The melting point of magnesium oxide ($MgO$) is much higher than that of sodium fluoride ($NaF$) because $M{g}^{2+}$ and $O^{2-}$ have higher charges and smaller or similar ionic radii, resulting in stronger electrostatic attractions in the lattice.

In the molten state, ionic compounds conduct electricity because the electrostatic lattice is broken, allowing delocalised valence electrons to move freely through the liquid.

Lithium iodide ($LiI$) has significant covalent character because the large iodide anion has high polarising power, which distorts the electron cloud of the small lithium cation.

Calcium chloride ($CaC{l}_2$) has a greater lattice dissociation enthalpy than calcium oxide ($CaO$) because there are twice as many anions per formula unit to form electrostatic bonds.