Ionic crystal structures adopt a form which compromises between three factors: relative sizes of cations and anions, lowest possible potential energy, and charge balance. Structures fall most often into either binary or ternary compounds. Binary compounds consist of an anion and a cation. Ternary compounds consist of three species. A few examples illustrate common ionic compounds. A complete treatment requires a much longer text. Regardless of the type of compound, the basic structure of ionic compounds derives from lattice structures already presented. How Atoms Pack. In most cases, anions occupy the lattice points, because anions are larger. Cations occupy voids in the structure. Solid Voids.