Why is Salt Square?

The salt in a shaker is typically made of very small crystals, whose shape you can’t see. But if you look at bigger crystals you can see that they look like tiny squares or cubes. You can make these bigger crystals by dissolving ordinary table salt in water and letting the water evaporate – leaving behind square salt crystals. But why are they square?

Sodium chloride, NaCl is the chemical name for the salt we eat. It is made up of equal amounts of sodium ions, which are positively charged, and chloride (chlorine) ions, which are negatively charged. A bit like how poles of a magnet that are the same repel each other, ions that have the same charge repel each other. When they pack together in a solid they need to form a shape which minimises the interaction between the ions that are charged the same (so sodium with sodium, chloride with chloride) and maximises the interaction between ions which are oppositely charged (sodium and chloride). It just so happens that for these ions – that shape is a cube.

This optimal shape doesn’t change, no matter how big the crystal is, it will still have the same essential shape. So tiny crystals are cubes, as are very large ones.