To describe covalent bonding, a quantum mechanical model called valence bond theoryhas been suggested by Linus Pauling (1954).

    Valence bond theory

    To describe covalent bonding, a quantum mechanical model called valence bond theory has been suggested by Linus Pauling (1954). It is explained as follows:

  1.     A covalent bond between two atoms is formed when the two atoms approach each other closely and one atom overlaps its valence orbital containing unpaired electron, the valence orbital of the other atom that contains the unpaired electron of opposite spin. The so formed paired electrons in the overlapping orbitals are attracted to the nuclei of both the atoms. This bonds the two atoms together.
        Eg: In the formation of H2 molecule, the 1s orbital of one ‘H’ atom containing an unpaired electron overlaps the ‘1s’ orbital of the other ‘H’ atom containing unpaired electron of opposite spin giving H-H bond and H2 molecule.
  2.     The greater the overlapping of the orbitals that form the bond, the stronger will be the bond. This gives a directional character to the bond when other than ‘s’ orbitals are involved.
  3.     Each bonded atom maintains its own atomic orbitals but the electron pair in the overlapping orbitals is shared by both the atoms involved in the overlapping.
  4.     If two atoms form multiple bonds between them the first bond is due to the overlap of orbitals along the inter-nuclear axis giving a stronger sigma(σ) bond. After formation of (σ) bond the other bonds are formed due to the overlap of orbitals side wise or laterally giving weaker π bonds. The ‘σ’ bond is stronger because the electron pair shared is concentrated more between the two nuclei due to end-end or head on overlap and attracted to both the nuclei. The π bond overlap gives a weaker bond due to the lateral overlap of ‘p’ orbitals which is not to greater extent.

Consider Cl-Cl molecule

17Cl- 1s2 2s2 2p6 3s2 3px 2 3py 2 3pz 1

    In the formation of Cl 2 molecule, the 3pz orbital of one chlorine atom containing an unpaired electron overlaps the 3pz orbital of other chlorine atom that contains unpaired electron of opposite spin.


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