Valence bond (VB) theory is basically one of the two basic theories in Chemistry with the other one being molecular orbital (MO) theory. Both these theories have been developed to use the techniques of quantum mechanics to explain chemical bonding. The valence bond theory mainly traces as to how the dissociated atoms’ atomic orbitals merge to come up with individual chemical bonds when a molecule forms. Molecular orbital theory on the other hand has orbitals that cover the entire molecule.
As per the valence bond theory, a covalent bond forms amid two atoms. This happens as the half filled valence atomic orbitals of both the atoms having one unpaired electron overlap. The structure of a valence bond is quite like a Lewis structure. Several valence bond structures are used at the place where a single Lewis structure cannot be written. Each VB structure symbolizes a specific Lewis structure. The amalgamation of valence bond structures is the key point of resonance theory.
As per the valence bond theory, chemical bond forms as the participating atoms’ atomic orbitals overlap. Due to the overlapping, it is probable that the electrons must be in the bond region. Bonds are viewed as weakly coupled orbitals in valence bond theory. It is easier to apply this theory in ground state molecules. During the formation of bonds, the inner-shell orbitals and electrons stay unchanged.
There are two kinds of overlapping orbitals. These are sigma and pi. Let us learn about these two in detail:
- Sigma Bonds: These bonds transpire when the orbitals of two shared electrons overlap each other head to head. For instance, a bond amid two s-orbital electrons is a sigma bond as two spheres are always coaxial.
- Pi Bonds: These bonds transpire when two orbitals overlap at the time they are parallel.
The modern valence bond theory complements molecular orbital theory that does not conform to the valence bond idea that electron pairs are localized amid two particular atoms in a molecule however they are disbursed in sets of molecular orbitals that can extend over the whole molecule. The modern valence bond theory sees aromatic properties of molecules because of spin coupling of the pie orbitals.
One of the significant facets of the valence bond theory is the state of maximum overlap that ends in the creation of the strongest of bonds. This theory is employed to describe the covalent bond formation in many molecules.