Molecular Geometry and VSEPR Model

Overview

This lecture introduces molecular geometry, explains how to predict molecular shape using the VSEPR model, and connects electron domains to hybridization and geometry.

VSEPR Model & Electron Domains

• The VSEPR model (Valence Shell Electron Pair Repulsion) predicts molecular shapes based on electron cloud repulsion.
• Electron domains are regions of electron density (bonds or lone pairs) around a central atom.
• Electron clouds repel each other, causing atoms to position themselves as far apart as possible.

Hybridization and Geometry Types

• The type of hybridization depends on the number of electron domains:
  • 2 domains: sp hybridization (linear geometry, 180° bond angle).
  • 3 domains: sp2 hybridization (trigonal planar geometry, 120° bond angle).
  • 4 domains: sp3 hybridization (tetrahedral geometry, 109.5° bond angle).
  • 5 domains: sp3d hybridization (trigonal bipyramidal geometry, 90° and 120° bond angles).
  • 6 domains: sp3d2 hybridization (octahedral geometry, 90° bond angles).
• The number of electron domains indicates the hybridization (count s and p/d characters as needed).

Lone Pairs & Molecular Geometry

• Lone pairs also count as electron domains and affect molecular geometry.
• Ammonia (NH3): Nitrogen has three bonds and one lone pair (sp3), giving trigonal pyramidal geometry.
• Water (H2O): Oxygen has two bonds and two lone pairs (sp3), giving bent geometry.
• Lone pairs occupy more space and alter bond angles compared to simple bonded structures.

Applying VSEPR: Steps

• Draw the Lewis dot structure to visualize the molecule.
• Count the total electron domains (bonds and lone pairs) around the central atom.
• Use this number to determine hybridization and electron domain geometry.
• Consider lone pairs to decide the actual molecular geometry.

Key Terms & Definitions

• VSEPR model — a method to predict molecular shape based on electron repulsions.
• Electron domain — any region of electron density (bond or lone pair) around a central atom.
• Hybridization — mixing atomic orbitals (s, p, d) to form new orbitals for bonding.
• Molecular geometry — the 3D arrangement of atoms in a molecule.

Action Items / Next Steps

• Practice drawing Lewis dot structures and counting electron domains for various molecules.