The factor is called the Madelung constant and is of order one. It is tabulated for many crystal structures. We assume that each ion is a hard sphere with radius and . Then we determine these values by calculating many different crystal structures and making sure that in every case we predict the correct stable structure. The structure of ionic materials can be predicted by assuming the ions are hard spheres with Pauling radii, which are tabulated in many periodic tables.
Negative ions are large, positive ions are small.