VSEPR Theory and Molecular Geometry

Introduction

• VSEPR Theory: Stands for "Valence Shell Electron Pair Repulsion."
  • Predicts molecular shape based on electron repulsion.
  • Electrons want to be as far apart as possible.

Linear Molecular Geometry

• Characteristics: Atoms arranged in a straight line.
• Examples:
  • BeCl₂ (Beryllium Chloride): Appears as a straight line.
  • CO₂ (Carbon Dioxide): Also linear.
  • Triiodide Ion (I₃⁻): Linear shape with three iodine atoms.
• Bond Angle: 180 degrees.

Trigonal Planar Structure

• Characteristics: Flat structure with three atoms around a central atom.
• Examples:
  • BH₃ (Boron Hydride): Boron in the center, three hydrogen atoms equidistant.
  • COCl₂: Carbon double-bonded to oxygen, plus two chlorine atoms.
• Bond Angle: Approximately 120 degrees.

Tetrahedral Structure

• Characteristics: Central atom surrounded by four atoms.
• Examples:
  • Methane (CH₄): Carbon in the center with four hydrogen atoms.
  • Silicon Tetrafluoride (SiF₄): Similar structure to methane.
• Bond Angle: Approximately 109.5 degrees.

Trigonal Pyramidal Structure

• Characteristics: Central atom with a lone pair and three surrounding atoms.
• Examples:
  • NH₃ (Ammonia): Nitrogen with a lone pair and three hydrogen atoms.
  • PH₃: Similar structure with phosphorus.
• Bond Angle: Approximately 107 degrees.

Bent Molecular Geometry

• Characteristics: Two atoms with lone pairs causing a bent shape.
• Examples:
  • Water (H₂O): Oxygen has two lone pairs, creating a bent shape.
  • Sulfur Dioxide (SO₂): Sulfur with one lone pair and two oxygens.
• Bond Angle:
  • Water: 104.5 degrees.
  • SO₂: Less than 120 degrees, similar to trigonal planar.

Key Comparisons

• Tetrahedral (e.g., Methane) vs. Trigonal Pyramidal (e.g., Ammonia) vs. Bent (e.g., Water):
  • Methane: 109.5 degrees.
  • Ammonia: 107 degrees (due to lone pair).
  • Water: 104.5 degrees (due to two lone pairs).
• Trigonal Planar (e.g., BH₃) vs. Bent (e.g., SO₂):
  • BH₃: 120 degrees.
  • SO₂: Slightly less than 120 degrees.