Short Answer

VSEPR theory (Valence Shell Electron Pair Repulsion theory) is a model used to predict the shape of molecules. It states that electron pairs around the central atom repel each other and therefore arrange themselves as far apart as possible. This arrangement determines the geometry of the molecule.

The theory helps explain why molecules have specific shapes such as linear, bent, tetrahedral, trigonal planar, and more. By considering both bonding pairs and lone pairs, VSEPR theory gives a clear idea of a molecule’s three-dimensional structure.

Detailed Explanation :

VSEPR Theory

VSEPR theory, which stands for Valence Shell Electron Pair Repulsion theory, is one of the simplest and most useful models for predicting the shapes of molecules. According to this theory, the shape of a molecule is determined by the repulsion between electron pairs in the valence shell of the central atom. Electron pairs repel each other because they are negatively charged. To minimise repulsion, they arrange themselves as far apart as possible in three-dimensional space.

The theory considers two types of electron pairs:

• Bonding pairs – electrons shared between atoms
• Lone pairs – electrons that belong only to the central atom

Since lone pairs repel more strongly than bonding pairs, they have a significant effect on the final shape of the molecule. VSEPR theory helps predict molecular geometry accurately by counting electron pairs and arranging them in the most stable pattern.

Basic Idea of VSEPR Theory

VSEPR theory is based on two main ideas:

1. Electron pairs repel each other

All electron pairs—bonding or lone—have negative charges and push each other apart.

2. Molecules adopt shapes that minimise repulsion

The arrangement that keeps electron pairs farthest apart is the most stable, and this becomes the molecular geometry.

This simple concept allows chemists to predict shapes without needing complex mathematics.

Steps to Use VSEPR Theory

To predict molecular shape using VSEPR theory:

1. Identify the central atom
   Usually the atom with the lowest electronegativity (except hydrogen).
2. Count the total electron pairs around the central atom
   This includes both bonding pairs and lone pairs.
3. Determine the electron-pair geometry
   This tells how electron pairs are arranged in space.
4. Determine the molecular geometry
   This shape depends only on the positions of atoms, not lone pairs.

This two-step process helps distinguish between electron geometry and actual molecular shape.

Electron Pair Repulsion Strength

VSEPR theory also explains that different electron pairs repel differently:

• Lone pair – lone pair repulsion (strongest)
• Lone pair – bonding pair repulsion (moderate)
• Bonding pair – bonding pair repulsion (weakest)

Because lone pairs repel more strongly, they push bonding pairs closer together, reducing bond angles.

Example:

• In methane (CH₄), the angle is 109.5°.
• In ammonia (NH₃), lone pair pushes bonds → angle becomes 107°.
• In water (H₂O), two lone pairs → angle becomes 104.5°.

Common Molecular Shapes Predicted by VSEPR

VSEPR theory helps identify the geometry based on the number of electron pairs:

1. Linear (2 electron pairs)

Bond angle: 180°
Example: CO₂

2. Trigonal Planar (3 electron pairs)

Bond angle: 120°
Example: BF₃

3. Tetrahedral (4 electron pairs)

Bond angle: 109.5°
Example: CH₄

4. Trigonal Bipyramidal (5 electron pairs)

Bond angles: 90° and 120°
Example: PCl₅

5. Octahedral (6 electron pairs)

Bond angle: 90°
Example: SF₆

These represent electron-pair geometries.
The presence of lone pairs changes the molecular geometry.

How Lone Pairs Affect Molecular Shapes

Lone pairs occupy hybrid orbitals just like bonding pairs, but they create stronger repulsion.
This leads to:

• Bent shape – when two lone pairs push bonding pairs (H₂O)
• Trigonal pyramidal – when one lone pair pushes three bonds (NH₃)
• Seesaw shape – one lone pair on a trigonal bipyramidal structure (SF₄)
• T-shaped – two lone pairs on trigonal bipyramidal (ClF₃)
• Square planar – two lone pairs on octahedral (XeF₄)

Thus, VSEPR explains both ideal structures and distorted shapes.

Importance of VSEPR Theory

VSEPR theory is extremely important because it:

• Predicts the 3D shape of molecules
• Helps understand bond angles
• Explains why lone pairs change geometry
• Supports understanding of polarity and dipole moments
• Helps determine reactivity and physical properties
• Simplifies molecular geometry for beginners

Even though more advanced theories exist, VSEPR remains the most widely used model for predicting shapes.

Conclusion

VSEPR theory states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible. This arrangement determines the molecular geometry. By considering both bonding pairs and lone pairs, VSEPR theory helps predict shapes such as linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. It is a simple but powerful tool for understanding the three-dimensional structure of molecules.