The melting point of an ionic compound depends on the strength of the electrostatic attractions. Two factors affect this:
Higher charge = stronger attraction = higher melting point
• NaCl (1+ and 1-): melts at 801°C
• MgO (2+ and 2-): melts at 2852°C
• MgO has higher charges, so MUCH stronger attractions!
Smaller ions = closer together = stronger attraction
• Smaller ions can get closer to each other
• This makes the electrostatic force stronger
• MgO has small ions AND high charges — extremely high MP!
"Explain why MgO has a higher melting point than NaCl."
Answer:
Mg²⁺ and O²⁻ have higher charges (2+/2-) than Na⁺ and Cl⁻ (1+/1-) [1]
This creates stronger electrostatic attractions between ions [1]
More energy is needed to overcome these stronger forces [1]
Quick Check: Why does solid sodium chloride not conduct electricity, but molten sodium chloride does?
In solid NaCl, ions are held in fixed positions in the lattice and cannot move — no movement of charge is possible. When molten, the lattice structure breaks down and ions are free to move and carry electrical charge.
This worked example covers Comparing Ionic Compounds within Ionic Compounds for GCSE Chemistry. Revise Ionic Compounds in Bonding & Structure for GCSE Chemistry with 21 exam-style questions and 21 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 5 of 12 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Practice questions for Ionic Compounds
What type of structure is found in all ionic compounds?
Explain why magnesium oxide conducts electricity when it is molten but not when it is solid.