Sunday, May 15, 2011


Structures of simple molecules like H2, F2 and HF etc can be readily explained by the electron pair theory. But the formation of the compound of Beryllium, Boron & carbon cannot be that easily explained on the basis of same theory.
The electronic configuration of Be, b & c are
        4Be = 1s2, 2s2
        5B = 1s2, 2s2 2px1
        6C = 1s2, 2s2 2px12py1
    On the basis of electron pair theory, Beryllium should be zero valent and behave like noble gas since it has no unpaired electron, boron with one unpaired electron (2Pxl) should be monovalent and carbon with two unpaired electrons (2px1, 2py1) should be divalent. But Be forms compounds like BeH2, BeF2
in which it is bivalent, B forms compounds
BF3, BCl3 in which it is trivalent and C forms compounds like CH4, CCl4, in which it is tetravalent. Thus, it is not possible to explain the valencies of the elements like Be, B and C, on the basis of the number of unpaired electrons in the ground state of their atoms. Hence, in order to explain the bi, tri and tetravalency of Be, B and C respectively and to explain the observed geometry of their molecules, a new concept, called Hybridization is introduced by Pauling. This concept, though hypothetical is extremely useful.
    "Hybridization may be defined as the phenomenon of mixing up of orbitals of an atom of nearly equal energy, giving rise to entirely new orbitals equal in number to the mixing orbitals & having same energies & identical shapes". The new set of orbitals so formed is known as Hybrid Orbitals. In order to minimize the repulsion among hybrid orbitals these rearrange themselves in space in definite orientation.

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