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
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Ionic solids have high melting points because of:
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Answer: Strong electrostatic (ionic) interactions
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The number of tetrahedral voids in an FCC unit cell:
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Answer: 8
-
The unit cell of NaCl is:
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Answer: FCC of Cl⁻ with Na⁺ in octahedral holes
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Number of atoms in a BCC unit cell:
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Answer: 2
Practice Questions
Practice 20 randomly selected NEET questions on Solid State. Answers are revealed after each question.
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What are the key concepts in Solid State for NEET?
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).
Which formulae are important for Solid State in NEET?
FCC relation (a and r): 4r = a√2; BCC relation (a and r): 4r = a√3; Density of crystal: ρ = ZM / (N_A · a³).
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