The molecular orbital theory of chemical valency VII. Molecular structure in terms of equivalent orbitals

Abstract

The twofold method of describing molecular structure, developed in earlier parts, is applied to a number of simple molecules in order to elucidate various properties of chemical interest. The molecules, methane, ammonia, water and hydrogen fluoride, are used to illustrate the properties of lone pairs of electrons and to show the significance of tetrahedral orbitals. Ethylene provides an example of the dual way of describing a double bond, either as a σ bond and a π bond, or as two equivalent bonds. The triple bonds of acetylene and nitrogen are similarly discussed, and an answer sought to the question why the lone pair of electrons on each atom of the nitrogen molecule differ from that of ammonia. Butadiene and benzene are considered as examples of conjugated systems in order to bring out the unlocalized character of the equivalent orbitals used to describe them, thus illustrating an essential difference between saturated and unsaturated molecules. There is also a short discussion of the boron hydrides and cyclopropane, each of which present some unusual features.