Provides a quantitative indication of an elements reactivity and is based on the charge of the valence electrons and the ionic radius. It is determined by the equation: (-eV) = kn/r. 'k' is a proportionality factor converting ?gstroms into centimeters and expressing the force exerted by the valence electrons in electron volts which is equated to 14.399. 'n' is the valence. 'r' is the ionic radius in ?gstroms.