Question

What is meant by VSEPR theory? Give various postulates. Explain the shape of PCl5 using VSEPR theory. Why are all P-Cl bond lengths not equal?

Answer 1

The VSEPR or valence shell electron pair repulsion theory helps in determining the shape of covalent molecules.

Postulates of this theory are as follows :

The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom.
Pairs of electrons in the valence shell tend to repel each other as their electron clouds are negatively charged.
The electron pairs tend to occupy those positions in space, which minimize repulsions and thus maximize distance from one another.
The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface at maximum distance from one another.
A multiple bond is treated as a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.
The VSEPR model is applicable to any such structure where two or more resonance structures can represent a molecule.

Decreasing order of repulsive interaction of electron pairs:

Lone pair (lp) − Lone pair (lp) > Lone pair (lp) − Bond pair (bp) > Bond pair (bp) − Bond pair (bp)

Reason for greater repulsion between lp − lp: The lone pairs occupy larger space than the bond pairs.

Molecules are classified into the following two categories to predict the geometrical shapes of molecules using VSEPR theory.

Molecules in which the central atom has no lone pair
Molecules in which the central atom has one or more lone pairs

For the hydronium ion, there are two lone pairs and two bond pairs in the valence shell of oxygen atom. There is also a coordinate bond between the H+ ion and oxygen atom of water molecule. Therefore the hydronium ion will have trigonal pyramidal geometry.

The structure of PCl5 is sp3d.It's three equatorial bonds are connected with an angle of 120 degree.one equatorial and axial are connected by an angle of 90 degree.hence the PCL bonds are unequal.
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