  We present an Atom-Pair Bond (APB) theory for the energy of a metallic bond
based on the ideas of covalent bonding proposed by Pauling. An expression is
derived which accurately predicts the signs of the heats of formation of binary
alloys. It also explains the characteristics of the Rajasekharan-Girgis lines
and their ability to predict accurately concomitant and mutually exclusive
structure types in phase diagrams. Evidence is provided to show that the charge
transfer on the atom-pair bond which is central to the present theory decides
the experimentally observed volume changes on alloying.
