The chemical bond between two identical atoms, as in the molecules H2, Cl2, 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+A- occurring, 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 [1]
