Chapter 4 Section 6: Molecular Geometry and Polarity

Objective

• Learning Goal: Predict the structures of small molecules using Valence Shell Electron Pair Repulsion (VSEPR) theory.

Overview

• VSEPR Theory: Used to predict the geometry of molecules based on electron pair repulsions.
• First Step: Draw the Lewis structure for the molecule.
• Practice Molecules: Water, nitrite, carbon tetrachloride, iodine pentafluoride, carbon dioxide, phosphorus pentachloride, alanine.

Steps for Predicting Geometry

1. Draw Lewis Structure: Visual representation of molecules showing all atoms, bonds, and lone pairs.
2. Count Regions of Electron Density: Includes single bonds, double bonds, triple bonds, and lone pairs.
3. Determine Electron Pair Geometry: Use the number of electron density regions to find geometry.
4. Determine Molecular Geometry:
   • Use lone pairs to modify electron pair geometry.
   • Lone pairs are not included in molecular geometry shape.

Examples

Water (H2O)

• Lewis Structure: Two hydrogen atoms bonded to oxygen with two lone pairs on oxygen.
• Electron Pair Geometry: Tetrahedral due to four regions of density.
• Molecular Geometry: Bent, with bond angle less than 109°.

Nitrite (NO2^-)

• Resonance: Two possible structures, real shape is an average.
• Geometry: Trigonal planar electron pair geometry, bent molecular geometry with bond angle less than 120°.

Carbon Tetrachloride (CCl4)

• Geometry: Tetrahedral electron pair and molecular geometry, bond angles 109°.
• Drawing: Use dashes and wedges to represent three-dimensional shape.

Iodine Pentafluoride (IF5)

• Hypervalent: More than eight electrons around central atom.
• Geometry: Octahedral electron pair geometry, square pyramidal molecular geometry.
• Bond Angles: Less than 90° due to lone pair.

Carbon Dioxide (CO2)

• Geometry: Linear with bond angle of 180°.

Phosphorus Pentachloride (PCl5)

• Hypervalent: Five regions of electron density.
• Geometry: Trigonal bipyramidal, axial and equatorial positions with 90° and 120° bond angles.

Alanine

• Multiple Central Atoms: Consider local geometry around each central atom.
• Example Atoms:
  • Carbon (1): Tetrahedral.
  • Nitrogen: Trigonal pyramidal, bond angle less than 109°.
  • Carbon (2): Trigonal planar.

Drawing Tips

• 3D Representation: Use wedges and dashes to show atoms coming in and out of the plane.
• Resonance Structures: Understand partial bond characteristics and average geometry.

Conclusion

• Practice: Essential for mastering VSEPR predictions, use textbook problems and online tools for additional exercises.