Solid State Chemistry: JEE Main Complete Guide

Solid State is a Physical Chemistry chapter that JEE Main tests with one to two questions per session — almost always from unit cell calculations, packing efficiency, and crystal defects. The chapter is formula-compact: mastering the five key unit cell results and the defect classification covers 90% of all JEE Main questions in this chapter. This guide covers the full syllabus systematically.

Crystal Systems and Unit Cells

Crystalline solids have a regular, ordered arrangement of particles. The unit cell is the smallest repeating unit. Three cubic systems: Simple Cubic (SC), Body-Centred Cubic (BCC), and Face-Centred Cubic (FCC, also called cubic close-packed CCP). For each: SC has 1 particle per unit cell (8 corner × 1/8 each). BCC has 2 particles (8 corner × 1/8 + 1 body centre × 1). FCC has 4 particles (8 corner × 1/8 + 6 face × 1/2). JEE Main frequently asks: how many particles per unit cell in a given crystal structure?

Coordination number (CN): SC: 6; BCC: 8; FCC: 12. Edge length and atomic radius: SC: a = 2r. BCC: 4r = a√3. FCC: 4r = a√2. Packing efficiency: SC: π/6 = 52.4%. BCC: π√3/8 = 68%. FCC/HCP: π/(3√2) = 74.05%. JEE Main tests packing efficiency directly and as a basis for void fraction calculation. For the Chemical Bonding context of ionic crystals, see our Chemical Bonding Guide.

Ionic Crystals and Radius Ratio

In ionic crystals, cations fit into voids created by anion packing. Voids: Tetrahedral voids (4 per FCC unit cell, located at body quarter-diagonals). Octahedral voids (4 per FCC unit cell, at edge centres and body centre). Radius ratio rules for CN: CN=4 (tetrahedral): 0.225 ≤ r₊/r₋ < 0.414. CN=6 (octahedral): 0.414 ≤ r₊/r₋ < 0.732. CN=8 (cubic): r₊/r₋ ≥ 0.732. Common ionic crystal structures: NaCl (rock salt) — FCC with CN=6 for both ions. CsCl — SC-like with CN=8. ZnS (zinc blende) — FCC with tetrahedral voids, CN=4. Take a free mock test on Solid State and Solutions to practise these calculation types.

Crystal Defects

Stoichiometric defects (point defects that maintain stoichiometry): Schottky defect — equal numbers of cation and anion vacancies. Decreases density. Common in ionic crystals with similar-sized ions (NaCl, KBr). Frenkel defect — cation moves to interstitial position, creating vacancy. Density unchanged. Common when cation is smaller than anion (AgBr, AgCl). Both defects are intrinsic/thermodynamic — present even in pure crystals.

Non-stoichiometric defects: Metal excess defect (extra metal ions in interstitial positions, with trapped electrons for electrical neutrality — gives F-centres, causing colour). Metal deficiency defect (fewer metal ions, with higher-valence ions to maintain charge balance). Impurity defects: substitution or interstitial foreign atoms. JEE Main tests: which defect is present in AgBr (Frenkel), why does NaCl show yellow colour when heated in Na vapour (metal excess defect / F-centres), which defect changes density (Schottky decreases density).

Electrical Properties of Solids

Conductors (metals): band gap = 0 (conduction band and valence band overlap). Insulators: large band gap (>3 eV). Semiconductors: small band gap (<3 eV). Intrinsic semiconductors: pure Si or Ge, conductivity increases with temperature (more electrons promoted to conduction band). Extrinsic semiconductors: doped. n-type: doped with pentavalent element (P, As in Si) — extra electrons are charge carriers. p-type: doped with trivalent element (B, Al in Si) — holes are charge carriers. JEE Main tests these semiconductor doping concepts and the p-n junction operation qualitatively.

Magnetic Properties

Diamagnetic: weakly repelled by magnetic field, all electrons paired (NaCl, benzene). Paramagnetic: weakly attracted, unpaired electrons (O₂, Cu²⁺). Ferromagnetic: strongly attracted, permanent magnetism (Fe, Co, Ni). Ferrimagnetic: weakly magnetic, spins partially aligned antiparallel but unequal (Fe₃O₄). Antiferromagnetic: equal and opposite spins, no net magnetism (MnO). JEE Main tests the classification of a given material based on its description.

Exam Strategy

Solid State questions are either formula-based (unit cell calculations, packing efficiency) or memory-based (defect classification, semiconductor doping). For formula questions: keep the five unit cell results (CN, particles per cell, packing efficiency, edge-radius relation) on a flash card. For memory questions: make a comparison table for Schottky vs. Frenkel defects. Two focused study sessions of 3 hours each cover the entire Solid State chapter. For the Ionic Equilibrium chapter that connects to this, see our Ionic Equilibrium Guide. Upgrade for ₹149/month for 100+ Solid State questions with full unit cell calculation solutions.