Vespa Theory and Molecular Geometry

Introduction to Vespa Theory

• Vespa stands for Valence Shell Electron Pair Repulsion.
• Electrons repel each other and aim to be as far apart as possible.
• Molecular shape can often be predicted based on electron repulsion.

Linear Molecular Geometry

• Characteristics:
  • Central atom with two other atoms on either side.
  • Forms a straight line.
  • Bond angle is 180 degrees.
• Examples:
  • Beryllium chloride (BeCl₂)
  • Carbon dioxide (CO₂)
  • Triiodide ion (I₃⁻)

Trigonal Planar Structure

• Characteristics:
  • Central atom with three surrounding atoms.
  • No lone pairs on the central atom.
  • Bond angle is 120 degrees.
• Examples:
  • Boron hydride (BH₃)
  • Carbonyl chloride (COCl₂)
  • Typically found in elements from Group 3A

Tetrahedral Molecular Structure

• Characteristics:
  • Central atom surrounded by four atoms.
  • Three-dimensional structure.
  • Bond angle is approximately 109.5 degrees.
• Examples:
  • Methane (CH₄)
  • Silicon tetrafluoride (SiF₄)

Trigonal Pyramidal Structure

• Characteristics:
  • Central atom with one lone pair and three surrounding atoms.
  • Bond angle is about 107 degrees.
• Examples:
  • Ammonia (NH₃)
  • Phosphine (PH₃)
  • Typically found in elements from Group 5A

Bent Molecular Geometry

• Characteristics:
  • Central atom with two atoms and lone pairs.
  • Bond angle varies, but often close to 104.5 degrees for water.
• Examples:
  • Water (H₂O)
  • Sulfur dioxide (SO₂)

Relationship Between Different Structures

• Tetrahedral vs. Trigonal Pyramidal vs. Bent
  • Tetrahedral has four atoms with an angle of 109.5 degrees.
  • Trigonal pyramidal has a lone pair, reducing the angle to 107 degrees.
  • Bent structure with two lone pairs has an angle of 104.5 degrees (e.g., H₂O).
• Trigonal Planar vs. Bent
  • Trigonal planar has three atoms with a 120-degree angle.
  • Bent (like in SO₂) can be close to 120 degrees due to lone pair effects.