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:

Domains Electron Geometry
2 Linear (180°)
3 Trigonal Planar (120°)
4 Tetrahedral (109.5°)
5 Trigonal Bipyramidal (90°, 120°)
6 Octahedral (90°)

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

Shape Formula Bond Angle
Linear CO2 180°
Trigonal Planar BF3 120°
Bent H2O 104.5°
Tetrahedral CH4 109.5°
Trigonal Bipyramidal PCl5 90°/120°
Octahedral SF6 90°

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.

 

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