On Bonding in Ionic Crystals

Abstract

This article details the description of ionic bonding derived from the electron localization function (ELF) topology in the alkali halide family (i.e., ionic sizes, shapes, critical points, atomic coordinations, etc.). This approach establishes a relationship between ELF topological characteristics and basic ionic and crystal properties. We show how many deeply rooted concepts about ionic bonding can be derived from a (counterintuitive) analysis of electron pairing. In other words, we show how principles of covalent bonding are also applicable to ionic bonds. For example, ionic compounds are found to have a definite valence shell, which is responsible for chemical activity and crystalline strains under pressure. The valence may be the outermost shell of the softest ion, or the outermost shells of both ions, cation and anion, if their hardnesses are similar. The valence shows partial electron pairs (shell electron pairs, SEPs) whose spatial distribution around the valence shell can be related to the classical definition of polarization. Core SEPs (which might include the outermost shell of the hardest ion) are arranged in the crystal maximizing the distance between them. Hence, analysis of electron pairing in the whole crystal reveals that covalent rules for geometry, the valence shell electron pair repulsion theory, is also applicable to ionic crystals. Furthermore, it is found to describe all electron pairs in the crystal, so that it is indeed an electron pair repulsion theory. Finally, we use global properties from the ELF topology (i.e., ionic volumes) to understand Pauling’s electronegativity scale.