Valence Bond Theory (VBT)

Valence Bond Theory (VBT)


Valence Bond Theory theory was presented by Heitler and London to explain the formation of covalent bond. The main postulates of this theory are:
(i) A covalent bond is formed by the overlapping of atomic orbitals of valency shell of two atoms.
(ii) The atomic orbital of only valence shell with unpaired electrons can take part in this process of overlapping.
(iii) The atomic orbital of valence shell with paired electrons do not participate in the process of overlapping and hence do not contribute towards bond formation (it forms (lone pair)
(iv) The strength of covalent bond depends upon the extent of overlapping. Greater is the overlapping stronger will be the covalent bond.

Types of overlapping and nature of covalent bonds

Depending upon the type of overlapping, the covalent bonds may be divided into two types. (A) Sigma (σ) (B) Pi (π) bond

 A. Sigma Bond


(a) This type of covalent bond is formed by end to end overlapping of bonding orbitals along the internuclear axis.
(b) The overlap is known as head on overlap or axial overlap.
(c) The sigma bond is formed by any one of the following types of combinations of atomic orbitals.
1. s-s overlapping
In this type two half-filled s-orbitals overlap along the internuclear axis as shown below.

2. s-p overlapping
It involves the overlapping of half-filled s-orbital of one atom with the half-filled p-orbital of other atom e.g. formation of H–F molecule involves the overlapping of 1s orbital of H with the half-filled 2pz - orbital of Fluorine.


3. p-p overlapping
It involves the coaxial overlapping between p-orbitals of one atom with the p-orbitals of the other e.g. ; formation of F2 molecule in which 2pz orbital of one F atom overlap coaxially with the 2pz orbitals of second F atom as shown below.


(B) Pi (π) bond

(a) This type of covalent bond is formed by the side wise (or lateral) overlapping of half-filled atomic orbitals of bonding atoms.
(b) The orbitals involved in overlap must be parallel and perpendicular to the internuclear axis.

(c) Since overlapping takes place on both sides of the internuclear axis, free rotation of atoms around a p bond is not possible.
(d) Furthermore, formation of a pi bond shortens the bond distance between the two atoms involved e.g. C–C, C=C and C=C, bond lengths are 1.54Å, 1.3Å and 1.20Å respectively.

                      




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