Section 13.13b: Understanding VSEPR Theory

Introduction to VSEPR Theory

• Key Principle: Electrons are negatively charged and repel each other.
• Bonds and Lone Pairs: Both are made out of electrons, hence they repel each other.
  • Lone pairs have greater repulsion than bonds.

Steric Number

• Definition: The number of electron groups around the central atom
  • Single, double, triple bonds, and lone pairs all count as one steric number.
  • Lone pairs are more repulsive.

Determining Geometry Using VSEPR

1. Draw the Lewis Dot Structure
2. Count Electronic Groups (e.g., CO2 and COCl2 examples)
   • Central atom's electron groups determine geometry
3. Geometry Examples:
   • Steric Number 2 (e.g., CO2): Linear geometry, 180° angle.
   • Steric Number 3 (e.g., COCl2, Ozone): Trigonal planar if no lone pairs, bent if one lone pair. Angle < 120°.
   • Steric Number 4 (e.g., CH4, NH3, H2O): Tetrahedral electronic geometry
     • Tetrahedral molecular geometry for no lone pairs.
     • Trigonal pyramidal with one lone pair.
     • Bent with two lone pairs.

Exploring Molecular Geometries with Models

• Simulation Tools: Gaucho Space for visualizing VSEPR models.
• Methane (CH4): Tetrahedral.
• Ammonia (NH3): Trigonal pyramidal.
• Water (H2O): Bent.

Advanced Geometries

• Steric Number 5 (e.g., PCl5): Trigonal bipyramidal geometry.

  • Seesaw if one lone pair.
  • T-shaped if two lone pairs.
  • Linear if three lone pairs.

• Steric Number 6 (e.g., SF6): Octahedral geometry.

  • Square pyramidal if one lone pair.
  • Square planar if two lone pairs.

Key Takeaways

• Remember electronic vs. molecular geometry.
• Lone pairs alter angles; typically less than stated angles without lone pairs.
• Practice example: SF4
  • Steric number 5 with one lone pair = Seesaw molecular geometry.

This summary provides a concise overview of VSEPR theory application, focusing on the electron repulsion affecting molecular and electronic geometry. Always consider steric numbers for accurate predictions of molecular shapes and angles.