The melting point of magnesium oxide is higher than that of sodium chloride because magnesium and oxide ions have larger ionic radii, which increases the strength of the electrostatic forces.

Ionic compounds conduct electricity when molten because the thermal energy breaks covalent bonds within the lattice to release mobile electrons.

The strength of the electrostatic attraction in an ionic lattice increases as the charges on the ions increase and the ionic radii decrease.

In the solid state, ionic compounds conduct electricity because the ions can vibrate rapidly about their fixed positions in the giant lattice.