This Valence bond theory is based on wave mechanical treatment of molecule and is useful in explaining the bond energies, bond lengths and shape of covalent molecules . The Theron was developed by Heitler and London in 7927 which was improved and extended by Pauling and Slater in 1931. According to this theory.
- A covalent bond is formed when half – filled atomic orbitals containing one electron overlap with the other half – filled atomic orbitals with one electron.
- The overlap of two atomic orbitals result in the formation of single bound orbital localised molecular orbital which is occupied by both the electrons.
- The two electrons that occupy a bond orbital must have opposite spin, must be paired.
- The attractive force or bond energy is due to the electron pairing.
- Each electron has the entire bond orbital available to it and may be considered to belong to both the atomic orbitals.
- As a result of overlapping there is maximum electron density somewhere between the two atoms.
- A large part of the bonding force of covalent bond result from the electrostatic attraction between the nuclei and the accumulatedelectrons clouds between them.
Limitations of valence bond theory:
The theory is concentrate on structure and magnetic properties of the molecule , but it fails.
- T o explain the paramagnetism of B2 and O2.
- To explain the single and double bonded structures in B2 and O2 respectively.
- The theory has little to offer im connection with the ekectronic sperctra and thermodynamics or kinetic od the sustem.
- This theory dose not consider the formation odd parinf of electrons moleciles or ions such as H+2 ion where no pairing of electron occur.
- Valence bond theory say nothinf about the relative strngth of the bonds O+2and O2. Whereas the molecular orbital theory treatment is simple and convincing.
- It does not explain bonding in electron deficient molecules.