(1) Valence Bond Theory (VBT)
Valence Bond Theory was for the first time proposed by Hietler and London and then modified by Pauling and Slater. It is based upon the pairing of electrons and the neutralization of their spins. The main postulates of this theory are as under-
(1) The covalent bonds are formed by overlapping of atomic orbitals, but in molecule atomic orbitals do not lose their identity. Thus, shape, size, and other parameters of atomic orbitals remain unchanged in the molecule. Thus, in molecules, the electrons are found in atomic orbitals. Hence, this theory is also called Atomic Orbital Theory (AOT).
(2) In overlapping, there occurs pairing of electrons and neutralization of their spins.
(3) The strength of a covalent bond depends upon the extent of overlapping between the atomic orbitals. The larger the extent of Overlapping, the stronger is the bond formed.
The extent of Overlapping is given by the overlap integral “S”. If ψA and ψB are the wave functions of combining atomic orbitals, then, the overlap integral “S” is given by the following expression-
S = ∫ ψA . ψB ∂r
(1) S > 0 (+ve), then there occurs attraction between the atoms, and stable covalent bonds are formed.
(2) S < 0 (-ve), then there occurs repulsion between the atoms, and covalent bonds are not formed.
(3) S = 0, then there occurs neither attraction nor repulsion between the atoms. Here also, stable covalent bonds are not formed.
(5) The inter-nuclear distance after the formation of covalent bonds is called the bond length of the respective bond.
(6) The energy released during the formation of a covalent bond is called the bond energy of the respective bond. It provides stability to the covalent bonds formed. The larger the value of bond energy, the stronger is the bond formed.
Limitations of VBT
This theory successfully explains the formation of molecules like H2, Cl2, NH3, CH4, etc. However, it has the following limitations-
(1) It could not explain the formation of odd election molecule ions like H2+ molecule ion.
(2) It could not explain the magnetic behavior of molecules. For example, according to this theory, the O2 molecule should not contain any unpaired election and therefore, it should be diamagnetic in nature. However, in practice, it is found to be paramagnetic.
(3) It does not give any idea about resonance.
Energy Profile of Covalent Bonds
The covalent bonds are formed by overlapping of atomic orbitals. Here, two half-filled atomic orbitals having electrons with opposite spin, come close together, share some common space and their unpaired elections get paired up in the shared space. Thus, a covalent bond gets formed.
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