Learn about the VSEPR Theory Definition, Postulates, Examples and Limitations
The Valence Shell Electron Pair Repulsion (VSEPR) theory is a fundamental concept in Chemistry used to predict the shape and geometry of molecules. According to this theory, electron pairs around the central atom in a molecule repel each other and arrange themselves to minimise repulsion, resulting in a specific molecular shape. By analysing the number of bonding and lone pairs of electrons, VSEPR theory provides insight into molecular geometry, bond angles, and overall molecular structure.
This theory is crucial for understanding chemical bonding, molecular interactions, and reactivity. This page aims to explain VSEPR theory in simple terms, covering its principles, applications, and how it helps predict the three-dimensional shapes of molecules.
What Is VSEPR Theory?
The Valence Shell Electron Pair Repulsion (VSEPR) theory explains how the arrangement of electron pairs around the central atom determines the shape and geometry of molecules. According to this theory, electron pairs repel each other and arrange themselves to minimise repulsion, creating specific molecular geometries. It is widely used in chemistry to predict and explain the 3D shapes of molecules.
Key Principles of VSEPR Theory
Electron Pair Repulsion: Lone pairs and bonding pairs of electrons repel each other and try to stay as far apart as possible.
Minimisation of Repulsion: The molecular shape is determined by minimising repulsion between electron pairs.
Lone Pair vs Bonding Pair: Lone pair-lone pair repulsion > Lone pair-bonding pair repulsion > Bonding pair-bonding pair repulsion.
Steric Number: The shape of the molecule is predicted using the steric number (sum of bonding pairs and lone pairs on the central atom).
What is the VSEP Number?
The VSEP Number (Valence Shell Electron Pair Number) refers to the total number of electron pairs surrounding the central atom in a molecule.
It includes both bonding pairs (shared electrons involved in chemical bonds) and lone pairs (non-bonding electrons localized on the central atom).
The VSEP number is crucial in predicting the molecular geometry of a compound using the VSEPR theory, as it helps determine how the electron pairs will arrange themselves to minimise repulsion.
How to Predict Molecular Geometry
Identify the Central Atom: Choose the least electronegative atom as the central atom.
Count Valence Electrons: Add the valence electrons of the central atom and the bonded atoms.
Determine Electron Pairs: Calculate the steric number (bonding pairs + lone pairs).
Use the VSEPR Model: Based on the steric number, determine the molecular shape.
Common Molecular Shapes and Examples
Linear (AX₂):
Example: CO₂, BeF₂
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Carbon Valancy = 4
No. of Oxygen Atoms = 2, Oxygen Forms Double Bond Constituting 1 Bond
Carbon (V.E) Oxygen (No Of Atoms)
X—---------------------------------O
X—---------------------------------O
X—---------------------------------O
X—---------------------------------O
So the Total No of Bonds Formed = 2 B.P + 0 L.P = 2 = Linear Geometry and SP
Hybridisation.
Note: 2 electrons contsitue 1 L.P
2 bonded atoms, 0 lone pair. Bond angle: 180°.
Trigonal Planar (AX₃):
Example: BF₃
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Boron Valancy = 3
No. of Fluorine Atoms = 3
Boron (V.E) Fluorine (No Of Atoms)
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
So the Total No of Bonds Formed = 3 B.P + 0 L.P = 5 =Triangular Planar Geometry and SP2
Hybridisation.
Three bonded atoms, no lone pairs. Bond angle: 120°.
Tetrahedral (AX₄):
Example: CH₄, NH₄⁺
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Carbon Valancy = 4
No. of Hydrogen Atoms = 4
Carbon (V.E) Hydrogen (No Of Atoms)
X—---------------------------------H
X—---------------------------------H
X—---------------------------------H
X—---------------------------------H
So the Total No of Bonds Formed = 4 B.P + 0 L.P = 4 Tetrahedral and SP3
Hybridisation.
Four bonded atoms, no lone pairs. Bond angle: 109.5°.
Trigonal Pyramidal (AX₃E):
Example: NH₃
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Nitrogen Valancy = 5
No. of Hydrogen Atoms = 3
Nitrogen (V.E) Hydrogen (No Of Atoms)
X—---------------------------------H
X—---------------------------------H
X—---------------------------------H
X—---------------------------------L.p
X—---------------------------------L.p
So the Total No of Bonds Formed = 3 B.P + 1 L.P = 4 Pyramidal Geometry and SP3
Hybridisation.
Note: 2 electrons contsitue 1 L.P
Three bonded atoms, one lone pair. Bond angle: ~107°.
Bent or V-Shaped (AX₂E₂):
Example: OF₂
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Oxygen Valancy = 6
No. of Fluorine Atoms = 2
Oxygen (V.E) Fluorine (No Of Atoms)
X—---------------------------------F
X—---------------------------------F
X—---------------------------------L.p
X—---------------------------------L.p
X—---------------------------------L.p
X—---------------------------------L.p
So the Total No of Bonds Formed = 2 B.P + 2 L.P = 4 Bent Geometry and SP3
Hybridisation.
