The development of various theories of chemical bonding is closely linked to the development of theories and experiments on the shape of the single atom. The first concrete theories were put forward after the discovery of the electron by Joseph John Thomson in 1897. In his model of the atom, Thomson imagined that chemical bonds were based on electrostatic forces created by the transfer from one atom to another. This initially led to the assumption that chemical bonds must always be polar in structure.
Based on the properties of organic compounds that could not be explained by polar bonds and experiments with channel beams, it soon became clear that there must also be a nonpolar bond. Gilbert Lewis first suggested in 1916 that the nonpolar bond was due to paired electrons. This theory was also compatible with the atomic models of Rutherford and Bohr, which had meanwhile replaced Thomson's model.
With the development of quantum mechanics and especially the establishment of the Schrödinger equation by Erwin Schrödinger in 1926, more precise theories of binding could be established. The first quantum mechanical theory was developed with the valence structure theory in 1927 by Walter Heitler and Fritz London. The original theory was initially valid only for the simplest molecule, the H2+ ion of two protons and one electron. Linus Pauling extended the theory extensively by introducing the orbital and hybridization, so that the theory could be applied to more complicated molecules.
Also in 1927, the more precise molecular orbital theory was established by Friedrich Hund and Robert Mulliken. This too was initially only applicable to simple molecules, but was gradually extended, for example in 1930 by Erich Hückel by a more precise explanation of multiple bonds with the explanation of the π-bond.
After the basic quantum mechanical theories were established, various researchers attempted to use these theories to explain observed phenomena in organic or inorganic chemistry. Important examples are the ligand field theory for complexes, published in 1951 by Hermann Hartmann and F. E. Ilse, and the Woodward-Hoffmann rules established in 1968 by Robert B. Woodward and Roald Hoffmann, with which a certain type of organic reactions, the pericyclic reactions, could be understood on the basis of molecular orbital theory.
With the development of the computer from about 1950 onwards, more complicated theoretical calculations on chemical bonding also became possible. An important development for this were, among others, those of the Roothaan-Hall equations by Clemens C. J. Roothaan and George G. Hall in 1951, which are important in the Hartree-Fock method. Finally, starting in 1964, another way to theoretically calculate chemical bonding was developed by Walter Kohn in the form of density functional theory. He received the Nobel Prize in Chemistry for this in 1998.
