Geometry of Molecules Lecture Notes

Introduction to Molecular Geometry

• Molecular geometry: 3D structure/arrangement of atoms in a molecule.
• Importance: Determines polarity, reactivity, phase of matter, color, magnetism, biological activity.
• Lewis Electron Dot Structure: First step in predicting molecule shapes.
  • Identifies bond pairs and lone pairs.
  • Basis for Valence-Shell Electron-Pair Repulsion (VSEPR) theory.

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

• VSEPR Theory: Predicts molecular shape based on electron pair repulsion.
• Electron group: Can be electron pair, lone pair, single unpaired electron, double bond, triple bond.
• Electron-group geometry vs. Molecular geometry:
  • Electron-group geometry: Based on number of electron groups.
  • Molecular geometry: Depends on number of electron groups and lone pairs.

Electron-Group Geometries

• 2 groups: Linear
• 3 groups: Trigonal-planar
• 4 groups: Tetrahedral
• 5 groups: Trigonal-bipyramidal
• 6 groups: Octahedral

VSEPR Notation

• AXn: No lone pairs; n = number of bonds.
• AXnEx: x = number of lone pairs; combines with AXn notation.

Geometry of Molecules Chart

• Lists electron-group and molecular geometries with VSEPR notation, ideal bond angles, and examples for 2 to 6 electron groups.

Examples of Molecular Geometry

• H2O: Tetrahedral electron-group geometry, bent molecular shape.
• CO2: Linear electron-group and molecular geometry.

Molecules with Multiple Central Atoms

• VSEPR applies to molecules with more than one central atom (e.g., Butane).
• Break down molecules to determine shape of each section.

Bond Angles

• Bond angles: Angles between adjacent lines representing bonds.
• Ideal bond angles minimize repulsion, e.g., tetrahedral = 109.5°.

Steps to Determine Molecular Shape (Using VSEPR Theory)

1. Draw Lewis Structure.
2. Count electron groups (bonds and lone pairs).
3. Name electron-group geometry.
4. Determine molecular geometry based on lone pairs.

Dipole Moments and Polarity

• Dipole moment: Product of partial charge and distance; indicates polarity.
• Polarity determination:
  • Net dipole = polar.
  • Symmetric structure = non-polar.
  • Rules for determining polarity:
    • No lone pairs = non-polar.
    • Linear/square planar = non-polar.
    • Different terminal atoms = polar.

Example Problems

Part I: Draw Lewis Structure, name shape, determine polarity/dipole moment.

• HClO3, SO3, PCl4, C2H4, SnCl3⁻

Part II: Name shape, determine if polar/non-polar.

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References

1. Petrucci, R. H., et al., General Chemistry, Ninth Edition.
2. Mason, P. E. & Brady, J. W., Tetrahedrality in Liquid Water.
3. Wiberg, K. B., Inverted geometries at carbon.
4. Molecular Geometries, Chemistry Foundations and Applications.