Molecular Shapes and Electron Domains

Overview

This lecture discussed how to determine the 3D shapes of molecules using electron domains, with examples including methane, ammonia, water, boron trifluoride, and hydrogen cyanide.

Determining Molecular Shape

• Lewis structures are 2D representations and do not show the true 3D shape of molecules.
• Electron domains refer to groups of electrons (bonds or lone pairs) around a central atom.
• Electron domains repel each other and arrange as far apart as possible in space.

Tetrahedral Electron Domain Examples

• Methane (CH₄): Carbon has four electron domains; shape is tetrahedral with bond angles of 109.5°.
• Ammonia (NH₃): Four electron domains (3 bonds, 1 lone pair); electron domain shape is tetrahedral, molecular shape is trigonal pyramidal; bond angle is ~107°.
• Water (H₂O): Four electron domains (2 bonds, 2 lone pairs); electron domain shape is tetrahedral, molecular shape is bent; bond angle is ~105°.

Trigonal Planar Electron Domain Example

• Boron trifluoride (BF₃): Boron has three electron domains (3 bonds, 0 lone pairs); both electron domain and molecular shape are trigonal planar with 120° bond angles.

Linear Electron Domain Example

• Hydrogen cyanide (HCN): Carbon has two electron domains (1 single, 1 triple bond); shape is linear with a 180° bond angle.

Comparing Electron Domain vs. Molecular Shape

• Electron domain shape includes all bonds and lone pairs on the central atom.
• Molecular shape considers only the positions of the atoms, not lone pairs.

Key Terms & Definitions

• Electron Domain — A group of electrons (single, double, triple bonds, or lone pairs) around a central atom.
• Tetrahedral — Shape with four domains distributed at 109.5° angles.
• Trigonal Planar — Shape with three domains in one plane at 120°.
• Linear — Shape with two domains at 180°.
• Trigonal Pyramidal — Molecular shape from tetrahedral electron domains with one lone pair.
• Bent — Molecular shape from tetrahedral electron domains with two lone pairs.

Action Items / Next Steps

• Refer to the lecture preview and review the summary table of molecular shapes.
• Draw the Lewis structures and determine the shape around the carbon atoms for: ethane (CH₃CH₃), ethene (CH₂=CH₂), and ethyne (CH≡CH).