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

What accounts for the higher melting point of magnesium oxide compared to sodium chloride?

1. MgO has a smaller ionic size than NaCl

2. MgO contains covalent bonds

3. MgO ions have a higher charge than the ions in NaCl

4. NaCl has a larger ion size than MgO

The correct answer is: MgO ions have a higher charge than the ions in NaCl

The higher melting point of magnesium oxide compared to sodium chloride can be attributed to the charges of the ions involved. Magnesium oxide (MgO) consists of magnesium ions (Mg²⁺) and oxide ions (O²⁻), while sodium chloride (NaCl) is made up of sodium ions (Na⁺) and chloride ions (Cl⁻). The key factor is that the ions in magnesium oxide carry a higher charge (±2) compared to the ions in sodium chloride (±1). This higher charge in MgO results in stronger electrostatic forces between the positively charged magnesium ions and the negatively charged oxide ions. Stronger ionic bonds require more energy to break, which corresponds to a higher melting point. The presence of these stronger charges means that as temperature increases, more energy is needed to overcome the ionic bonds in magnesium oxide than in sodium chloride, where the bonds are weaker due to the lower ionic charges. This fundamental difference in the ionic charge and the resultant strength of the ionic bonds is what explains the significant difference in melting points between these two compounds.