Chapter 8.4, Problem 63SSC

Interpretation Introduction

Interpretation:

The effect on the spacing of the shared bonding orbitals of a lone pair needs to be explained.

Concept introduction:

VSEPR theory stands for Valence Shell Electron Pair Repulsion Theory. It helps to predict the molecular shape or geometry of the molecule with the help of the number of bond pairs or lone pairs present in it.

Answer to Problem 63SSC

The presence of a lone pair on the central atom will change the bond angle of the molecule.

Explanation of Solution

According to the VSEPR theory, the presence of lone pair on the central atom of a molecule causes deviation from standard molecular geometry. Thus, valence electrons provide a Lewis structure, which gives an idea about electron pair geometry and hybridization.

The presence of a lone pair on a central atom affects the molecular geometry of a molecule. Since the order of repulsion is:

Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

Hence as the number of lone pair increases on the central atom, they move towards bond pairs and it decreases the spacing between bond pair which further decreases the bond angle of the molecule. For example, both water and ammonia have sp³ hybridization but they have different bond angles due to differences in the number of lone pairs on central N and O atom. In ammonia molecule, N has one lone pair so here lone pair-bond pair repulsion decreases the bond angle from 109° to 107°, whereas due to the presence of two lone pairs on O atom, the lone pair-lone pair repulsion is more therefore the bond angle of water molecule will be less than ammonia molecule.

Conclusion

The presence of a lone pair on the central atom will change the bond angle of the molecule. In the absence of a lone pair, the molecule will show the standard molecular geometry.