Crystalline solids are composed of atoms, ions, or molecules that are arranged in an extended regular repeating array.

Huge numbers of atoms are required to build a cubic millimeter of a solid. The figure shows a model of a plane of atoms in a crystal of platinum metal. To model the actual number of atoms in the 2 mm circle on the crystal, the magnified image would need to be about 8,000,000 atoms wide rather than 8 atoms wide as drawn. A model covered with a layer of 1/2 inch marbles would require a circle with a diameter of about 60 miles.