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Molecular Shapes and Electron Domains

Sep 8, 2025

Overview

This lecture explains how to determine molecular shapes using electron domain theory, including examples and the relationship between electron domains and bond angles.

Limitations of Lewis Structures

  • Lewis structures are 2D representations and do not show the actual 3D shape of molecules.
  • Molecular shape depends on electron domain distribution, not just the drawn structure.

Electron Domains and Molecular Geometry

  • Electron domains (groups of electrons from bonds or lone pairs) determine molecule shape.
  • Domains arrange as far apart as possible in 3D space for stability.

Examples of Molecular Shapes

Methane (CHâ‚„)

  • Carbon has four electron domains (four single bonds) arranged tetrahedrally.
  • Molecular shape: Tetrahedral.
  • Bond angle: 109.5°.

Ammonia (NH₃)

  • Nitrogen has four electron domains (three bonds, one lone pair).
  • Electron domains are tetrahedral, but molecular shape is trigonal pyramidal.
  • Bond angle: ~107° (slightly less than 109.5° due to lone pair).

Water (Hâ‚‚O)

  • Oxygen has four electron domains (two bonds, two lone pairs).
  • Electron domains are tetrahedral; molecular shape is bent.
  • Bond angle: ~105° (smaller than 109.5° due to two lone pairs).

Boron Trifluoride (BF₃)

  • Boron has three electron domains (three single bonds).
  • Electron domains and molecular shape are trigonal planar.
  • Bond angle: 120°.

Hydrogen Cyanide (HCN)

  • Carbon has two electron domains (one single bond, one triple bond).
  • Electron domains and molecular shape are linear.
  • Bond angle: 180°.

Electron Domain vs. Molecular Shape

  • Electron domain shape may differ from molecular shape because lone pairs are not counted as atoms.
  • Types: Tetrahedral, trigonal planar, and linear.

Key Terms & Definitions

  • Electron Domain — A group of electrons (bond or lone pair) around a central atom.
  • Tetrahedral — Four domains, 3D shape like a tripod plus top; bond angle 109.5°.
  • Trigonal Planar — Three domains spread in a plane, bond angle 120°.
  • Linear — Two domains in a straight line, bond angle 180°.
  • Lone Pair — Non-bonding pair of electrons on an atom.

Action Items / Next Steps

  • Review the lecture preview and summary table for molecular shapes on Blackboard.
  • Practice: Draw Lewis structures and determine the shapes for ethane (CH₃CH₃), ethene (CHâ‚‚=CHâ‚‚), and ethyne (CH≡CH).
  • Prepare to discuss these molecules in the next class.

Full transcript