An ionic bond arises when atoms form oppositely charged ions and those ions are held together by electrostatic attraction.

A common route is electron transfer between atoms with very different tendencies to hold valence electrons. Sodium and chlorine, for example, can form $\mathrm{Na^+}$ and $\mathrm{Cl^-}$. The attraction between isolated ions has the Coulomb form

$$U(r)\propto -\frac{|q_1q_2|}{r},$$

but real ions cannot collapse into one another: strong short-range repulsion appears when their electron clouds overlap. The equilibrium spacing is where attraction and repulsion balance.

Ionic solids are collective

In an ionic crystal, an ion is attracted to many neighbors of opposite charge and repelled by ions of like charge. It is therefore misleading to picture a solid as a collection of independent ion pairs. The crystal structure arranges charges so that the total electrostatic energy is low while local charge neutrality is maintained.

This explains several broad property trends. Ionic solids often have high melting temperatures because separating the ions costs substantial electrostatic energy. They are commonly electrical insulators as solids because the ions are locked in place and electrons are not freely mobile, although molten salts and ionic solutions can conduct by ion motion.

Ionic crystals are also often brittle. If slip shifts charged planes into an unfavorable alignment, like charges can be brought close together and strongly repel, encouraging fracture rather than extensive plastic slip.

Ionic bonding is a bonding mechanism; the detailed geometry of a particular ionic crystal is a separate structural question.