Lecture on Molecular Geometry

Importance of Molecular Geometry

• Understanding the 3D arrangement of molecules is crucial for understanding their chemical behavior.
• VSEPR Model (Valence Shell Electron Pair Repulsion) is used to predict molecular shapes.

VSEPR Model Basics

• Atoms are surrounded by electron clouds, which repel each other due to like charges.
• Molecules adopt geometries that minimize repulsion by maximizing the distance between electron clouds (or domains).

Hybridization and Electron Domain Geometry

Carbon Dioxide (CO2)

• Electron Domains: 2
• Hybridization: sp
• Geometry: Linear (180° bond angle)

Boron Trifluoride (BF3)

• Electron Domains: 3
• Hybridization: sp2
• Geometry: Trigonal Planar (120° bond angles)

Methane (CH4)

• Electron Domains: 4
• Hybridization: sp3
• Geometry: Tetrahedral (109.5° bond angles)

Trigonal Bipyramidal Geometry

• Electron Domains: 5
• Hybridization: sp3d
• Geometry: Trigonal Bipyramidal (90° and 120° bond angles)

Octahedral Geometry

• Electron Domains: 6
• Hybridization: sp3d2
• Geometry: Octahedral (90° bond angles)

Lone Pairs and Molecular Geometry

• Lone pairs are considered electron domains.
• They affect molecular geometry differently than bonded atoms.

Ammonia (NH3)

• Electron Domains: 4 (3 bonds, 1 lone pair)
• Hybridization: sp3
• Geometry: Trigonal Pyramidal (due to lone pair)

Water (H2O)

• Electron Domains: 4 (2 bonds, 2 lone pairs)
• Hybridization: sp3
• Geometry: Bent (due to two lone pairs)

Key Differences in Similar Molecules

• CO2 (linear) vs. H2O (bent): Differences in geometry despite having similar atoms due to lone pairs on oxygen in water.

Summary

• Count electron domains (covalent bonds plus lone pairs) to determine hybridization.
• Hybridization indicates electron domain geometry.
• Molecular geometry varies within electron domain geometry depending on lone pairs.
• Use Lewis dot structures to count electron domains and predict geometries.

Additional Notes

• Comprehension check suggested.
• Encouragement to subscribe for more tutorials and contact via email: [email protected].