Valence Shell Electron Pair Repulsion (VSEPR) Theory

VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY (SHAPES AND GEOMETRY OF MOLECULES)

Molecules exist in a variety of shapes. A number of physical and chemical properties of molecules arise from and are affected by their shapes. For example, the angular shape of the water molecules explains its many characteristic properties while a linear shape does not.

The determination of the molecular geometry and the development of theories for explaining the preferred geometrical shapes of molecules is an integral part of chemical bonding. The VSEPR theory (model) is a simple treatment for understanding the shapes of molecules.

Strictly speaking VSEPR theory is not a model of chemical bonding. It provides a simple recipe for predicting the shapes of molecules. It is infact an extension of the Lewis interpretation of bonding and is quite successful in predicting the shapes of simple polyatomic molecules.

The basic assumptions of the VSEPR theory are that:

Pairs of electrons in the valence shell of a central atom repel each other

1. These pairs of electrons tend to occupy position in space that minimize repulsions and thus maximize distance between them.

2. The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface at maximum distance from one another.

3. A multiple bonds are treated as a single super pair.

4. Where two or more resonance structures can depict a molecule, the VSEPR model is applicable to any such structures

For the prediction of geometrical shapes of molecules with the help of VSEPR model, it is convenient to divide molecules into two categories

Regular Geometry

Molecules in which the central atom has no lone pairs

Irregular Geometry

Molecules in which the central atom has one or more lone pairs, the lone pair of electrons in molecules occupy more space as compared to the bonding pair electrons. This causes greater repulsion between lone pairs of electrons as compared to the bond pairs repulsions. The descending order of repulsion

(lp – lp) > (lp – bp) > (bp – bp)

where lp-Lone pair; bp-bond pair

Regular Geometry

Irregular Geometry

Example . Why the bond angle of H – C – H in methane (CH₄) is 109° 28' while

H – N – H bond angle in NH₃ is 107° though both carbon and nitrogen are sp³ hybridized

Solution: In CH₄ there are 4 bond pair of electrons while in NH₃ are 3 bond pair of electrons and 1 lone pair of electrons. Since bond pair bond pair repulsion is less than lone pair bond pair repulsion, in NH₃ bond angle is reduced from 109°28' to 107°.