Valence Bond Theory
Valence Bond Theory
As we know
the Lewis approach helps in writing the structure for molecules but it fails to
explain the formation of chemical bond. It also does not give any reason for
the difference in bond dissociation enthalpies and bond lengths in molecules
like H2 (435.8 kJ mol-1, 74 pm) and F2 (150.6
kJ mol-1, 42 pm), although in both the cases a single covalent bond
is formed by the sharing of an electron pair between the respective atoms. It
also gives no idea about the shapes of polyatomic molecules.
Similarly
the VSEPR theory gives the geometry of simple molecules but theoretically, it
does not explain them and also it has limited applications. To overcome these
limitations the two important theories based on quantum mechanical principles
are introduced. These are valence bond (VB) theory and molecular orbital (MO)
theory.
Valence bond
theory was introduced by Heitler and London (1927) and developed further by
Pauling and others. A discussion of the valence bond theory is based on the
knowledge of atomic orbitals, electronic configurations of elements, the
hybridization of atomic orbitals and the principles of variation and
superposition. A rigorous treatment of the VB theory in terms of these aspects
is beyond the scope of this book. Therefore, for the sake of convenience,
valence bond theory has been discussed in terms of qualitative and non-mathematical
treatment only. To start with, let us consider the formation of hydrogen
molecule which is the simplest of all molecules.
Consider two
hydrogen atoms A and B approaching each other having nuclei NA and NB
and electrons present in them are represented by eA and eB.
When the two atoms are at large distance from each other, new attractive and
repulsive forces begin to operate.
Attractive
forces arise between:
Nucleus of
one atom and its own electron that is NA – eA and NB
–eB.
Nucleus of
one atom and electron of other atom i.e., NA – eB, NB
– eA.
Similarly
repulsive forces arise between
(i)
Electrons
of two atoms like eA – eB
(ii)
Nuclei
of two atoms NA – NB
Attractive
forces tend to bring the two atoms close to each other whereas repulsive forces
tend to push them apart.
Experimentally
it has been found that the magnitude of new attractive force is more than the
new repulsive forces. As a result, two atoms approach each other and potential
energy decreases. Ultimately a stage is reached where the net force of
attraction balances the force of repulsion and system acquires minimum energy. At
this stage two hydrogen atoms are said to be bonded together to form a stable
molecule having the bond length of 74 pm.
Since the
energy gets released when the bond is formed between two hydrogen atoms, the
hydrogen molecule is more stable than that of isolatded hydrogen atoms. The
energy so released is called as bond enthalpy, which is corresponding to minimum
in the curve depicted in the below figure. Conversely, 435.8 kJ of energy is
required to dissociate one mole of H2 molecule.
H2(g)
+ 435.8 kJ mol-1 - H(g) + H(g)
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