Molecular And Electron Geometries

Electron Domains and Molecular Geometry

Introduction

  • Discussing the relationship between electron domains and molecular geometry.
  • Electron domains categorize as bonding or nonbonding domains.

Two Electron Domains

  • Electron Geometry: Linear
    • When there are two bonding domains, both are bonding domains due to carbon having four valence electrons.
    • Bond Angle: 180 degrees
      • Atoms spread out as far as possible.
    • Example: Carbon dioxide ( ext{CO}_2)
      • Configuration: Linear

Three Electron Domains

  • Electron Geometry: Trigonal Planar
    • All atoms in the same plane.
    • Can lead to two molecular geometries: (
      • Trigonal Planar when all atoms bond.
      • Bent when there are lone pairs.
    • Example: ext{BF}_3 (Boron trifluoride)
      • Configuration: Trigonal planar, bond angles of 120 degrees.
    • Example: ext{SO}_2 (Sulfur dioxide)
      • Configuration: Bent due to lone pairs.

Lewis Structure of SO2

  • Constructing Lewis Structure: Needs Octet completion.
    • Initially, atoms might not have a full octet; need to shift electrons and create double bonds.
    • Aim for formal charges to be zero for best structure.
  • Formal Charge Calculation: Example results in 0 for ext{SO}_2.
  • Optimal structures often yield a formal charge of zero to minimize energy and maximize stability.

Influence of Lone Pairs

  • Lone pairs exert greater electron repulsion, pushing bonded groups closer together.
  • Effect on Geometry: Results in bond angles less than 120 degrees in bent configurations.
  • Conceptual Visual: Balloons tied together representing molecular shape.

Expanded Octets

  • Elements with atomic number n=3 or greater (e.g., Sulfur) can expand their octet to form stable structures.
  • Example: Ozone cannot expand its octet, requiring formal charge calculations.

Electron Domains and Bond Angles

  • Lone pairs occupy more space than bonding pairs due to stronger repulsion.
    • Order of space occupancy: Lone pairs > Triple bonds > Double bonds > Single bonds.
  • Geometry Dynamics: Varies from electron to molecular geometry depending on the number of lone pairs.

Tetrahedral Geometry

  • Electron Geometry: Tetrahedral when there are four bonded pairs.
    • Example: Methane ( ext{CH}_4)
      • Bond angle: 109.5 degrees.
  • Variations:
    • Trigonal Pyramidal: Example: Ammonia ( ext{NH}_3)
      • Electron geometry: Tetrahedral; molecular geometry: trigonal pyramidal; bond angle: ~107 degrees due to lone pair repulsion.
    • Bent Geometry: Example: Water ( ext{H}_2 ext{O})
      • Electron geometry: Tetrahedral; molecular geometry: bent; bond angle: ~104.5 degrees due to two lone pairs.

Five Electron Domains

  • Electron Geometry: Trigonal bipyramidal
    • Example: ext{PF}_5 (Phosphorus pentafluoride)
      • Bond angle: close to 90 degrees due to repulsion; interactions need careful consideration.

Six Electron Domains

  • Electron Geometry: Octahedral
    • Example: Contains multiple configurations: Square pyramidal and square planar.
    • Bond Angles: Approximately 90 degrees.
      • Square planar structure balances forces from pairs above and below.

Conclusion and Additional Resources

  • Use tools like QR 10 for visualizing molecular structures in 3D.
  • Encouragement to explore structures and their electronic configurations through hands-on applications.