where is the permittivity of a vacuum Z1 and Z2 are the valences of
the two ion types, and e is the electronic charge
4 Sometimes bonding energies are expressed per atom or per ion. Under these circumstances
the electron volt (eV) is a conveniently small unit of energy. It is, by definition, the
energy imparted to an electron as it falls through an electric potential of one volt. The
joule equivalent of the electron volt is as follows: 1.602 1019 J 1 eV.