Saturday, May 14, 2011

Valence Bond Theory (Modern Approach of Covalent Bond)


        This theory was developed by Heitler and London in 1927. Later, Pauling and Slater modified this theory. The various postulates of this theory are as follows;
(i).    Covalent bonds are formed by the overlapping of half filled atomic orbitals present in the valence shell of the atoms participating in bonding.
(ii).    The orbitals undergoing overlapping must have electrons with opposite spins.
(iii).    Overlapping of atomic orbitals results in the decrease of energy and formation of covalent bond.
(iv).    The strength of a covalent bond depends on the extent of overlapping. The greater the overlapping more is the energy released and consequently, stronger will be the covalent bond.
Let us now apply the valence bond theory to the formation of few covalent molecules;
    (a).    Formation of fluorine (F2) molecule
        Fluorine molecule is formed by the overlap of two half filled p-orbitals of two fluorine atoms as shown below:
    (b).    Formation of hydrogen fluoride (HF) molecule
    Hydrogen fluoride molecule is formed by the overlapping of 1s-orbital of hydrogen atom with one of 2p-orbital of fluorine atom which is half filled as shown below:
    (c).    Formation of oxygen (O2) molecule
    Oxygen molecule is formed when two half filled 2p-orbitals of each oxygen atom overlap with the 2p-orbitals, of the other oxygen atom to form a double bond.
    (d).    Formation of water (H2O) molecule
        Water molecule is formed when two half-filled 2p-orbitals of oxygen atom overlap with two half filled 1s-orbitals of two hydrogen atoms.
    (e).    Formation of nitrogen (N2) molecule
        Nitrogen molecule is formed when the three half filled 2p-orbitals of one nitrogen atom overlap with three similar orbitals of the other nitrogen atom to constitute a triple bond.
    (f).    Formation of ammonia (NH3) molecule
        Ammonia molecule is formed when three half filled p-orbitals of nitrogen atom overlap with three half filled 1s-orbitals of three hydrogen atoms.  

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