Why do ionic compounds often form crystal structures?

Ionic compounds form crystal structures because this arrangement allows for the most stable, low-energy state for the substance.12

In an ionic solid, atoms transfer electrons to become oppositely charged ions—positively charged cations and negatively charged anions. Because opposite charges attract and like charges repel, these ions naturally orient themselves in a repeating, periodic crystalline array where every cation is surrounded by anions and vice versa.31

This specific packing maximizes the electrostatic attraction between the ions, releasing a significant amount of energy known as lattice energy. The formation of this ordered structure is driven by the need to minimize the overall energy of the system; the energy released during the creation of the crystal lattice is often great enough to compensate for the energy required to form the ions in the first place. Consequently, the ions pack together in the pattern that results in the greatest possible lattice energy.12