VSEPR Theory and Molecular Geometry

Overview

This lesson covers VSEPR theory as the foundation for predicting molecular geometry, including the electron domain and molecular geometries for molecules with 2–6 electron domains, and how lone pairs affect these geometries.

VSEPR Theory Basics

• VSEPR (Valence Shell Electron Pair Repulsion) theory states that electron groups around a central atom arrange to minimize repulsion.
• Electron domains include bonded atoms (single, double, or triple bonds each count as one domain) and lone pairs.
• The shape of a molecule depends on the number of electron domains around the central atom.

Electron Domain Geometries & Bond Angles

• 2 domains: Linear geometry, 180° bond angle.
• 3 domains: Trigonal planar geometry, 120° bond angle.
• 4 domains: Tetrahedral geometry, 109.5° bond angle.
• 5 domains: Trigonal bipyramidal geometry, 90°, 120°, and 180° bond angles.
• 6 domains: Octahedral geometry, 90° and 180° bond angles.

Molecular Geometries (Based on Lone Pairs)

• If all electron domains are bonding, molecular geometry = electron domain geometry.
• Lone pairs change molecular geometry but not the electron domain geometry.

Three Domains (Trigonal Planar):

• 0 lone pairs: trigonal planar.
• 1 lone pair: bent (<120° bond angle).

Four Domains (Tetrahedral):

• 0 lone pairs: tetrahedral.
• 1 lone pair: trigonal pyramidal (<109.5°).
• 2 lone pairs: bent (<109.5°).

Five Domains (Trigonal Bipyramidal):

• 0 lone pairs: trigonal bipyramidal.
• 1 lone pair: seesaw.
• 2 lone pairs: T-shaped.
• 3 lone pairs: linear.

Six Domains (Octahedral):

• 0 lone pairs: octahedral.
• 1 lone pair: square pyramidal.
• 2 lone pairs: square planar.

Lone Pair & Multiple Bond Effects

• Lone pairs create greater repulsion, decreasing bond angles slightly.
• Double and triple bonds may increase repulsion, slightly altering bond angles.

Determining Geometry from Lewis Structures

• Count electron domains (bonded atoms + lone pairs) on central atom.
• Assign electron domain geometry, then molecular geometry based on bonding/non-bonding domains.

Key Terms & Definitions

• VSEPR Theory — Explains molecular shapes based on electron pair repulsion.
• Electron Domain — Region where electrons are found (bonding or lone pair).
• Molecular Geometry — Arrangement of only atoms (not lone pairs) around the central atom.
• Bond Angle — Angle between two bonds in a molecule.

Action Items / Next Steps

• Memorize all electron domain and molecular geometries, including characteristic bond angles.
• Practice drawing Lewis structures to determine electron domains and molecular geometry.