Critical Survey of the method of ionic-homopolar resonance

Abstract

Some of the theoretical difficulties in the method of ionic-covalent resonance are discussed. They include our ignorance of the fundamental energies, and also of the orbitals used. If these are hybrids, as usually occurs, considerable care is required in using the conventional theory because: (1) the atomic radius, and (2) the effective electronegativity of a hybrid depend on the degree of mixing of the basic atomic orbitals. In polyatomic molecules the lone-pair electrons play a substantial part in determining the total dipole moment, and there are further difficulties associated with (1) independence, (2) partial delocalization, and (3) possible 'bent' character of the bonds. As a result many bonds (e.g. CH, NH, OH) are intrinsically much less ionic than is usually supposed. In addition the dipole moment of a molecule does not depend in any simple way upon the formal charges associated with the atoms; nor does it provide a completely satisfactory basis for assigning individual bond moments. The paper concludes with an outline of some possible improvements which merit further research.