The theory given by Lewis explained about the structure of molecules but it could not explain the chemical bond formation. Similarly, VSEPR theory explained the shape of simple molecules but it had limited application and failed to explain the geometry of complex molecules. Hence to overcome these limitations, valence bond theory was introduced.
Valence Bond Theory:
This theory was introduced by Heitler and London which is based on concepts of atomic orbitals, electronic configuration of elements, the overlapping of atomic orbitals, hybridization of atomic orbitals. The overlapping of atomic orbitals results in the formation of a chemical bond and the electrons are localized in the bond region due to overlapping. Valence bond theory describes the electronic structure of molecules. The theory says that electrons fill the atomic orbitals of an atom within a molecule and nucleus of one atom is attracted to the electrons of another atom.
Postulates of Valence Bond Theory:
- The overlapping of two half filled valence orbitals of two different atoms result in the formation of the covalent bond. Due to overlapping, the electron density between two bonded atoms increases and this gives 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 and it is 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.
Limitations of Valence Bond Theory:
- It fails to explain the tetravalency of carbon.
- This theory does not discuss energies of electrons.
- The assumptions about the electrons being localized to specific locations.
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