The Solid State — chemistry Class 12 Notes (CBSE & HBSE)
Free NCERT chemistry notes for The Solid State (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — The Solid State (CBSE & HBSE)
Classification of solids into crystalline and amorphous, crystal lattice and unit cells, packing efficiency, density calculations, and crystal defects in Class 12 Chemistry.
Classification and Types of Solids
The Solid State
Classification of Solids
Solids are substances with definite shape and volume due to strong intermolecular forces. Based on the arrangement of constituent particles, solids are classified as crystalline or amorphous.
Crystalline Solids: Have a regular, repeating three-dimensional arrangement of particles (crystal lattice). They are true solids with sharp melting points, anisotropy, and clean cleavage. Examples: NaCl, KCl, diamond, quartz, metals.
Amorphous Solids: Lack long-range order. Called pseudo-solids or super-cooled liquids. Isotropic, no sharp melting point. Examples: Glass, rubber, plastics, wax.
Types of Crystalline Solids
| Type | Particles | Binding Forces | Melting Point | Conductivity | Examples |
|---|---|---|---|---|---|
| Ionic | Cations + Anions | Electrostatic | High (600-3000 C) | Poor (solid), Good (melt) | NaCl, MgO, CaCl2 |
| Covalent | Atoms | Covalent bonds | Very high | Poor (except graphite) | Diamond, SiO2, SiC |
| Molecular | Molecules | Van der Waals, H-bonds | Low | Poor | Ice, CO2, glucose |
| Metallic | Metal ions + e- sea | Metallic bond | Variable | Good | Cu, Fe, Na, Mg |
Special Cases
Graphite: Covalent solid conducting electricity due to delocalized pi electrons between sp2 carbon layers. Soft due to weak Van der Waals between layers. Ice: Molecular solid with high mp (0 C) relative to other molecular solids due to extensive H-bonding (O-H...O).
Anisotropy vs Isotropy
Anisotropy in crystalline solids: different properties in different directions (different arrangement of particles). Isotropy in amorphous solids: same properties in all directions (random arrangement).
Bragg's Law
X-ray diffraction determines crystal structure. nλ = 2d sinθ where n = order, λ = wavelength, d = interplanar spacing, θ = angle.
Crystal Lattice, Unit Cells and Packing
Crystal Lattice and Unit Cells
Crystal Lattice (Space Lattice)
A crystal lattice is a regular, repeating three-dimensional arrangement of points in space. There are 14 Bravais lattices in 7 crystal systems.
Unit Cell
Smallest repeating structural unit that generates the complete crystal lattice by repetition in 3D.
Cubic Unit Cells:
| Unit Cell | Atoms/Cell | CN | Examples |
|---|---|---|---|
| Simple Cubic (SC) | 1 | 6 | Po |
| Body-Centred Cubic (BCC) | 2 | 8 | Na, K, Cr, W, Fe(alpha) |
| Face-Centred Cubic (FCC) | 4 | 12 | Cu, Ag, Au, Al, Ni |
FCC atom count: 8 corners x (1/8) + 6 faces x (1/2) = 1 + 3 = 4 BCC atom count: 8 corners x (1/8) + 1 body centre = 1 + 1 = 2
Packing Efficiency
Packing efficiency = (Volume of atoms / Volume of unit cell) x 100
| Crystal | Packing % | Void Space |
|---|---|---|
| SC | 52.4% | 47.6% |
| BCC | 68% | 32% |
| FCC/HCP | 74% | 26% |
Radii Relations
- SC: a = 2r
- BCC: 4r = sqrt(3) x a
- FCC: 4r = sqrt(2) x a
Density Formula
rho = (Z x M) / (a^3 x NA) Z = atoms/cell, M = molar mass, a = edge length, NA = Avogadro's number
Close Packing
- 2D square: CN = 4; 2D hexagonal: CN = 6
- ABAB stacking: HCP (hexagonal close packed)
- ABCABC stacking: CCP/FCC (cubic close packed)
Voids
Tetrahedral void: 4 spheres around it; r(void)/r(sphere) = 0.225; count = 2N Octahedral void: 6 spheres around it; r(void)/r(sphere) = 0.414; count = N
Radius Ratio Rules
| r+/r- | CN | Structure | Example |
|---|---|---|---|
| 0.225-0.414 | 4 | Tetrahedral | ZnS |
| 0.414-0.732 | 6 | Octahedral | NaCl |
| 0.732-1.000 | 8 | BCC | CsCl |
Crystal Defects and Properties
Crystal Defects
Types of Defects
Point defects are irregularities around individual lattice sites.
1. Stoichiometric Defects (maintain stoichiometry)
Vacancy defect: Some lattice sites are empty. Decreases density. Heating causes vacancies.
Interstitial defect: Extra particles in interstitial positions. Increases density. Non-ionic solids.
Frenkel Defect: Smaller ion (usually cation) leaves its site and occupies an interstitial position. Density unchanged. Ionic solids where r+/r- is small. Examples: AgCl, AgBr, ZnS, AgI
Schottky Defect: Equal numbers of cations and anions are missing. Density decreases. Ionic solids where r+ ≈ r-. Examples: NaCl, KCl, CsCl, KBr Note: AgBr shows BOTH Frenkel and Schottky defects.
Frenkel vs Schottky
| Property | Frenkel | Schottky |
|---|---|---|
| Displaced ion | Cation to interstitial | Cation and anion missing |
| Density | Unchanged | Decreases |
| Condition | r+ << r- | r+ ≈ r- |
| Examples | AgCl, ZnS | NaCl, KCl |
2. Non-Stoichiometric Defects (change stoichiometry)
Metal excess defect (F-centres): Extra metal cations present. Anionic vacancies trap electrons (F-centres from German Farbe = color). These electrons absorb visible light and give color to crystals. Examples: NaCl becomes yellow, ZnO becomes yellow on heating.
Metal deficiency defect: Less metal than stoichiometric proportion. Example: FeO (actually Fe0.95O), NiO, VO.
Electrical Properties (Band Theory)
| Property | Conductor | Semiconductor | Insulator |
|---|---|---|---|
| Band gap | 0 (overlapping) | 0.5-3 eV | > 3 eV |
| Examples | Metals | Si, Ge, GaAs | Diamond, glass |
n-type semiconductor: Doped with Group 15 element (As, P, Sb) - extra electrons conduct. p-type semiconductor: Doped with Group 13 element (Al, In, Ga) - electron holes conduct.
Magnetic Properties
- Diamagnetic: All electrons paired; repelled by magnetic field (NaCl, H2O)
- Paramagnetic: Unpaired electrons; attracted (O2, Cu2+, Cr3+)
- Ferromagnetic: Permanent magnetism, magnetic domains aligned (Fe, Co, Ni)
- Antiferromagnetic: Adjacent moments cancel (MnO, Cr2O3)
- Ferrimagnetic: Unequal opposite moments (Fe3O4, MgFe2O4)
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for The Solid State.