The modern theory of valency

Abstract

NEARLY 100 years ago, in 1852, it was stated for the first time, by E. Frankland, that atoms have a definite combining power, which determines the formulas of compounds. Then in 1858 Archibald S. Couper introduced the idea of the valency bond and drew the first structural formulas, and August Kekulé showed that carbon is quadrivalent. This simple valency-bond theory permitted great progress to be made in structural organic chemistry, but structural inorganic chemistry remained largely undeveloped until the present century. With the discovery of the electron and the elucidation of the electronic structure of atoms, it became possible for Gilbert Newton Lewis in 1916 to identify the covalent bond with a pair of electrons shared by two atoms and counting as part of the outer shell of each, and thus to lay the basis for the development of the modern theory of valency, to which Sidgwick, Robinson, and many other chemists have contributed. The modern theory of valency is not simple-it is not possible to assign in an unambiguous way definite valencies to the various atoms in a molecule or crystal. It is instead necessary to dissociate the concept of valency into several new concepts-ionic valency, covalency, metallic valency, oxidation number-that are capable of more precise treatment; and even these more precise concepts in general involve an approximation, the complete description of the bonds between the atoms in a molecule or crystal being given only by a detailed discussion of its electronic structure. Nevertheless, these concepts, of ionic valency, covalency, etc., have been found to be so useful as to justify our considering them as constituting the modern theory of valency.