Bond Approach to Chemical Bonding

Application of valence bond theory to more complex molecules usually proceeds by writing as many plausible Lewis structures as possible, and assuming that the actual molecule is a hybrid of these canonical forms. The molecular wave function is then given by the summation of the products of the individual wave functions and weighting factors proportional to the contribution of the canonical forms to the overall structure. As a simple example, the hydrogen chloride molecule would be considered a hybrid of the limiting canonical forms H-Cl, H Cr, and H Cr. The mathematical treatment of molecular structure in terms of valence bond theory can be expanded to encompass more complex molecules. However, as the number of atoms and electrons increases the mathematical expression of the structure, the wave function, rapidly becomes complex. For this reason, qualitative concepts which arise from the valence bond treatment of simple molecules have been applied to larger molecules. The key ideas which are used to adapt the concepts of valence bond theory to complex molecules are hybridization and resonance. In this qualitative form, valence bond theory describes molecules in terms of orbitals which are mainly localized between two atoms. The shapes of these orbitals are assumed to be similar to orbitals described by more quantitative treatment of simpler molecules. 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to a question of specific concern to organic chemistry, the tetrahedral orientation of the bonds to carbon. Pauling reasoned that since covalent bonds require mutual overlap of orbitals, stronger bonds result from better overlap. Orbitals that possess directional properties such as p orbitals should therefore be more effective than spherically symmetric s orbitals. 

SECTION 1.1. VALENCE BOND APPROACH TO CHEMICAL BONDING 

Covalent bond formation between two atoms involving overlap of a p orbital of one atom with an 5 or p orbital of another is illustrated in Fig. 1.1. The electron distribution that results is cylindrically symmetric with respect to the internuclear axis and defines a cr bond. 

The descriptive valence bond approach to the bonding in ethylene and acetylene and their congeners is analogous to that of methane. In ethylene (Fig. 1.3), each carbon bears three ligands and sp hybridization, wherein three sp orbitals are 

---