Determining the Shape of a Molecule: VSEPR Theory

Introduction

• Continuation from Lewis Dot Diagrams to molecular shapes.
• Example molecule: Aspirin.
  • Features: flat sections, triangular sections, and sections protruding from the page.

VSEPR Theory

• VSEPR Theory: Valence Shell Electron Pair Repulsion theory.
• Focuses on valence shell electrons (practiced through Lewis Dot Diagrams).
• Electrons act in pairs and sometimes in groups, forming "electron domains."

Electron Domains

• Example: Nitrate molecule has three electron domains around nitrogen.
• Domains repel each other to maximize distance, forming specific shapes.

Common Molecular Shapes

• Trigonal Planar:
  • Three domains.
  • Expected angles: ~120°.
• Tetrahedral:
  • Four domains.
  • Expected angles: ~109.5°.
• Linear:
  • Two domains.
  • Expected angles: 180°.

Lone Pairs vs. Bonded Pairs

• Lone pairs repel more than bonded pairs.
• Example: Molecule with four electron domains and two lone pairs.
  • Lone pairs can distort traditional angles (e.g., from 109°).

Examples of Shape Determination

• Methane (CH₄):

  • Lewis Dot Diagram shows four electron domains.
  • Forms a tetrahedral shape with bond angles of 109°.

• Ammonia (NH₃):

  • Eight electrons; forms a tetrahedral arrangement.
  • One lone pair causes a trigonal pyramidal shape.
  • Bond angles less than 109° due to lone pair repulsion.

• Water (H₂O):

  • Eight electrons; forms a tetrahedral-like arrangement.
  • Two lone pairs create a bent or V-shape.
  • Bond angles less than 109°.

• Ozone (O₃):

  • Eighteen electrons; forms a trigonal planar shape.
  • One lone pair results in a bent shape.
  • Bond angles less than 120° due to lone pair.

• Carbon Dioxide (CO₂):

  • Sixteen electrons; forms a linear shape.
  • Two electron domains; angles of 180°.

Summary

• Start with a Lewis Dot Diagram to determine electron domains.
• Consider how electron domains will spread to maximize distance.
• Evaluate the effect of lone and bonded pairs on angles and molecular shape.

Conclusion

• Understanding molecular shapes requires considering electron pair repulsion, domain arrangements, and lone vs. bonded pairs.