In chemistry, valence bond (VB) theory is one of two basic theories—along with molecular orbital (MO) theory—that use quantum mechanics methods to explain chemical bonding. According to VB theory, a covalent bond forms from the overlap of the half-filled valence orbitals in atoms containing an unpaired electron.
Mechanisms of Bonding in VB Theory
VB theory dictates that overlapping atomic orbitals of participating atoms form a chemical bond. Because of the overlap, it is probable that electrons are found in the bond (where the orbitals overlap) region.
Sigma and Pi Bonds
There are two types of overlapping orbitals: sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap between the nuclei of two atoms; Pi bonds occur when the two overlapping orbitals are outside of the space between the nuclei (above, below, in front, and in back).
Comparing VB and MO
VB theory complements molecular orbital (MO) theory, which does not adhere to the VB concept that electron pairs are localized between two specific atoms in a molecule. MO theory suggests that electrons are distributed in sets of molecular orbitals that can extend over the entire molecule. MO theory can predict magnetic and ionization properties in a straightforward manner. VB theory produces similar results, but is more complicated.
An important aspect of the VB 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. In the F2 molecule, the F–F bond is formed by the overlap of pz orbitals of the two F atoms, each containing an unpaired electron. Since the nature of the overlapping orbitals is different in H2 and F2 molecules, bond strength and bond lengths differ between H2 and F2 molecules.
In an HF molecule, the covalent bond forms from the overlap of the 1s orbital of H and the 2pz orbital of F, each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond in HF.