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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| Comparison and sigma and pi bonds |