Theory
The drawback of Lewis concept was it was unable to explain the shapes of the molecules.
The valence shell electron pair repulsion (VSEPR) theory provides a simple method to predict the shapes of covalent molecules.
The postulates of VSEPR theory are:
- The geometry and shape of the molecule depends upon the number of electron pairs (bonded as well as non-bonded) in the valence shell of the central atom.
- The electron pairs surrounding the central atom repel one another as their orbitals/electron clouds are negatively charged.
- In order to minimise repulsion, these electron pairs tend to occupy such positions in space where the distance between them is maximum.
- A multiple bond is treated as if it is a single electron pair.
- Where two or more resonance structures can represent a molecule, the VSEPR model is applicable to any such structure.
- The repulsive interaction of electron pairs follow the order: lp-lp > lp-bp > bp-bp. The electron cloud containing lone pair is more spread out and occupy more space. This results in deviations from the idealised shapes and bond angles.
When lone pairs are included in the geometry of the molecules, the structures becaome as below.
