# Molecular and Ionic Compound Structure
AP Chemistry Unit 2 covers how atoms bond to form compounds. You need to draw Lewis structures, predict molecular geometry with VSEPR, understand bond polarity, and explain ionic lattice properties.
1. Lewis Structures
Steps
- Count total valence electrons (adjust for charges)
- Draw single bonds to connect atoms
- Distribute remaining electrons as lone pairs
- Form multiple bonds if needed (central atom < 8 electrons)
Special Cases
- Resonance: multiple equivalent Lewis structures (e.g. O₃, CO₃²⁻)
- Expanded octet: Period 3+ elements (e.g. PCl₅, SF₆)
- Incomplete octet: BF₃ (6e⁻ around B)
Formal Charge
Best structure minimises formal charges and places negative FC on more electronegative atom.
2. VSEPR Theory
| Electron Groups | Bonding | Lone | Shape | Angle |
|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° |
| 3 | 3 | 0 | Trigonal planar | 120° |
| 3 | 2 | 1 | Bent | <120° |
| 4 | 4 | 0 | Tetrahedral | 109.5° |
| 4 | 3 | 1 | Trigonal pyramidal | <109.5° |
| 4 | 2 | 2 | Bent | <109.5° |
| 5 | 5 | 0 | Trigonal bipyramidal | 90°/120° |
| 6 | 6 | 0 | Octahedral | 90° |
3. Bond and Molecular Polarity
Bond polarity: ΔEN > 0.4 → polar bond; ΔEN > 1.7 → ionic
Molecular polarity: polar bonds + asymmetric shape → polar molecule
- Symmetric molecules (e.g. CO₂, BF₃, CCl₄) → nonpolar
- Asymmetric (e.g. H₂O, NH₃, CHCl₃) → polar
4. Ionic Compounds
Lattice energy: energy released when gaseous ions form a solid lattice.
- Larger with: higher ion charges, smaller ionic radii
Properties: high MP, conduct when molten/dissolved, brittle, crystalline.
5. Metallic Bonding
Sea of delocalised electrons around positive metal ions. Properties: conductors, malleable, lustrous, generally high MP.
6. Practice Questions
- Draw the Lewis structure for XeF₄ and predict its shape.
- Explain why CO₂ is nonpolar but SO₂ is polar.
- Rank NaCl, MgO, and KBr in order of increasing lattice energy. Explain.
- Draw resonance structures for NO₃⁻.
- Predict whether PF₃ is polar or nonpolar.
Want to check your answers and get step-by-step solutions?
Summary
- Lewis structures: count electrons, satisfy octet, use formal charges
- VSEPR: electron group geometry determines molecular shape
- Polarity: bond + molecular geometry
- Ionic lattice energy depends on charge and radius
- Metallic bonding: delocalised electrons → conductivity, malleability