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VSEPR Theory and Molecular Geometry Basics

Apr 22, 2025

Lecture on VSEPR Theory and Crystal Solids

Overview of Lecture Topics

  • VSEPR Theory
  • Crystal Solids
  • Introduction to Redox Reactions and Energetics
  • Kinetic Theory
  • Rates of Reactions and Log Graphs

Lewis Structures

  • Essential for understanding molecular geometries.
  • Represents atoms, bonded and free electrons.
  • Helpful in constructing VSEPR structures.

Steps to Construct Lewis Structures

  1. Central Atom: Identify and write it down (e.g., Carbon in Methane).
  2. Count Valence Electrons: For all atoms involved.
  3. Connect Using Bonds: Use single lines to represent sigma bonds.
  4. Lone Pairs: Position remaining valence electrons around atoms.
  5. Formal Charge: Calculate and adjust to stabilize the molecule.

Formal Charge Calculation

  • Formula: Valence Electrons - Non-bonding Electrons - (Bonding Electrons/2)
  • Aim for the structure with the least formal charge.

Examples of Molecules

  • Methane (CH₄): Central carbon with four hydrogens.
  • Nitrogen (N₂): Example of shared electron pairs.
  • Carbon Dioxide (CO₂): Central carbon with oxygen atoms.
  • Carbonate Ion (CO₃²⁻): More complex, with negative charges.

VSEPR Theory (Valence Shell Electron Pair Repulsion)

  • Predicts the geometry of covalent molecules.
  • Based on electron and bond pair interactions.
  • Electrons spread out to minimize repulsion.

Key Postulates

  1. Electron pairs are arranged to minimize repulsion.
  2. Central atom links all others in a polyatomic molecule.
  3. Valence electrons determine molecular shape.
  4. The nature of electrons (bonding or lone pairs) affects molecule shape.
  5. Lone pairs cause more repulsion than bond pairs.

Molecular Shapes

  • Linear: Two bonds, no lone pairs.
  • Trigonal Planar: Three bonds, no lone pairs.
  • Tetrahedral: Four bonds, no lone pairs.
  • Trigonal Bipyramidal: Five bonds, no lone pairs.
  • Octahedral: Six bonds, no lone pairs.

Bond Angles and Geometry

  • Water (H₂O): Bent shape due to lone pairs, Bond angle ~104.5°.
  • Ammonia (NH₃): Trigonal pyramidal shape, Bond angle ~107.3°.

Wedge and Dash Notation

  • Represents 3D molecular structures on 2D surfaces.
    • Wedge: Bonds coming out towards you.
    • Dash: Bonds going away from you.

Learning Tools

  • Tables and Diagrams: Useful for memorizing different molecular geometries.
  • Simulation Tools: Aid in visualizing molecular shapes and geometry.

Conclusion

  • Practice constructing Lewis and VSEPR structures.
  • Understand differences in molecular geometry and bond angles.

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