Question

How do ionic and molecular compounds compare in terms of melting points?

Answer 1

Ionic compounds typically have much higher melting points than molecular compounds.
Ionic compounds are held together by electric attractions between positive and negative ions. These are ionic bonds, and these bonds keep the ions together in a regular $3d$ pattern (a crystal lattice).
To melt an ionic substance, we have to disrupt these bonds. This requires a lot of energy.
Molecules are held together by covalent bonds, which are strong. But we do not need to break these covalent bonds when melting a molecular substance. In molecular solids, each molecule is neutral and it is attracted to its neighboring neutral molecule by an intermolecular force ($3$ types: dispersion forces, dipole-dipole force, and hydrogen bonding).

To melt a molecular substance, we need to break these weak intermolecular forces between neutral molecules, which is why ionic compounds generally have much higher melting points than molecular compounds.

For example, sodium chloride boils at ${1413}^{\circ }C$. Acetic acid $\left(C{H}_{3}COOH\right)$ is a molecular compound with almost the same molecular mass as $NaCl$, but it boils at ${118}^{\circ }C$.