There occur as many Madelung constants formula_3 in a crystal structure as ions occupy different lattice sites. For example, for the ionic crystal NaCl, there arise two Madelung constants – one for Na and another for Cl. Since both ions, however, occupy lattice sites of the same symmetry they both are of the same magnitude and differ only by sign. The electrical charge of the Na+ and Cl− ion are assumed to be onefold positive and negative, respectively, formula_9 and formula_10. The nearest neighbour distance amounts to half the lattice constant of the cubic unit cell formula_11 and the Madelung constants become