Valence Shell Electron Pair Repulsion (VSEPR) Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the shape of individual molecules on the number of electron pairs surrounding their central atoms. The idea is that electron pairs, whether bonding or non-bonding, repel one another and will spread out as far apart as possible to minimize the repulsion.

1.0Historical Development of VSEPR Theory

1. Sidgwick and Powell’s Contribution (1940):

Sidgwick and Powell proposed that the repulsion between electron pairs in the valence shell of the central atom determines the shape of a molecule. They emphasized that both bonding and lone pairs repel each other, leading to a spatial arrangement that minimizes repulsion. This idea offered a straightforward explanation for molecular shapes, advancing the understanding of chemical bonding.

2. Nyholm and Gillespie’s Refinement (1957):

Ronald Nyholm and Ronald Gillespie expanded Sidgwick and Powell's ideas, refining VSEPR theory by highlighting the role of lone pairs in determining molecular shape. They recognized that lone pairs exert more repulsion than bonding pairs, leading to distortions in geometry. Their key contributions included classifying electron pairs into bonding and lone pairs, which allowed for more accurate predictions of molecular shapes, including deviations caused by lone pairs. They also introduced specific geometries like linear, trigonal planar, tetrahedral, and modified shapes like bent and trigonal pyramidal.

2.0Postulates of VSEPR theory

Nyholm and Gillespie (1957) refined the VSEPR model, indicating the greater repulsion between lone pairs of electrons compared to bond pairs. This leads to deviations from idealized shapes and alterations in bond angles. The VSEPR theory categorizes molecules based on whether the central atom has lone pairs or not. 

• The shape of a molecule is determined by the number of valence shell electron pairs, whether bonded or nonbonded, around the central atom.
• Electron pairs present in the valence shell repel each other due to their negative charges.
• These electron pairs arrange themselves in space to minimize repulsion, maximizing the distance between them.
• The valence shell is considered a spherical surface, with electron pairs located on this surface at a maximum distance from each other.
• Multiple bonds are treated as a single electron pair, and the electron pairs of multiple bonds are considered a single "super pair."
• When multiple resonance structures are possible for a molecule, the VSEPR model applies to all such structures.

3.0Principles of the VSEPR Theory

• Electron pairs repel each other: 
• Electron pairs (bonding pairs or lone pairs) repel each other and tend to arrange themselves in a way that minimizes repulsion.
• Geometry is determined by electron pair repulsion: 
• The geometric shape of a molecule is determined by the arrangement of electron pairs around the central atom, which is influenced by their mutual repulsions.
• The steric number determines molecular geometry: 
• The steric number of an atom is the sum of the number of atoms which are bonded to the central atom including the number of lone pairs on the central atom. The steric number determines the molecular geometry according to the VSEPR theory.

Based on these principles, the VSEPR theory predicts various molecular geometries, including:

• Linear
• Trigonal planar
• Tetrahedral
• Trigonal bipyramidal
• Octahedral
• Bent
• Trigonal pyramidal

4.0Planarity of Molecules-

• To identify the planarity of a molecule, first, we predict the hybridisation of the molecule and draw its shape.
• In a planar structure, all atoms are present in the same plane, and the number of such planar is one.
• In nonpolar structures, we first visualise the structure in 3D, then carefully check the maximum number of atoms in each plane.

5.0Shapes of molecules based on VSEPR theory

6.0Shapes of molecules based on VSEPR theory with Lone Pair

(Molecules having lone pairs on the Central atom, where E represents lone pair)

7.0VSEPR Theory Chart-

8.0Limitations of VSEPR Theory

While VSEPR theory is useful for predicting the shapes of many molecules, it has some limitations:

Doesn't Consider Electron Delocalization: 

In cases where electrons are delocalized over several atoms (as in resonance structures), VSEPR may not accurately predict geometry.

Limited to Main Group Elements: 

The theory primarily applies to compounds of main group elements and may not work as well for transition metals with complex d-orbital interactions.

Doesn’t Account for Bond Strengths: 

VSEPR doesn’t consider differences in bond strengths (single, double, triple bonds) which can also affect molecular geometry.