rock-salt structure
/ ROK-sawlt STRUK-cher /
Picture a three-dimensional checkerboard where the black squares are one kind of ball and the white squares another, alternating perfectly in every direction — left-right, front-back, and up-down. Every red ball is surrounded on all six sides by green balls and vice versa. That endlessly alternating arrangement is the rock-salt structure, named for ordinary table salt.
The rock-salt structure consists of two kinds of ion arranged so that each one is surrounded by six of the other, sitting at the corners of an octahedron — a coordination number of six. Geometrically it is two interpenetrating face-centered cubic lattices, one of each ion, shifted by half a cube edge so they slot together. The pattern is held together not by shared electrons but by the electrical attraction between oppositely charged ions, the ionic bond, which pulls plus toward minus throughout the crystal.
The rock-salt structure matters because it is the textbook home of ionic crystals: besides sodium chloride, it is taken by many salts and oxides such as magnesium oxide and the lead and metal compounds used in detectors and ceramics. Their hardness, brittleness, high melting points, and tendency to cleave along flat planes all flow from those strong, undirected ionic attractions. One honest caveat: real salt is never a single flawless block — it has vacancies and impurities, and dissolving it in water tears the lattice apart into free-floating ions.
Tap a salt crystal with a blade and it splits along perfectly flat faces. That cleavage happens because the planes of alternating sodium and chloride ions part cleanly — a direct, visible consequence of the rock-salt arrangement.
Salt cleaves along flat planes because of its alternating ionic layers.
Despite the name, the rock-salt structure has a coordination number of six, not eight. The eight-fold alternating structure is a different one, the cesium-chloride structure, which looks superficially similar but is not the same.