Introduction
Solids have definite volume/shape; particles in fixed positions with regular order. Properties depend on nature/arrangement of constituents.
Crystalline solids: Long-range order, sharp MP, anisotropic.
Amorphous solids: Short-range order, no sharp MP, isotropic.
Types of Solids
Crystalline
Definite MP/heat of fusion
Rigid, plane cleavage
Anisotropic, symmetry
Amorphous
No definite MP/fusion
Irregular cleavage
Isotropic, no symmetry
Examples
| Quartz (Crystalline SiO₂) | Amorphous SiO₂ |
|---|---|
| Network solid, sharp MP 1710°C | Polymeric, no sharp MP |
| Diamond: Hard, insulator, density 3.5 | Graphite: Soft, conductor, density 2.3 |
Classification of Crystalline Solids
| Type | Bonding | Examples | Nature | MP/BP | Conductivity |
|---|---|---|---|---|---|
| Ionic | Coulombic | NaCl, ZnS | Hard, brittle | High | Molten/aq |
| Covalent | Sharing | Diamond, Quartz | Very hard | Very high | Insulator (graphite conducts) |
| Molecular | Intermolecular | I₂, CO₂ | Soft | Low | Insulator |
| Metallic | Metallic | Cu, Au | Ductile | High | Conductor |
| Atomic | Dispersion | Noble gases | Soft | Very low | Poor |
Crystallography
Cube: 23 symmetry elements (9 planes, 13 axes, 1 center).
Laws
Interfacial Angles
Constant (Steno's)
Rational Indices
Weiss indices na, nb, nc
Symmetry
Same elements
Miller indices: h = a/intercept_x, etc. d_{hkl} = a / √(h²+k²+l²)
Space Lattice & Unit Cell
14 Bravais lattices, 7 crystal systems.
Cubic Analysis
| Cell | Atoms/Unit Cell | CN |
|---|---|---|
| SC | 1 | 6 |
| BCC | 2 | 8 |
| FCC | 4 | 12 |
ρ = ZM / (N₀ a³)
PF: SC=52%, FCC=74%, BCC=68%
X-Ray & Packing
Bragg: nλ = 2d sinθ
Close Packing
| Type | Arrangement | Space | CN |
|---|---|---|---|
| HCP | ABAB | 74% | 12 |
| CCP (FCC) | ABCABC | 74% | 12 |
| BCC | Not close | 68% | 8 |
Voids: Trigonal r/R=0.155
Key Points
JEE Essentials
- Crystalline: anisotropic, sharp MP
- FCC: max packing 74%
- CN: FCC=12, BCC=8, SC=6
- Bragg for structure
- 7 systems, 14 lattices
JEE Weightage
2-3 questions typical.
WeightageHigh