VSEPR model: VSEPR theory

What is the VSEPR theory? What are the applications of VSEPR theory?

Answer:

VSEPR theory :

This theory was given by Sidgwick and Powell in 1940 and was further improved by Nyholm and
Gillespie in 1957. The basic concept of this theory is as follows :
“The electron pair surrounding the central atom repel one another and move so far apart from
one another that there are no further repulsions between them. As a result, the molecule has
minimum energy and maximum stability.”

The basic postulates of the VSEPR theory which help to find the shape of a molecule are :

1. The shape of a molecule containing only two atoms is always linear.
2. For molecules containing 3 or more atoms, one of the atoms is called the central atom to which
other atoms are linked.

3. If the central atom is linked to similar atoms and is surrounded by bond pairs of electrons only, the repulsions between them are similar. As a result, the shape of the molecule is symmetrical and the molecule is said to have a regular geometry.

4. If the central atom is linked to different atoms or is surrounded by bond pair as well as lone pairs of electrons, the repulsion between them are different. As a result, the molecule has an irregular or distorted geometry. The order of repulsions between electron pairs is as follows :

Lone pair‐Lone pair > Lone pair‐Bond pair > Bond pair‐Bond pair.

5. The exact shape of the molecule depends upon the total number of electron pairs present around the central atom.

VSEPR theory

Applications of VSEPR theory to some real molecules (shapes of molecules) :

On the basis of this theory shape of molecules can be predicted without knowing type of hybridisation.

To find the shape of a molecule, follow the steps given below :

(i) Identify the central atom and count the number of valence electrons.

(ii) Then find the number of electron pair shared.

(iii) While counting the number of electron pairs for ion, value of negative charge is added and positive charge is subtracted.

(iv) On the basis of total number of electron pairs (bond pairs + lone pairs) predict the geometry of the molecule.

 

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