Valence Bond Theory

The valence bond theory explains the covalent bond formation for two half-filled orbitals. The bond strength depends on several factors, including the extent of overlap of the two atomic orbitals. The bond strength is directly proportional to the extent of overlap.

In simple words, the greater the amount of overlap between two orbitals, the stronger the covalent bond will be.

JEE asks many questions based on the comparison of bond strength for two different pairs of atomic orbitals forming a covalent bond. For example, why H–F bond stronger than the F–F bond?

The concept of delocalization or resonance can be explained for quantum mechanical atomic models in which electrons are considered to be spread over the entire molecular orbital.

The fundamental assumption of the valence bond theory contradicts the delocalization. The valence bond theory assumes that a covalent bond forms from the overlap of atomic orbitals on adjacent atoms, and electron density is localized between two specific nuclei.

That is why VBT cannot explain energy stabilization due to the resonance property.

The central atom of nitrogen has 5 valence electrons as per the electronic configuraion. During the formation of ${\mathrm{N}\mathrm{H}}_3$ , 3 valence electrons forms three sigma bonds with hydrogen and one lone electron pair is left.

• The steric number  of the ammonia molecule: SN=3 bonds +1 lone pair $=4$ total electron domains.
• As per the steric number, there is ${\mathrm{s}\mathrm{p}}^3$ hybridisation in ammonia.
• The lone pair causes repulsion, which leads to a trigonal pyramidal geometry with bond angles of ${107}^{?}$ .