<P> In chemistry, valence bond (VB) theory is one of two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding . It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed . In contrast, molecular orbital theory has orbitals that cover the whole molecule . </P> <P> In 1916, G.N. Lewis proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as Lewis structures . In 1927 the Heitler--London theory was formulated which for the first time enabled the calculation of bonding properties of the hydrogen molecule H based on quantum mechanical considerations . Specifically, Walter Heitler determined how to use Schrödinger's wave equation (1926) to show how two hydrogen atom wavefunctions join together, with plus, minus, and exchange terms, to form a covalent bond . He then called up his associate Fritz London and they worked out the details of the theory over the course of the night . Later, Linus Pauling used the pair bonding ideas of Lewis together with Heitler--London theory to develop two other key concepts in VB theory: resonance (1928) and orbital hybridization (1930). According to Charles Coulson, author of the noted 1952 book Valence, this period marks the start of "modern valence bond theory", as contrasted with older valence bond theories, which are essentially electronic theories of valence couched in pre-wave - mechanical terms . Resonance theory was criticized as imperfect by Soviet chemists during the 1950s . </P> <P> According to this theory a covalent bond is formed between the two atoms by the overlap of half filled valence atomic orbitals of each atom containing one unpaired electron . A valence bond structure is similar to a Lewis structure, but where a single Lewis structure cannot be written, several valence bond structures are used . Each of these VB structures represents a specific Lewis structure . This combination of valence bond structures is the main point of resonance theory . Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond . Because of the overlapping, it is most probable that electrons should be in the bond region . Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules . The inner - shell orbitals and electrons remain essentially unchanged during the formation of bonds . </P> <P> The overlapping atomic orbitals can differ . The two types of overlapping orbitals are sigma and pi . Sigma bonds occur when the orbitals of two shared electrons overlap head - to - head . Pi bonds occur when two orbitals overlap when they are parallel . For example, a bond between two s - orbital electrons is a sigma bond, because two spheres are always coaxial . In terms of bond order, single bonds have one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds contain one sigma bond and two pi bonds . However, the atomic orbitals for bonding may be hybrids . Often, the bonding atomic orbitals have a character of several possible types of orbitals . The methods to get an atomic orbital with the proper character for the bonding is called hybridization . </P>

State the main features of valence bond theory