Vacancy, (from the Latin vacans — standing empty, free), the absence of an atom or an ion at a point of a crystal lattice. Vacancies are in thermodynamic equilibrium with the lattice; they appear and disappear as a result of the thermal motion of the atoms. Vacancies move about the crystal at random, changing places with neighbouring atoms. The movement of vacancies is the chief cause of the diffusion of atoms in a crystal. To each temperature there corresponds a definite equilibrium concentration of vacancies. The number of vacancies in crystals of metals near the melting temperature — 1 — 2% of the number of atoms. At room temperature in Al there is one vacancy to every 1012 atoms, and in Ag and Cu fewer than one vacancy. Despite their low concentration, vacancies appreciably affect the physical properties of a crystal: they lower its density, increase its electrical conductivity and so on (see Defects).