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Solid State

Crystal structures, unit cells, packing and defects

Key Concepts
  • Crystalline solids have long-range order; amorphous solids lack it
  • Unit cell types: simple cubic (Z=1), BCC (Z=2), FCC (Z=4)
  • Packing efficiency: FCC/HCP = 74%, BCC = 68%, SC = 52%
  • Schottky defect: missing ions (density decreases); Frenkel: ion displaced to interstitial (density unchanged)
  • Semiconductors: intrinsic (pure), n-type (donor impurity), p-type (acceptor impurity)
Important Formulae
FCC relation (a and r) 4r = a√2
BCC relation (a and r) 4r = a√3
Density of crystal ρ = ZM / (N_A · a³)
Quick Tips
  • Coordination number: SC = 6, BCC = 8, FCC = 12.
  • Ionic solids conduct electricity in molten state or aqueous solution, not in solid state.
  • CsCl structure: Cs⁺ in body centre, Cl⁻ at corners (coordination number 8:8).
Sample Practice Questions

  1. Ferromagnetic materials below Curie temperature:

    • Repel magnetic fields
    • Have permanent magnetic dipoles aligned in domains
    • Have random dipoles
    • Show no magnetic property
    Show answer

    Answer: Have permanent magnetic dipoles aligned in domains

  2. Radius ratio for octahedral void:

    • 0.155 to 0.225
    • 0.225 to 0.414
    • 0.414 to 0.732
    • 0.732 to 1
    Show answer

    Answer: 0.414 to 0.732

  3. In a face-centred cubic (FCC) unit cell, number of atoms per unit cell:

    • 1
    • 2
    • 4
    • 8
    Show answer

    Answer: 4

  4. Ionic solids have high melting points because of:

    • Covalent bonds
    • Van der Waals forces
    • Strong electrostatic (ionic) interactions
    • Hydrogen bonds
    Show answer

    Answer: Strong electrostatic (ionic) interactions

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Practice Questions

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