What principle of VSEPR theory?

Short Answer

The basic principle of VSEPR theory is that electron pairs around the central atom repel each other and therefore arrange themselves as far apart as possible. These repulsions determine the three-dimensional shape of a molecule. Both bonding pairs and lone pairs influence the geometry.

By reducing repulsion between electron pairs, the molecule adopts the most stable arrangement. This principle explains common molecular shapes like linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Detailed Explanation :

Basic Principle of VSEPR Theory

The basic principle of VSEPR (Valence Shell Electron Pair Repulsion) theory is that electron pairs in the valence shell of a central atom repel each other and position themselves to minimise repulsion. Because electrons carry negative charges, they naturally push away from one another. This repulsion controls how atoms are arranged around the central atom, which in turn determines the overall shape or geometry of the molecule.

VSEPR theory is built on the idea that molecules prefer to be in the lowest energy and most stable arrangement, and the best way to achieve this is by keeping electron pairs as far apart as possible. This simple idea allows chemists to predict the shapes of molecules with great accuracy.

Key Principle: Electron Pair Repulsion

The central idea of VSEPR theory can be summarised as:

“Electron pairs around a central atom repel each other and arrange themselves to minimise repulsion.”

This includes:

  • Bonding pairs (shared electrons forming bonds)
  • Lone pairs (non-bonding electrons on the central atom)

Both types of electron pairs influence shape, but lone pairs repel more strongly than bonding pairs.

  1. Why Electron Pairs Repel

Electrons are negatively charged, so they push away from each other due to electrostatic forces.
When electron pairs repel one another, the molecule adjusts until repulsion is minimized.
This leads to a stable three-dimensional structure.

Examples:

  • Two electron pairs → farthest apart → linear (180°)
  • Three electron pairs → triangle shape → trigonal planar (120°)
  • Four electron pairs → tetrahedral arrangement → 109.5°

These shapes come directly from the repulsion principle.

  1. Lone Pair vs Bond Pair Repulsion

Not all electron pairs repel equally.
VSEPR theory states:

Lone pair–lone pair repulsion > lone pair–bond pair repulsion > bond pair–bond pair repulsion

Reason:

  • Lone pairs occupy more space because they are only attracted to one nucleus.
  • Bonding pairs are shared between two atoms, so they are held more tightly.

This difference affects molecular geometry.

Examples:

  • CH₄: tetrahedral, 109.5°
  • NH₃: trigonal pyramidal (one lone pair), ~107°
  • H₂O: bent shape (two lone pairs), ~104.5°

The angles decrease because lone pairs push bonding pairs closer together.

  1. Molecular Geometry vs Electron Geometry

VSEPR distinguishes two types of geometry:

Electron Geometry

Based on all electron pairs (bonding + lone pairs).

Molecular Geometry

Based only on the arrangement of atoms; lone pairs are not counted in the visible shape.

Example:

  • H₂O has 4 electron pairs (tetrahedral electron geometry)
  • But the molecular geometry is bent, because only two pairs form bonds.

This difference arises directly from the repulsion principle.

  1. Predicting Shapes Using VSEPR

The basic principle of repulsion allows us to predict molecular shapes:

Number of electron pairs Shape
2 Linear
3 Trigonal planar
4 Tetrahedral
5 Trigonal bipyramidal
6 Octahedral

Lone pairs alter the final shape, causing variations such as:

  • Bent
  • Trigonal pyramidal
  • Seesaw
  • T-shaped
  • Square planar

All these shapes are consequences of electron repulsion.

  1. Why the Basic Principle Works

The repulsion-based principle works because it reflects the real physical behaviour of electrons:

  • Electrons seek maximum distance from each other
  • Lower repulsion leads to lower energy
  • Lower energy structures are more stable

Thus, geometry and stability are closely linked.

This principle is simple but powerful, making VSEPR theory widely used in chemistry education and practice.

Conclusion

The basic principle of VSEPR theory is that electron pairs around a central atom repel each other and arrange themselves as far apart as possible to reduce repulsion. This principle governs molecular geometry and explains why molecules take shapes such as linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Understanding electron pair repulsion allows chemists to predict molecular shapes easily and accurately.