Class 11 Chapter 3 VSEPR Theory And Shapes of molecules explained

Introduction

In Chapter 3 of Class 11 Chemistry, the Valence Shell Electron Pair Repulsion (VSEPR) theory explains how electron pairs around a central atom arrange themselves to minimize repulsion, determining the geometry of molecules.

VSEPR Theory Fundamentals

Proposed by Gillespie and Nyholm, VSEPR theory states that:

• Electron pairs (bonding and lone pairs) repel each other and adopt positions as far apart as possible.
• The geometry around the central atom depends on the total number of electron domains.
• Lone pairs exert greater repulsion than bonding pairs, slightly altering bond angles.

Electron Domains & Repulsions

Electron domains include bonding pairs and lone pairs. The repulsion strength order is:

• Lone Pair–Lone Pair (LP–LP) > Lone Pair–Bond Pair (LP–BP) > Bond Pair–Bond Pair (BP–BP)

Standard electron-domain geometries for 2–6 domains:

Common Molecular Shapes

Depending on the number of bonding and lone pairs, molecules adopt distinct shapes:

• Linear: AB₂ (e.g., CO₂)
• Trigonal Planar: AB₃ (e.g., BF₃)
• Bent: AB₂E (e.g., SO₂)
• Tetrahedral: AB₄ (e.g., CH₄)
• Trigonal Pyramidal: AB₃E (e.g., NH₃)
• Bent (Tetrahedral): AB₂E₂ (e.g., H₂O)
• Trigonal Bipyramidal: AB₅ (e.g., PCl₅)
• Seesaw: AB₄E (e.g., SF₄)
• T-shaped: AB₃E₂ (e.g., ClF₃)
• Octahedral: AB₆ (e.g., SF₆)
• Square Pyramidal: AB₅E (e.g., BrF₅)
• Square Planar: AB₄E₂ (e.g., XeF₄)

Examples & Geometry Table

Factors Affecting Shape

• Number of lone pairs on the central atom
• Electronegativity differences
• Multiple bonding (double/triple bonds occupy more space)
• Steric hindrance from bulky substituents

Conclusion

VSEPR theory provides a straightforward way to predict molecular geometry by considering electron pair repulsions. Mastery of these concepts enables understanding of molecular polarity, reactivity, and physical properties in chemical systems.