Overview
This lecture introduces VSEPR theory to determine molecular shapes by considering electron pair repulsion around a central atom, focusing on bonded atoms and lone pairs.
VSEPR Theory Basics
- VSEPR stands for Valence Shell Electron Pair Repulsion.
- Electron pairs on a central atom repel each other and arrange as far apart as possible.
- Bond angles depend on the number and type of electron groups on the central atom.
Key Terminology
- "BA" (bonded atoms): Number of atoms bonded to the central atom.
- "LP" (lone pairs): Number of non-bonding electron pairs on the central atom.
Determining Molecular Geometry
- Count bonded atoms (BA) and lone pairs (LP) on the central atom, ignoring bonds on outer atoms.
- Double or triple bonds count as one bonded atom, not multiple.
- BA and LP are combined as "electronic groups" for determining geometry.
Common Geometries and Bond Angles
- 2 BA, 0 LP: Linear geometry, 180° bond angle (e.g., beryllium dihydride).
- 2 BA, 1 LP: Bent geometry, bond angle < 120° (e.g., sulfur dioxide).
- 3 BA, 0 LP: Trigonal planar geometry, 120° bond angle (e.g., boron trihydride).
- 3 BA, 1 LP: Trigonal pyramidal geometry, bond angle < 109.5° (e.g., ammonia).
- 4 BA, 0 LP: Tetrahedral geometry, 109.5° bond angle (e.g., carbon tetrafluoride).
- 2 BA, 2 LP: Bent geometry, bond angle < 109.5° (e.g., water).
Drawing Structures
- "Shape drawing" shows correct bond angles, unlike Lewis structure which focuses on connectivity.
- Only use bond angles of 180°, 120°, or 109.5° in shape drawings as per VSEPR predictions.
- Always start with the Lewis structure to determine BA and LP.
Examples
- Water: 2 BA, 2 LP ⇒ Bent geometry, bond angle < 109.5°.
- Formaldehyde: 3 BA, 0 LP ⇒ Trigonal planar geometry, bond angles = 120°.
- Carbon tetrafluoride: 4 BA, 0 LP ⇒ Tetrahedral geometry, bond angles = 109.5°.
- Phosphorus trihydride: 3 BA, 1 LP ⇒ Trigonal pyramidal geometry, bond angle < 109.5°.
- Beryllium diiodide: 2 BA, 0 LP ⇒ Linear geometry, bond angle = 180°.
Key Terms & Definitions
- VSEPR — Theory explaining molecular shapes based on electron pair repulsion.
- Bonded Atom (BA) — An atom directly connected to the central atom.
- Lone Pair (LP) — Non-bonding electron pair on the central atom.
- Electronic Group — Any bonded atom or lone pair around a central atom.
Action Items / Next Steps
- Practice determining BA and LP for central atoms using Lewis structures.
- Use the VSEPR table to assign molecular geometry to given compounds.
- Draw both Lewis structures and shape drawings for practice molecules.