The chemical bond between two identical atoms, as in the molecules H2, C12, etc., may be considered as an example of a normal covalent bond, involving an electron pair shared by the two atoms. The wave function representing this bond cannot necessarily be closely approximated by a function of the Heitler-London type, with the electrons staying on different atoms, but may contain ionic terms, corresponding to the two electrons of the bond on the same atom, the term representing the configuration A+Aoccurring, of course, with the same coefficient as that for A -A +. The contribution of these ionic terms to the wave function for the normal state of the hydrogen molecule has been discussed by Slater.In the wave function representing the bond between unlike atoms A and B, the ionic terms A +B -and A -B + will occur with the same coefficient, of the order of magnitude of those for A :A and B: B, if the two atoms have the same degree of electronegativity. We propose to call such a function a normal covalent bond wave function, and the bond a normal covalent bond; and to make the postulate that the energies of normal covalent bonds are additive, that is, A:B = '/2(A :A + B: B), where the symbols A: B, etc., mean the energies of the normal covalent bonds. This postulate requires that the energy change for a reaction such as 1/2A2 + '/2B2 = AB involving only normal covalent substances with single bonds be zero. The energy of the normal covalent bond A: B would be given by the integral f t*H4dr, with V the normalized normal covalent wave function. Inasmuch as the energy integral for any wave function for a system must be equal to or greater than the energy of the lowest state of the system, the energy of the actual bond between A and B will either be equal to that for a normal covalent bond A B, or, in case the PROC. N. A. S.
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