VSEPR Theory and Molecular Geometry

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a simple but remarkably powerful model for predicting the three-dimensional shape of a covalent molecule directly from its Lewis structure, without invoking orbitals at all.

The central postulate of VSEPR theory is this: the electron pairs — both bonding pairs and lone pairs — around a central atom repel one another electrostatically, and they arrange themselves in space so as to be as far apart as possible, minimising this mutual repulsion. The observed shape of the molecule is simply the geometric arrangement that achieves this minimum-repulsion configuration for the given number of electron pairs.

Applying VSEPR theory is a short, mechanical procedure: count the number of electron pairs (bonding and lone, together called the steric number) around the central atom, identify the base geometry that spaces that many points as far apart as possible on a sphere, and then note that the molecular shape is described only by the positions of the atoms, even though lone pairs still occupy space and still influence the geometry.

The five base electron-pair geometries met at Class 11 level, and the molecules that illustrate them in this chapter, are:

When some of the electron pairs around the central atom are lone pairs rather than bonding pairs, the molecular shape becomes a "distorted" version of the base geometry, because the different pair-pair interactions are not all equally repulsive. VSEPR theory ranks them as:

A lone pair, held by only one nucleus, occupies a broader, more diffuse region of space than a bonding pair, which is pulled in by two nuclei at once — so a lone pair pushes neighbouring pairs away more forcefully than a bonding pair does. This is exactly why, among the four tetrahedral-electron-pair-count molecules $\text{CH}_4$ (no lone pairs, $109.5^{\circ}$), $\text{NH}_3$ (one lone pair, $107^{\circ}$) and $\text{H}_2\text{O}$ (two lone pairs, $104.5^{\circ}$), the bond angle steadily shrinks as lone pairs replace bonding pairs, one at a time, around the same total of four electron pairs. $\text{NH}_3$'s molecular shape is described as trigonal pyramidal (three N–H bonds visible, the lone pair occupying the fourth, "invisible" position), while $\text{H}_2\text{O}$'s is described as angular or bent (two O–H bonds visible, two lone pairs occupying the other two positions). …