Depending on the radii of the anions and cations, some ionic compounds have threefold, fourfold, sixfold, or eightfold coordination about the cation (three nearest-neighbor anions). This Demonstration allows the user to explore the ratio of the cation to anion radii in order to determine the ideal ratio. Because of electrostatic considerations, the minimum (and ideal) ratio occurs when the anions just make contact with one another and the cation just makes contact with each anion.