<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> <P> Modern valence bond theory now complements molecular orbital theory, which does not adhere to the valence bond idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule . Molecular orbital theory can predict magnetic and ionization properties in a straightforward manner, while valence bond theory gives similar results but is more complicated . Modern valence bond theory views aromatic properties of molecules as due to spin coupling of the π orbitals . This is essentially still the old idea of resonance between Kekulé and Dewar structures . In contrast, molecular orbital theory views aromaticity as delocalization of the π - electrons . Valence bond treatments are restricted to relatively small molecules, largely due to the lack of orthogonality between valence bond orbitals and between valence bond structures, while molecular orbitals are orthogonal . On the other hand, valence bond theory provides a much more accurate picture of the reorganization of electronic charge that takes place when bonds are broken and formed during the course of a chemical reaction . In particular, valence bond theory correctly predicts the dissociation of homonuclear diatomic molecules into separate atoms, while simple molecular orbital theory predicts dissociation into a mixture of atoms and ions . For example, the molecular orbital function for dihydrogen is an equal mixture of the covalent and ionic valence bond structures and so predicts incorrectly that the molecule would dissociate into an equal mixture of hydrogen atoms and hydrogen positive and negative ions . </P> <P> Modern valence bond theory replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over a large number of basis functions, either centered each on one atom to give a classical valence bond picture, or centered on all atoms in the molecule . The resulting energies are more competitive with energies from calculations where electron correlation is introduced based on a Hartree--Fock reference wavefunction . The most recent text is by Shaik and Hiberty . </P> <P> An important aspect of the Valence Bond theory is the condition of maximum overlap, which leads to the formation of the strongest possible bonds . This theory is used to explain the covalent bond formation in many molecules . </P>

The chemistry of the theory of valence forces