text: The bond valence model can be reduced to the traditional ionic model if certain conditions are satisfied.  These conditions require that atoms be divided into cations and anions in such a way that (a) the electronegativity of every anion is equal to, or greater than, the electronegativity of any of the cations, (b) that the structure is electroneutral when the ions carry charges equal to their valence, and (c) that all the bonds have a cation at one end and an anion at the other.  If these conditions are satisfied, as they are in many ionic and covalent compounds, the electrons forming a bond can all be formally assigned to the anion.  The anion thus acquires a formal negative charge and the cation a formal positive charge, which is the picture on which the ionic model is based.  The electrostatic flux that links the cation core to its bonding electrons now links the cation core to the anion.  In this picture, a cation and anion are bonded to each other if they are linked by electrostatic flux, with the flux being equal to the valence of the bond.  In a representative set of compounds Preiser et al. have confirmed that the electrostatic flux is the same as the bond valence determined from the bond lengths using Eq. 2.
  The association of the cation bonding electrons with the anion in the ionic model is purely formal.  There is no change in physical locations of any electrons, and there is no change in the bond valence.  The terms "anion" and "cation" in the bond valence model are defined in terms of the bond topology, not the chemical properties of the atoms.  This extends the scope of the ionic model well beyond compounds in which the bonding would normally be considered as "ionic".  For example, methane, CH, obeys the conditions for the ionic model with carbon as the cation and hydrogen as the anion (or vice versa, since carbon and hydrogen have the same electronegativity).
  For compounds that contain cation-cation or anion-anion bonds it is usually possible to transform these homoionic bonds into cation-anion bonds either by treating the atoms linked by the homoionic bond as a single complex cation (e.g., Hg), or by treating the bonding electrons in the homoionic bond as a pseudo-anion to transform a cation-cation bond into two cation - pseudo-anion bonds, e.g., Hg-e-Hg.