The structure of a crystal is described in terms of its smallest unit, which is call a "unit cell." All crystals are made up of unit cells that have been stacked to form a macrostructure. These cells form a repeating pattern of atoms, ions or molecules called "the lattice."
A crystal structure assumes a three-dimensional geometrical arrangement that depends on the relative sizes of the atoms in this structure. Elemental metal crystals have the simplest structures as all of their ions are the same size; salts have more more complicated arrangements as they comprise ions of different diameters.
The simplest salt crystals are those featuring an equal number of positive and negative ions. Rock salt, sodium chloride, for example, features one chloride ion for every sodium ion. However, chloride ions are much larger than sodium ions so chlorides occupy most of the bulk of the crystal. The most energetically favorable lattice configuration for sodium chloride sees the sodium ions arranged at 90 degrees to the chloride ions in all three directions. This structure is called the "faced centered cubic" or "fcc pattern." Further examples of salts with fcc arrangements include lithium chloride, potassium bromide, silver(I) chloride and magnesium oxide.
Further complications arise when the number of positive and negative ions in a salt differ. Calcium fluoride assumes a configuration called "fluorite"; featuring a repeating unit of eight calcium ions arranged in a cube. Inside this cube is another structure featuring alternating calcium and fluoride ions. Other crystal structures include antifluorite found in potassium oxide; wurtzite, as found in silver(I) iodide; and rutile, as found in magnesium fluoride.
The ions in all crystals are held in place by very strong electrostatic bonds. In salts, these bonds are called "ionic bonds"; in metals they are called "metallic bonds." Because of the strengths of these bonds, crystals tend to have very high melting and boiling points. Other physical properties of crystals vary considerably depending on the configuration of their ions. Some crystals are excellent conductors of heat or electricity. Others have excellent optical properties; some are very hard while others are very brittle.