Unoccupied sites within a crystal that are usually occupied by an atom. A vacancy may move when a neighboring atom or Atom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. moves to occupy the vacant site (the vacancy shift in the opposite direction to the site that was the source of the atom). The surrounding bonded Mutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal. ensures that the neighboring atoms do not collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they can better form bonds with atoms in the other directions.