Overview
This lecture explains VSEPR theory and how it predicts the molecular geometry and bond angles of molecules by considering electron repulsion.
VSEPR Theory Basics
- VSEPR stands for Valence Shell Electron Pair Repulsion.
- Electron pairs around a central atom repel each other and arrange as far apart as possible, determining molecular shape.
Linear Molecular Geometry
- Linear molecules have a central atom bonded to two atoms, forming a straight line.
- Example: BeCl₂, CO₂, and triiodide ion (I₃⁻).
- Bond angle in linear geometry is 180°.
Trigonal Planar Geometry
- Central atom is bonded to three atoms in a flat, triangular arrangement.
- Examples: BH₃, COCl₂, AlCl₃.
- Bond angle is 120° (360° ÷ 3).
- No lone pairs on the central atom.
Tetrahedral Geometry
- Central atom bonded to four atoms in 3D space.
- Examples: CH₄ (methane), SiF₄ (silicon tetrafluoride).
- Bond angle is approximately 109.5°.
Trigonal Pyramidal Geometry
- Central atom bonded to three atoms and has one lone pair.
- Examples: NH₃ (ammonia), PH₃.
- Bond angle is about 107° (slightly less than tetrahedral due to lone pair).
- Common with Group 5A elements.
Bent Molecular Geometry
- Central atom has two bonded atoms and one or two lone pairs, creating a "bent" shape.
- Examples: H₂O (water, two lone pairs), SO₂ (sulfur dioxide, one lone pair).
- Water's bond angle is 104.5°, SO₂’s is less than 120°.
- Lone pairs decrease bond angle compared to similar structures without lone pairs.
Comparing Molecular Geometries & Bond Angles
- Tetrahedral: 4 atoms, 109.5° (CH₄).
- Trigonal pyramidal: 3 atoms + 1 lone pair, ~107° (NH₃).
- Bent: 2 atoms + 2 lone pairs, 104.5° (H₂O).
- Trigonal planar: 3 atoms, 120° (BH₃).
- Bent (from trigonal planar): 2 atoms + 1 lone pair, <120° (SO₂).
Key Terms & Definitions
- VSEPR Theory — Model predicting molecular shape based on electron pair repulsion.
- Lone Pair — A pair of valence electrons not involved in bonding.
- Bond Angle — Angle between two bonds at the central atom.
Action Items / Next Steps
- Review the example molecules and practice drawing their Lewis structures.
- Memorize the common bond angles and molecular geometries for upcoming tests.