VSEPR theory

Learning Objectives：

• Definition of VSEPR.

• Prediction of shapes and geometries of molecules.

• Bent’s rule and its application in understanding the structural parameters.

The Valence Shell Electron Pair Repulsion Theory (VSEPR)

VSEPR theory is an improved and extension of Lewis model but predicts the shapes of polyatomic molecules. This model was first suggested by Nevil Sidgwick and Herbet Powell in 1940 and later improved by Ronald Gillespie and Ronald Nyholm.

Prediction of molecular shapes and geometries was made easy by this model through the following simple steps.

1. Draw the Lewis structure.

2. Count the total number of bonds and lone pairs around the central atom. (Each single bond would involve one pair of electrons).

3. Arrange the bonding pairs and lone pairs in one of the standard geometries to minimize the electron-electron repulsion.

1. Lone pair electrons stay closer to the nucleus and also they spread out over a larger space than bond pairs and hence large angles between lone pairs.

2. The repulsion follows the order LP—LP > LP—BP > BP—BP.

4. Multiple bonds should be considered as a single bonding region.

   
   

Steric numbers

Another term called steric number is often used in VSEPR theory.

Steric number (SN) = No. of attached atom + No. of lone pairs. Since the lone pair—lone pair repulsions are maximum, the most stable geometry can be obtained by maximizing the distance between steric numbers on the central atom.

Molecular shapes are eventually determined by two parameters: Bond distance, separation between the nuclei of two bonded atoms in a straight line and the bond angle, the angle between any two bonds containing a common atom.

While mentioning the molecular shapes lone pairs may be ignored, however, while defining the geometry both the lone pairs and bond pairs should be considered.

For example: in water molecule the central oxygen atom is in tetrahedral environment with two lone pairs and two O—H bonds (or two bond pairs). The shape of the water molecule is therefore bent (two lone pairs are ignored).

Similarly, in ammonia, the nitrogen atom is in tetrahedral environment with three bonded pairs (three N—H bonds) and one lone pair. The shape of NH3 molecule is pyramidal.

Predicting the molecular geometries

To begin with, draw the Lewis structure.

Count the number of bonding pairs and lone pairs around the central atom.

Arrange the bonding pairs and the lone pairs in one of the standard geometries thereby minimizing electron—electron repulsion.

Multiple bonds count as a single bonding region.

What is Bent’s rule:
More electronegative substituents ‘prefer’ hybrid orbitals having less s-character, and more electropositive substituents ‘prefer’ orbitals having more s-character.

The bond angles in CH4, CF4 and CH2F2 can be explained using Bent’s rule. While a carbon in CH4 and CF4 uses four identical sp³ hybrids for bonding, in CH2F2 the hybrids used are not identical.

The C-F bonds are formed from sp3 + x hybrids, with slightly more p-character and less s-character than an sp³ hybrid, and the hydrogen are bonded by sp3 - x hybrids, with slightly less p-character and slightly more s-character. Increasing the amount of p-character in the C-F bonds decreases the F-C-F bond angle, because for bonding by pure p-orbitals the bond angle would be decreased to 90°.

Molecular shapes determined by VSEPR theory

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