valence bond theory

valence bond theory

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.

  1. A covalent bond is formed when half – filled atomic orbitals containing one electron overlap with the other half – filled atomic orbitals with one electron.
  2. The overlap of two atomic orbitals result in the formation of single bound orbital localised molecular orbital which is occupied by both the electrons.
  3. The two electrons that occupy a bond orbital must have opposite spin, must be paired.
  4. The attractive force or bond energy is due to the electron pairing.
  5. Each electron has the entire bond orbital available to it and may be considered to belong to both the atomic orbitals.
  6.  As a result of overlapping there is maximum electron density somewhere between the two atoms.
  7.  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.

  1. T o explain the paramagnetism of B2 and O2.
  2. To explain the single and double bonded structures in  B2 and O2 respectively.
  3. The theory has little to offer im connection with the ekectronic sperctra and thermodynamics or kinetic od the sustem.
  4. 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.
  5. 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.
  6. It does not explain bonding in electron deficient molecules.