Each sphere has a coordination number of twelve. The simplest arrangement of spheres, which will reproduce the entire crystal structure when repeated is called a unit cell . The unit cells for the packing arrangements are shown below. In most ionic compounds, the anions are much larger than the cations, and it is the anions which form the crystal array. The smaller cations reside in the holes between the anions.
Ions try to surround themselves with as many ions of opposite charge as closely as possible.
Usually in the packing arrangement, the cation is just large enough to allow te anions to surround it without touching one another. The cation to anion ratio must reflect the stoichiometry of the compound. For MgCl 2 the lattice must be an array of chloride anions with only half that number of magnesium ion. The packing arrangement adopted by an ionic compound is determined by the comparative sizes of the ions. Consider a lattice in which the anions assume a cubic array.