What are the structures of ionic solids?
Regardless of the nature of a compound, a crystal can always be described as an array of unit cells. Any molecule, not just metals, will attempt to pack in the most compact structure; nature seeks the most efficient packing. Intermolecular interactions, however, may be more important than packing efficiency. Intermolecular hydrogen bonding, for example, is often quite important. To simplify this topic, let’s restrict our attention to ionic solids comprising monatomic ions (e.g., NaCl, KI, CaO, TiO2). The crystal structures of ionic solids composed solely of monatomic ions is governed by two relatively simple factors. • The most efficient packing of the ions is desired. • Because like charges repel, it is necessary to prevent two anions or two cations from coming in contact with each other. Just as covalent bonding and non-spherical atoms created exceptions to the closest-packing rule for metals, these same issues can be important in the packing of ionic solids. Consequently there are m
