Why Do Ionic Compounds Have High Melting Points?

Ionic compounds have high melting points because strong electrostatic attractions act between oppositely charged ions throughout a giant three-dimensional lattice. Considerable thermal energy is required to overcome enough of these attractions for the ions to move past one another.

This explanation is required at both IB Chemistry SL and HL in Structure 2.1, the ionic model.

The Reasoning

When electrons are transferred from metal atoms to non-metal atoms, positive cations and negative anions form. These ions arrange themselves in a repeating ionic lattice, with each ion surrounded by oppositely charged ions.

The attraction is not limited to individual ion pairs. It acts in all directions throughout the lattice, producing strong ionic bonding across the solid. When heated, the ions gain kinetic energy, but melting occurs only after sufficient energy has been supplied to disrupt the ordered lattice and allow the ions to move.

The strength of attraction can be represented qualitatively using Coulomb's law:

$$E \propto \frac{|q_+q_-|}{r}$$

Here, attraction increases with greater ionic charges and decreases as the distance, $r$, between ion centres increases.

For example, ions with charges of $+2$ and $-2$ generally attract more strongly than ions with charges of $+1$ and $-1$, provided their radii are comparable.

A common misconception is that melting breaks the ions themselves. It does not: melting overcomes electrostatic attractions and disrupts the lattice, while the ions remain chemically intact.

Exam Technique

For an explain question, identify the giant ionic lattice, describe strong electrostatic attractions between oppositely charged ions, and conclude that substantial thermal energy is required. Avoid writing only “ionic bonds are strong,” because this does not explain the lattice-wide mechanism.