Note :2 electrons contsitue 1 L.P
Two bonded atoms, two lone pairs. Bond angle: ~104.5°.
Trigonal Bipyramidal (AX₅):
Example: PCl₅
Lets Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Phosphorus Valancy = 5
No. of Chlorine Atoms = 5
Phosphorus (V.E) Chlorine (No Of Atoms)
X—---------------------------------Cl
X—---------------------------------Cl
X—---------------------------------Cl
X—---------------------------------Cl
X—---------------------------------Cl
So the Total No of Bonds Formed = 5 B.P + 0 L.P = 5 =TBP Geometry with, Sp3d Hybridisation.
Five bonded atoms, no lone pairs. Bond angles: 90° and 120°.
Octahedral (AX₆):
Example: SF₆
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Sulphur Valancy = 6
No. of Fluorine Atoms = 6
Sulphur (V.E) Fluorine (No Of Atoms)
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
So the Total No of Bonds Formed = 6 B.P + 0 L.P = 6 = Octahedral Geometry with, Sp3d2 Hybridisation.
Six bonded atoms, no lone pairs. Bond angle: 90°.
Square Planar (AX₄E₂):
Example: XeF₄
Let's Consider Metal Ion as X with their Valance Electron and Ligand with no of atoms
Xenon Valancy = 8
No. of Fluorine Atoms = 4
Xenon (V.E) Fluorine (No Of Atoms)
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
X—---------------------------------F
X—---------------------------------L.P
X—---------------------------------L.P
X—---------------------------------L.P
X—---------------------------------L.P
So the Total No of Bonds Formed = 4 B.P + 2 L.P = 6 = Square Planar Geometry with, Sp3d2 Hybridisation.
Four bonded atoms, two lone pairs. Bond angle: 90°.
The various molecular geometries predicted by the VSEPR theory are illustrated in the diagram below.
Advantages of VSEPR Theory
Predicts the 3D shapes of molecules effectively.
Explains molecular geometry based on repulsions between electron pairs.
Simple compared to quantum mechanical models.
Limitations of VSEPR Theory
Does not account for bond lengths or multiple bonds.
Less effective for predicting the shapes of larger and more complex molecules.
Ignores the influence of electron delocalisation and resonance.
Applications of VSEPR Theory
Predicting molecular geometry in simple molecules like CH₄ and NH₃.
Understanding bond angles and reactivity in chemical reactions.
Explaining molecular polarity and dipole moments.
Conclusion
The VSEPR theory provides a simple yet powerful way to understand the shapes and geometries of molecules by focusing on electron pair repulsions. It complements Lewis structures by predicting the 3D arrangements of atoms. Although it has limitations, the theory is a cornerstone in chemistry for studying molecular interactions, bonding, and reactivity. Mastering the VSEPR model, students and professionals can better analyse and predict molecular behaviour in diverse chemical contexts.
FAQs on What is VSEPR Theory with Examples
1. What is VSEPR Theory Class 11?
In Class 11 chemistry, VSEPR theory is introduced to explain molecular shapes. It states that electron pairs in the valence shell of the central atom repel each other and try to arrange themselves to minimize repulsion, leading to specific molecular geometries.
2. What are the Postulates of VSEPR Theory?
The postulates of VSEPR theory include:
The shape of a molecule depends on the number of electron pairs around the central atom.
Electron pairs repel each other and occupy positions that maximize distance between them.
Lone pairs occupy more space than bonding pairs, affecting bond angles.
Multiple bonds are treated as a single electron region for shape determination.
3. What are the Limitations of VSEPR Theory?
The limitations of VSEPR theory include:
It does not accurately predict the geometries of transition metal complexes.
It does not consider d-orbital participation in bonding.
The theory does not account for size differences of substituent groups, which may affect molecular shape.
It often underestimates the influence of lone pairs, leading to incorrect bond angles.
4. Explain VSEPR Theory with an Example.
A common example of VSEPR theory is methane (CH₄). The central carbon atom forms four single bonds with hydrogen atoms. According to VSEPR theory, these electron pairs repel each other and arrange themselves tetrahedrally, leading to a bond angle of 109.5°.
5. What is the VSEPR Theory Chart?
A VSEPR theory chart helps predict molecular shapes based on the number of bonding and lone pairs. Some examples:
2 bonding pairs, 0 lone pairs → Linear (e.g., CO₂)
3 bonding pairs, 0 lone pairs → Trigonal planar (e.g., BF₃)
4 bonding pairs, 0 lone pairs → Tetrahedral (e.g., CH₄)
3 bonding pairs, 1 lone pair → Trigonal pyramidal (e.g., NH₃)
2 bonding pairs, 2 lone pairs → Bent (e.g., H₂O)
This chart helps visualise molecular geometries and their bond angles.
6. What is the Full Form of VSEPR?
The full form of VSEPR is Valence Shell Electron Pair Repulsion. It is a theory that predicts molecular shapes based on electron pair repulsion around the central atom.
