronger electrostatic forces between ions

Magnesium oxide has a higher melting point than sodium chloride primarily due to the stronger electrostatic forces between the ions in magnesium oxide. The melting point of an ionic compound is determined by the strength of the ionic bonds that hold the lattice structure together. In magnesium oxide, the ions present are magnesium ions (Mg²⁺) and oxide ions (O²⁻). The charges on these ions are higher (2+ for magnesium and 2- for oxide) compared to the sodium ions (Na⁺) and chloride ions (Cl⁻) in sodium chloride, which only have charges of 1+. The greater ionic charges in magnesium oxide lead to a stronger attraction between the positive and negative ions, resulting in a more stable and tightly held lattice structure. Consequently, more energy is required to break these strong electrostatic forces when transitioning from a solid to a liquid state, which translates to a higher melting point for magnesium oxide. In contrast, sodium chloride, while also having ionic bonds, experiences weaker attractions due to its lower ionic charges, resulting in a lower melting point.