text: The bond valence model is an extension of the electron counting rules and its strength lies in its simplicity and robustness.  Unlike most models of chemical bonding, it does not require a prior knowledge of the atomic positions and so can be used to construct chemically plausible structures given only the composition.  The empirical parameters of the model are tabulated and are readily transferable between bonds of the same type.  The concepts used are familiar to chemists and provide ready insight into the chemical restraints acting on the structure.  The bond valence model uses mostly classical physics, and with little more than a pocket calculator, it gives quantitative predictions of bond lengths and places limits on what structures can be formed.
  However, like all models, the bond valence model has its limitations.  It is restricted to compounds with localized bonds; it does not, in general, apply to metals or aromatic compounds where the electrons are delocalized.  It cannot in principle predict electron density distributions or energies since these require the solution of the Schoedinger equation using the long-range Coulomb potential which is incompatible with the concept of a localized bond.