Structure of Ionic Solids

Ions don't bond in pairs the way covalent atoms do. A sodium ion has no particular chlorine partner; it just pulls on every chloride nearby and pushes away every other sodium. The arrangement that wins is the one that packs the most opposite-charge neighbors as close as possible while keeping same-charge ions as far apart as geometry allows. For NaCl, that arrangement is a repeating cubic lattice: each Na+ is surrounded by six Cl-, and each Cl- by six Na+, alternating in all three directions. This is why the ratio of attractive to repulsive interactions stays favorable throughout the whole crystal, not just between one pair.

That rigid, alternating order also explains how the crystal behaves when you hit it. A sharp blow shifts one plane of ions relative to the next. In a metal, atoms can slide past each other and the shape just deforms. In an ionic crystal, shifting a plane suddenly lines up same-charge ions face to face. The sudden repulsion splits the crystal cleanly along that plane: cleavage, not denting. The same fixed positions that make the lattice strong also make it brittle.

Those fixed positions are also why solid NaCl won't conduct electricity: the ions can't move to carry charge, they're locked in place by the surrounding lattice. Melt the solid or dissolve it in water and the lattice breaks apart, freeing the ions to migrate and carry current. This links back to bonding generally: ionic bonding is really electrostatic attraction throughout a 3-D array, not a localized bond between two atoms, which is also why swapping in a larger or smaller ion changes the whole lattice's spacing and stability, not just one connection.