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