‘Effective’ Ionic Charge in Crystals

Abstract

A RECENT communication by Mooser and Pearson¹ has directed attention to the confusion which can result from two distinct usages of the term ‘ionic’. As an example they cite discussions of the bonding in zinc blende, where the tetrahedral bonds may be said to arise from resonance involving (i) Zn²⁻S²⁺ and (ii) Zn²⁺S²⁻. The first is the extreme covalent structure which gives each atom four valence electrons ; the second is conventionally called the fully ionic structure since both atoms have closed-shell configurations. As Mooser and Pearson point out, both (i) and (ii) are physically ionic, and the convention which describes ‘neutral bonded’ Zn⁰S⁰ as 50 per cent ionic, because it involves equal resonance contributions from (i) and (ii), can be most misleading.