Heitler and London introduced Valence Bond Theory. This is primarily based on the concepts of atomic orbitals, electronic configuration of elements, overlapping of atomic orbitals and hybridization of atomic orbitals.The overlapping of atomic orbitals results in the formation of a chemical bond.The electrons are localized in the bond region due to overlapping. Valence bond theory describes the electronic structure of molecules.
POSTULATES OF VBT
• The overlapping of two half-filled valence orbitals of two different atoms results in the formation of the covalent bond. The overlapping causes the electron density between two bonded atoms to increase. This gives the property of stability to the molecule.
• In case the atomic orbitals possess more than one unpaired electron, more than one bond can be formed and electrons paired in the valence shell cannot take part in such a bond formation.
• A covalent bond is directional. Such a bond is also parallel to the region of overlapping atomic orbitals.
• Based on the pattern of overlapping, there are two types of covalent bonds: sigma bond and a pi bond. The covalent bond formed by sidewise overlapping of atomic orbitals is known as pi bond whereas the bond formed by overlapping of atomic orbital along the inter nucleus axis is known as a sigma bond.