Ionic structure building rules

This rule is the expression of the fundamental principle of ionic structure building, namely, that in these structures the ions may be treated to a first approximation as rigid spheres in contact, each of characteristic and constant radii, and that the way in which these spherical ions are packed together is determined by their relative sizes. 

The early structure-building rules, based on ionic bonding guidelines, are still of value, however, in understanding some of the patterns underlying the multiplicity of crystal structures that are known. A simple assumption is that crystals are built of hard spherical ions linked by nondirectional ionic bonding. In terms of this idea, a structure is made up of... 

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