The Valence bond theory was first put forward by Heitler and London in 1927. It is based on the Lewis concept of a bond being formed by the pairing of electrons. It is the first quantum mechanical theory to explain the nature of bond formation in the hydrogen molecule.
Importance of Valance Bond Theory:
(1) Delocalization of electrons over the two nuclei.
(2) Shielding effect of electrons.
(3) The essentially covalent character of the bond.
(4) The presence of partial ionic nature in a covalent bond.
(5) The concept of resonance and the connection between resonance energy and molecular stability.
Limitations of Valence Bond Theory:
(1) This theory only explains the formation of a covalent bond in which a shared pair of electrons comes from two bonding atoms. However, it offers no explanation for the formation of a “coordinate covalent bond” in which both the electrons are contributed by one of the bonded atoms.
(2) On the basis of this theory, oxygen molecule should be diamagnetic. However, experimentally the molecule is found to be paramagnetic having two unpaired electrons. Thus, this theory fails to explain paramagnetism of oxygen molecule.
(3) This theory does not explain the bonding in electron deficient molecules like B2H6 (diborane) in which the central atom possesses less number of electrons than required for an octet of electrons.
Importance of Valance Bond Theory:
(1) Delocalization of electrons over the two nuclei.
(2) Shielding effect of electrons.
(3) The essentially covalent character of the bond.
(4) The presence of partial ionic nature in a covalent bond.
(5) The concept of resonance and the connection between resonance energy and molecular stability.
Limitations of Valence Bond Theory:
(1) This theory only explains the formation of a covalent bond in which a shared pair of electrons comes from two bonding atoms. However, it offers no explanation for the formation of a “coordinate covalent bond” in which both the electrons are contributed by one of the bonded atoms.
(2) On the basis of this theory, oxygen molecule should be diamagnetic. However, experimentally the molecule is found to be paramagnetic having two unpaired electrons. Thus, this theory fails to explain paramagnetism of oxygen molecule.
(3) This theory does not explain the bonding in electron deficient molecules like B2H6 (diborane) in which the central atom possesses less number of electrons than required for an octet of electrons.
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