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Valence Bond Theory

Lewis approach is used in writing the structure of molecules. But it fails to explain the formation of chemical bond. It does not explain 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 (155 KJ mol-1, 144 pm), although in both the cases a single covalent bond is formed by the sharing of an electron pair between the respective atoms. It does not explain about the shapes of polyatomic molecules.

VSEPR theory gives the geometry of simple molecules. Theoretically, it does not explain them and also it has limited applications. To overcome these limitations the two important theories called valence bond (VB) theory and molecular orbital (MO) theory are introduced.

For the sake of convenience, valence bond theory has been discussed in detail in terms of qualitative and non-mathematical treatment only. To begin with, let us consider the formation of hydrogen molecule which is the simplest of all molecules.

Consider two hydrogen atoms A and B. These two hydrogen atoms have nuclei NA and NB. Electrons present in them are denoted as eA and eB. Assume that these two hydrogen atoms are approaching each other. When the two atoms are at large distance from each other, there will be no interaction between them. If these two atoms approach each other, new attractive and repulsive forces begin to operate.

Attractive forces arise between
  1. Nucleus of one atom and its own electron that is NA - eA and NB - eB.
  2. Nucleus of one atom and electron of other atom. i.e., NA - eB , NB - eA.
Similarly repulsive forces arise between
  1. Electrons of two atoms like eA - eB,
  2. Nuclei of two atoms NA - NB.
Attractive forces have the tendency to bring the two atoms close to each other whereas repulsive forces will have the tendency to push them apart
  1. Experimentally it has been discovered that the magnitude of new attractive force is more than the new repulsive forces.
  2. As a result, potential energy decreases due to the approach of two atoms.
  3. The stage is reached where the net force of attraction balances the force of repulsion and system acquires minimum energy.
  4. At this stage two hydrogen atoms are said to be bonded together
  5. As a result of bonding it forms a stable molecule having the bond length of 74 pm.
  6. The hydrogen molecule is more stable than that of isolated hydrogen atoms. Since, the energy gets released when the bond is formed between two hydrogen atoms.
  7. The energy released is called as bond enthalpy, which is corresponding to minimum in the curve depicted in. Conversely, 435.8 kJ of energy is required to dissociate one mole of H2 molecule.

The potential energy curve for the formation of H2 molecule as a function of internuclear distance of the H atoms. The minimum in the curve corresponds to the most stable state of H2.

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