CBSE Class 12 Chemistry: Solid State — Complete Notes 2026

Types of Solids

• Crystalline: regular, repeating arrangement; sharp melting point; anisotropic (NaCl, diamond)
• Amorphous: irregular arrangement; softens over temperature range; isotropic; glass, rubber, plastics
• Four types of crystalline: ionic (NaCl), covalent/network (diamond, SiO₂), metallic (Cu, Fe), molecular (ice, CO₂)

Crystal Systems and Unit Cells

• Unit cell: smallest repeating unit of crystal lattice; 7 crystal systems (cubic, orthorhombic, etc.)
• Cubic unit cells: simple cubic (SC), body-centred cubic (BCC), face-centred cubic (FCC/CCP)
• FCC: atoms at 8 corners (8×⅛) + 6 faces (6×½) = 4 atoms per unit cell; most efficiently packed

Packing Efficiency

• Simple cubic: 52.4% (least efficient); BCC: 68%; FCC/CCP: 74% (most efficient)
• HCP: also 74%; AB layer arrangement; both FCC and HCP are close-packed structures
• Coordination number: SC = 6; BCC = 8; FCC = 12; higher coordination = more efficient packing

Density Calculation

• ρ = (Z × M) / (Nₐ × a³) where Z = atoms per unit cell, M = molar mass, a = edge length, Nₐ = Avogadro number
• Example: NaCl (FCC): Z = 4 formula units, a = 564 pm → calculate density
• Can rearrange to find edge length from density, or find molar mass

Crystal Defects

• Point defects: Schottky (cation + anion vacancy; ionic crystals); Frenkel (cation moved to interstitial site; smaller cations; AgCl)
• Interstitial defect: extra atoms fit in gaps; increases density; Substitutional: foreign atom replaces host atom (alloys)
• F-centres: anion vacancies occupied by electrons; causes colour (NaCl becomes yellow, KCl becomes lilac)

Electrical Properties

• Conductors: overlapping conduction and valence bands; metals
• Insulators: large band gap (>3 eV); no electrons in conduction band at normal temperature; diamond
• Semiconductors: small band gap (~1 eV); conductivity increases with temperature; Si, Ge; n-type (extra electrons), p-type (holes)

CBSE Board Focus

• Solid State: 5–6 marks; packing efficiency, density calculation, Schottky vs Frenkel defects
• Numerical: find edge length from density or vice versa; find atoms per unit cell
• Semiconductors: p-type vs n-type doping; draw band gap diagrams