VSEPR Theory and Molecular Geometries

Introduction to VSEPR Theory

• VSEPR (Valence Shell Electron Pair Repulsion) model is used to predict the shape of molecules.
• Focuses on electrons in the valence shell (outermost electrons).
• Electron pairs repel each other, which determines the shape of the molecule.

Basic Concepts

• Electron Pairs: Negative charges repel each other.
• Central Atom: Other atoms are added to a central atom in a molecule forming shapes based on repulsion.

Molecular Shapes and Geometries

Linear Geometry

• Steric Number: 2
• Lone Pairs: 0
• Example: CO2
• Bond Angle: 180 degrees
• Characteristics:
  • Two atoms attached to the central atom.
  • Shape remains linear regardless of single, double, or triple bonds.

Trigonal Planar Geometry

• Steric Number: 3
• Lone Pairs: 0
• Example: BF3
• Bond Angle: 120 degrees
• Characteristics:
  • Three atoms bonded to the central atom, no lone pairs.

Bent Geometry (Steric Number 3)

• Steric Number: 3
• Lone Pairs: 1
• Example: SO2
• Characteristics:
  • Two atoms and one lone pair.
  • Lone pairs push atoms leading to a bent shape.

Tetrahedral Geometry

• Steric Number: 4
• Lone Pairs: 0
• Example: CH4
• Bond Angle: 109.5 degrees
• Characteristics:
  • Four atoms bonded to the central atom.

Trigonal Pyramidal Geometry

• Steric Number: 4
• Lone Pairs: 1
• Example: NH3
• Characteristics:
  • Three atoms and one lone pair.
  • Lone pair pushes atoms to form a pyramidal shape.

Bent Geometry (Steric Number 4)

• Steric Number: 4
• Lone Pairs: 2
• Example: H2O
• Characteristics:
  • Two atoms and two lone pairs.
  • Results in a bent shape, bond angle slightly less than 109.5 degrees.

Trigonal Bipyramidal Geometry

• Steric Number: 5
• Lone Pairs: 0
• Characteristics:
  • Five atoms attached to the central atom.

Octahedral Geometry

• Steric Number: 6
• Lone Pairs: 0
• Characteristics:
  • Six atoms attached to the central atom.

Practice and Application

• Use Lewis structures to visualize molecular geometries.
• Determine steric number and count lone pairs and bonded atoms on the central atom.
• Understand geometry by visualizing atoms and lone pairs spreading out.

Conclusion

• VSEPR theory helps understand how lone pairs and atoms repel each other.
• Use Lewis structure and steric numbers to determine molecular geometry.
• Develop intuition on how atoms and lone pairs influence molecular shape.

Key Practice: Draw Lewis structures, visualize 3D shapes, and use steric numbers to define geometry.