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VSEPR Theory


A simple theory to account for the molecular shape of covalent molecules was developed by Gillespie and Nyholm in 1957. This theory, known as VSEPR (abbreviation of valence shell electron-pair repulsion) theory, predicts the shape of a molecule by considering the most stable configuration of the bond angles in the molecule. The guiding rules of this theory are as follows.
  1. Electron pairs in the valence shell of the central atom of a molecule, whether bonding or lone pairs, are regarded as occupying localized orbitals. These orbitals arrange themselves in space so as to minimize the mutual electronic repulsions.
  2. The magnitude of the different types of electronic repulsions follows the following order lone pair—lone pair > lone pair—bond pair > bond pair—bond pair
According to the first rule, the electronic repulsion between two pairs of electrons will be minimum if they are as far apart as possible. Based on this fact, the geometrical arrangements of pairs of electrons around the central atom are given as in the Figure. 

The description of the geometrical arrangements shown in figure. along with the corresponding examples of molecules is given in the following Table.
 

Figure

 

 

Example

 

Number of valence

Basic shape

 

 

electrons around the central atom

electron pairs

 

bonding pairs

 

lone pairs

 

(i)

BeCl2

4

2

2

0

Linear

(ii)

 

BCl

3

6

 

3

 

3

 

0

 

Triangular planar

(iii)

CH4

 

NH3

 

H2O

8

 

8

 

8

4

 

4

 

4

4

 

3

 

2

0

 

1

 

2

Tetrahedron

(iv)

 

 

PF5

 

SF4

 

ClF3

10

 

10

 

10

5

 

5

 

5

5

 

4

 

3

0

 

1

 

2

Trigonal bipyramid

 

 

(v)

 

 

SF6

 

IF5

12

 

12

6

 

6

6

 

5

0

 

1

Octahedron

 


The actual shape of the molecules containing lone pairs is a little distorted from the basic shape. This is primarily due to rule 2 mentioned above. Taking the examples of CH4, NH3 and H2O molecules, we find that the basic shape of these molecules is a tetrahedron. Experimentally, it is found that the bond angles in CH4, NH3 and H2O are approximately equal to 109O, 107O and 105O respectively. This progressive decrease in bond angle is due to the fact that CH4, NH3 and H2O contain 0, 1 and 2 lone pairs, respectively. In NH3 bond pair in place of lone pair the shape would have been tetrahedral but one lone pair is present and due to the repulsion between lone pair-bond pair (Which is more than bond pair- bond pair repulsion) the angle between bond pairs is reduced to 107o from 109.5o.
 
In H2O the shape should have been tetrahedral if there were all bond pair but two lone pair are present so the shape is distorted tetrahedral or angular. The reason is lone pair-lone pair repulsion is more than lone pair – bond pair repulsion which is more than bond pair- bond pair repulsion. Thus, the angle is reduced to 104.5o from 109.5o.




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