What holds a salt crystal together?

A grain of table salt is a huge organized gathering of charged particles. Sodium ions carry a positive charge and chloride ions carry a negative charge. Their electric attractions help hold them in a repeating three-dimensional arrangement called a crystal lattice. There is no tiny package containing just one sodium and one chloride hidden inside each grain: the pattern continues through the crystal.

An ordinary sodium atom becomes a sodium ion when it loses an electron; a chlorine atom becomes a chloride ion when it gains one. The atoms' nuclei remain unchanged, so the elements retain their identities. The formula NaCl records the one-to-one ratio of the two kinds of ion. It does not mean that a crystal is made from isolated NaCl molecules. In the actual three-dimensional structure, each ion has several nearest neighbors of the opposite charge.

Attraction is only part of the explanation. Like charges repel, and bringing electron clouds too close also raises the energy. A stable arrangement balances these effects at particular separations. Separating all the ions requires energy. This helps explain why solid salt has a high melting temperature compared with many everyday molecular substances. The drawing shows a flat slice through a much larger structure; real ions are not colored marbles joined by sticks.

The arrangement also explains a surprising weakness. When layers in an ionic crystal are displaced, like charges can become neighbors across a plane. Their repulsion helps the crystal split instead of bending smoothly like a metal. A material can be hard to scratch yet brittle when struck. Hardness and toughness are different properties.

In solid salt, the ions cannot travel freely through the material, so they do not carry an ordinary electric current across it. When salt melts, or when enough dissolves in water, ions can move and transport charge. Water also changes the surroundings of the ions and can stabilize them separately. The same particles therefore behave differently when their arrangement and freedom to move change. A salt grain's shape, brittleness and electrical behavior all begin with its invisible organization.