VALENCE BOND THEORY (VBT):
This theory was proposed by Heitler and London (1927) and later on was developed by Pauling. This concept is based on wave-mechanical treatment of molecules and has been successful in explaining the bond energies, bond lengths and shapes of covalent molecules.
• BASIC ASSUMPTION:
According to Valence bond theory, a covalent bond between two atoms is formed when the atoms come so close that a partially filled atomic of one atom overlap with a partially filled atomic orbital of the other. The two overlapping atomic orbitals retain their identity.
~POSTULATES OF VBT:
• Overlapping:
A bond formed between two atoms is formed by the overlap of half filled atomic orbitals of two atoms.
• Identity:
The two overlapping atomic orbitals retain their identity.
• Opposite spin:
The two overlapping orbitals must have electrons of opposite spin.
• Unpaired electrons:
The number of bonds formed by an atom is equal to the number of unpaired electrons present in the Valence shell of the atom.
• Single and multiple bonds:
If only two orbitals overlap, a single bond is formed. If additional orbitals of atoms overlap then multiple bonds are formed.
• Symmetry:
In order to form a bond, the overlapping orbitals must have same symmetry with respect to the bond axis.
• Energy:
In overlapping of orbitals energy is released. The greater the overlap between the orbitals, the greater is the energy released and the stronger will be the bond formed.
"BOND AXIS"
" A line joining the nuclei of two bonded atoms is called bond axis".
•>DRAWBACKS OF VALENCE BOND THEORY:
It fails to explain;
1. The formation of coordinate covalent bond.
2. The formation of odd electron molecules or ions.
3. The para magnetic behaviour of oxygen molecule.
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