Although the bond valence model is mostly used for validating newly determined structures, it is capable of predicting many of the properties of those chemical structures that can be described by localized bonds 
In the bond valence model, the valence of an atom, V, is defined as the number of electrons the atom uses for bonding.  This is equal to the number of electrons in its valence shell if all the valence shell electrons are used for bonding.  If they are not, the remainder will form non-bonding electron pairs, usually known as lone pairs.
The valence of a bond, S, is defined as the number of electron pairs forming the bond.  In general this is not an integral number.  Since each of the terminal atoms contributes equal numbers of electrons to the bond, the bond valence is also equal to the number of valence electrons that each atom contributes.  Further, since within each atom, the negatively charged valence shell is linked to the positively charged core by an electrostatic flux that is equal to the charge on the valence shell, it follows that the bond valence is also equal to the electrostatic flux that links the core to the electrons forming the bond.  The bond valence is thus equal to three different quantities: the number of electrons each atom contributes to the bond, the number of electron pairs that form the bond, and the electrostatic flux linking each core to the bonding electron pair.