VSEPR Theory and Molecular Geometry

Introduction

• VSEPR stands for Valence Shell Electron Pair Repulsion.
• Predicts molecular shape based on electron repulsion.
• Electrons repel each other, aiming to be as far apart as possible.

Types of Molecular Geometry

Linear Molecular Geometry

• Example: BeCl2 (Beryllium chloride)
  • Structure: Straight line.
  • Bond angle: 180 degrees.
• Example: CO2 (Carbon dioxide)
  • Structure: Straight line.
  • Generic Structure: Central atom with two atoms on sides.
• Example: Triiodide ion
  • Structure: Linear with three iodide atoms.

Trigonal Planar Structure

• Definition: Flat structure with three atoms.
• Example: BH3
  • Boron at the center, spaced evenly by three hydrogen atoms.
  • Bond angle: 120 degrees (360/3).
• Example: COCl2
  • Carbon double-bonded to oxygen, attached to two chlorine atoms.
  • No lone pairs on the central atom.

Tetrahedral Molecular Structure

• Definition: Central atom surrounded by four other atoms.
• Example: Methane (CH4)
  • Carbon in the center, 3D structure.
  • Bond angle: 109.5 degrees.
• Example: Silicon tetrafluoride (SiF4)
  • Silicon surrounded by four fluorine atoms.
  • Similar geometry to methane.

Trigonal Pyramidal Structure

• Definition: Central atom with three atoms and a lone pair.
• Example: NH3 (Ammonia)
  • Nitrogen with one lone pair, three hydrogen atoms.
  • Bond angle: 107 degrees.
• Example: PH3
  • Group 5 elements tend to have this structure.
• Comparison:
  • Trigonal pyramidal has lone pair on central atom, while trigonal planar does not.

Bent Molecular Geometry

• Example: Water (H2O)
  • Oxygen has two lone pairs causing bent shape.
  • Bond angle: 104.5 degrees.
• Example: Sulfur dioxide (SO2)
  • Sulfur has one lone pair.
  • Structure similar to trigonal planar but with a lone pair.
  • Bond angle less than 120 degrees.

Comparison of Structures and Angles

• Tetrahedral: Four atoms, bond angle 109.5°.
• Trigonal Pyramidal: Three atoms + one lone pair, bond angle 107°.
• Bent Water: Two lone pairs, bond angle 104.5°.
• Trigonal Planar: No lone pair, bond angle 120°.
• Bent SO2: Two atoms + one lone pair, bond angle slightly less than 120°.