rock salt structure

Look at a grain of table salt under a magnifier and you will see tiny perfect cubes. That cubic shape is no accident — it is the outward sign of the most famous arrangement in all of inorganic chemistry, the rock salt structure, named after the mineral form of NaCl. It is the picture most people first meet when they learn what an ionic crystal is, and dozens of compounds copy it.

Picture the chloride ions arranged in cubic close packing — a face-centred cubic array. Now fill every one of the octahedral holes with a sodium ion. The result is strikingly symmetric: each sodium is surrounded by six chlorides at the corners of an octahedron, and each chloride is likewise surrounded by six sodiums. We say it is 6:6 coordinate. An equivalent way to see it: two interpenetrating face-centred cubic lattices, one of Na+ and one of Cl-, offset by half a cell edge. The unit cell is cubic with four NaCl formula units (Z = 4), the count you reach by sharing the corner, edge and face ions among neighbouring cells.

The rock salt structure is adopted by a huge range of 1:1 compounds whose ions are reasonably matched in size: most alkali halides (LiCl, NaCl, KBr and so on), the silver halides AgF/AgCl/AgBr, the oxides and sulfides of magnesium, calcium and the like (MgO, CaO, MnS), and many transition-metal monoxides (FeO, NiO). It is favoured when the radius ratio puts the cation in the range that prefers six-coordination. When the cation is too big (CsCl) or too small (ZnS), other structures win — so rock salt is one fork in a family of choices, not the only option.