E AB E In (a) the lp is present at axial position so there are three lp—bp repulsions at °. In(b) the lp is in an equatorial position, and there are two lp—bp repulsions. Hence, arrangement (b) is more stable. The shape shown in (b) is described as a distorted tetrahedron, a folded square or a see-saw.
See- saw AB E (More stable) Table . Shapes of Molecules containing Bond Pair and Lone Pair Shape Reason for the shape acquired Arrangement of electrons No. of lone pairs No. of bonding pairs Molecule type In (a) the lp are at equatorial position so there are less lp- bp repulsions as compared to others in which the lp are at axial positions.
So structure (a) is most stable. (T-shaped). T-shape AB E Shape Reason for the shape acquired Arrangement of electrons No. of lone pairs No.
of bonding pairs Molecule type . Valence Bond Theory As we know that Lewis approach helps in writing the structure of 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 H ( . kJ mol - , pm) and F ( kJ mol - , 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 ( ) 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 (Units ), the overlap criteria of