Class 12 Chemistry - The Solid State
1. Classification of Solids
| Type | Particles | Forces | Examples |
|---|---|---|---|
| Molecular | Molecules | Van der Waals | Ice, Dry ice |
| Ionic | Ions | Electrostatic | NaCl, MgO |
| Metallic | Metal ions | Metallic bond | Fe, Cu, Ag |
| Covalent | Atoms | Covalent bond | Diamond, SiO₂ |
2. Crystal Lattice
Lattice = Regular arrangement of constituent particles in 3D
Unit Cell:
Smallest repeating unit that shows full symmetry
Types:
- Primitive - Particles at corners only
- Body-centered - Corners + center
- Face-centered - Corners + face centers
- End-centered - Corners + two face centers
3. Number of Atoms in Unit Cell
| Position | Contribution |
|---|---|
| Corner | 1/8 |
| Edge | 1/4 |
| Face | 1/2 |
| Body center | 1 |
Calculations:
- Simple Cubic: 8 × 1/8 = 1 atom
- BCC: 8 × 1/8 + 1 = 2 atoms
- FCC: 8 × 1/8 + 6 × 1/2 = 4 atoms
4. Packing Efficiency
$$\text{Packing Efficiency} = \frac{\text{Volume of atoms}}{\text{Volume of unit cell}} × 100$$
| Structure | Efficiency |
|---|---|
| Simple Cubic | 52.4% |
| BCC | 68% |
| FCC/HCP | 74% |
5. Defects in Solids
Point Defects:
- Vacancy Defect - Missing particle
- Interstitial Defect - Extra particle in void
- Schottky Defect - Equal cation & anion vacancies
- Frenkel Defect - Ion displaced to interstitial site
✔️ Important Formulas
- Density: ρ = (Z × M)/(a³ × NA)
- Edge length relation: a = 2r (simple), a = 4r/√3 (BCC), a = 2√2r (FCC)
- Coordination number: SC=6, BCC=8, FCC=12
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