Molecular Geometry and VSEPR Model

Overview

This lecture introduces molecular geometry and the VSEPR model, explaining how electron domains determine molecular shapes, bond angles, and hybridization.

VSEPR Model and Electron Domains

• VSEPR stands for Valence Shell Electron Pair Repulsion, used to predict molecular shape.
• Electron clouds around atoms repel each other, causing atoms to arrange to minimize repulsion.
• Electron domains include both covalent bonds and lone pairs around a central atom.

Hybridization and Geometry Types

• Number of electron domains equals number of hybridized orbitals used by the central atom.
• 2 electron domains: sp hybridization, linear geometry, 180° bond angle (e.g., CO₂).
• 3 electron domains: sp² hybridization, trigonal planar geometry, 120° bond angles (e.g., BF₃).
• 4 electron domains: sp³ hybridization, tetrahedral geometry, 109.5° bond angles (e.g., CH₄).

Higher Electron Domains and Bond Angles

• 5 electron domains: sp³d hybridization, trigonal bipyramidal geometry, 90° and 120° bond angles.
• 6 electron domains: sp³d² hybridization, octahedral geometry, 90° bond angles.

Effects of Lone Pairs on Molecular Geometry

• Lone pairs also count as electron domains but occupy more space than bonds.
• Ammonia (NH₃): 3 bonds + 1 lone pair, sp³ hybridization, trigonal pyramidal geometry.
• Water (H₂O): 2 bonds + 2 lone pairs, sp³ hybridization, bent geometry.

Assigning Molecular Geometry

• Draw the correct Lewis dot structure to identify electron domains.
• Count total electron domains to determine hybridization and electron domain geometry.
• Different molecular geometries arise when lone pairs replace bonded atoms.

Key Terms & Definitions

• VSEPR Model — Theory that predicts molecular shapes based on electron pair repulsion.
• Electron Domain — Region where electrons are likely found, including bonds and lone pairs.
• Hybridization — Mixing atomic orbitals to form new orbitals for bonding.
• Linear Geometry — Shape with 180° bond angle, two electron domains.
• Trigonal Planar — Shape with 120° bond angles, three electron domains.
• Tetrahedral — Shape with 109.5° bond angles, four electron domains.
• Trigonal Bipyramidal — Shape with 90° and 120° bond angles, five electron domains.
• Octahedral — Shape with 90° bond angles, six electron domains.

Action Items / Next Steps

• Practice drawing Lewis dot structures and counting electron domains.
• Review and memorize common molecular geometries and bond angles.