~~The study of the structure of molcculcs was originally earned on by chemists using methods of investigation that were essentially chcmical in nature, relating to the chemical composition of a substancc, the existence of isomers, the nature of the chemical reactions in which a substance takes part and so on. From the consideration of facts of this kind Frankland, Kekulc, Couper and Butlcrov were led a century ago to formulate the theory of valcncc and to write the first structural formulas for molcculcs, van't Hoff and lc Bel were led to bring classical organic stereochemistry into its final form by their brilliant postulate of the tctrahcdral orientation of the four-valence bonds of the carbon atom, and Werner was led to his development of the theory of the stereochemistry of complex inorganic substanccs.
In the nineteenth century the valcncc bond was represented by a line drawn between the symbols of two chcmical elements, which expressed in a concisc way many chemical facts, but which had only qualitative significance with regard to molecular structure. The nature of the bond was completely unknown. After the discovery of the electron, numerous attempts were made to develop an electronic theory of the chcmical bond. These culminated in the work of Lewis, who in his 1916 paper, which forms the basis of the modern clcctronic theory of valcncc, discussed not only the formation of ions by the completion of stable shells of electrons but also the formation of a chcmical bond, now called the covalent bond, by the sharing of the two electrons between two atoms. Lewis further emphasized the importance of the phenomena of the pairing of unshared as well as of shared electrons and of the stability of the group of eight electrons (shared or unshared) about the lighter atoms. These ideas were then further developed by many investigators; the work of Langmuir was especially valuable in showing the great extent to which the facts of chemistry could be coordinated and clarified by the application of the new ideas. Many of the features of the detailed theory that is discusscd were suggested in the papers of Langmuir and others written in the decade
following 1916, or in the book Valence and Structure of Atoms and Molecules written by Lewis in 1923.
All of these early studies, however, contained, in addition to suggestion that have since been incorporated into the present theory, many others that have been discarded.
