Several factors influence the coordination number of an atom, but the most important of these is its size; larger atoms have larger coordination numbers.  The coordination number depends on the surface area of the atom, and so is proportional to r.  If V is the charge on the atomic core (which is the same as the valence of the atom when all the electrons in the valence shell are bonding), and N is the corresponding average coordination number, V/N is proportional to the electric field at the surface of the core, represented by S in Eq. 5:
:                           (Eq. 5)
Not surprisingly, S gives the same ordering of the main group elements as the electronegativity, though it differs in its numerical value from traditional electronegativity scales.  Because it is defined in structural terms, S is the preferred measure of electronegativity in the bond valence model,