Valence bond theory or VBT
This theory was developed by Heitler and London in the year 1927 and modified by Pauling and Slater in the year 1931.
(1) To form the covalent bond, two atoms should come close to each other so that the orbitals of one overlaps with the orbital of the other.
(2) The Orbitals having unpaired electrons of the anti spin overlaps with each other.
(3) After the overlapping a new localized bond orbital is formed which has maximum probability of finding the electrons.
(4) The Covalent bond is formed due to the electrostatic attraction between radii and the accumulated electrons cloud and by the attraction between spins of an anti spin electrons.
(5) The Greater is the overlapping, the less will be bond length, more will be attraction and more will be the bond energy and the stability of bond will also increase.
(6) The extent of the overlapping depends upon: Nature of the orbitals which are involved in the overlapping, and nature of overlapping.
(7) The closer the valence shells are to the nucleus, the more will be overlapping and the bond energy will also be high.
(8) Between the two sub shells of the same energy level, the sub shell which is quite more directionally concentrated shows much more overlapping. The Bond energy: 2s-2s < 2s-2p < 2p-2p
(9)The s-orbitals are spherically symmetrical and hence show only head on the overlapping. On the other hand, -orbitals are directionally concentrated and hence show either head on overlapping or the lateral overlapping. The overlapping of different type gives sigma (s) and pi (p) bond.
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Sigma (s) bond
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Pi (p) bond
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It results from the end to end overlapping of two s-orbitals or two p-orbitals or one s and one p-orbital.
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It result from the sidewise (lateral) overlapping of two p-orbitals.
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Stronger
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Less strong
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Bond energy 80 kcals
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Bond energy 65 kcals
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More stable
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Less stable
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Less reactive
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More reactive
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Can exist independently
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Always exist along with a s-bond
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The electron cloud is symmetrical about the internuclear axis.
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The electron cloud is above and below the plane of internuclear axis.
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