Last Updated: May 2026
NEET 2027 — Solid State chapter overview
For NEET 2027 Solid State Chemistry (NCERT Class 12, Chapter 1 of Inorganic Solid State), expect 1-2 questions with high-yield numerical practice on packing efficiency and density. The chapter is concept-light but formula-heavy — once the unit-cell formulas are memorised and the defect classification is clear, a 90% accuracy on this topic is realistic.
Crystalline vs Amorphous Solids
| Property | Crystalline | Amorphous |
|---|---|---|
| Order | Long-range | Short-range |
| Melting point | Sharp | Range |
| Anisotropy | Anisotropic | Isotropic |
| Heat of fusion | Definite | Indefinite |
| Examples | NaCl, ZnS, Cu | Glass, rubber, plastic |
Crystal Systems and Bravais Lattices
Seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, rhombohedral. Total 14 Bravais lattices. The cubic system has 3 sublattices: simple cubic (SC), body-centred cubic (BCC), face-centred cubic (FCC).
Unit Cell Numerics — Master Table
| Type | Atoms/Cell (Z) | Coord. No. | Packing Efficiency | Atom radius (a) |
|---|---|---|---|---|
| Simple Cubic (SC) | 1 | 6 | 52.4 % | r = a/2 |
| Body Centred (BCC) | 2 | 8 | 68.0 % | r = (√3/4)a |
| Face Centred (FCC) | 4 | 12 | 74.0 % | r = (a/2√2) = a×0.354 |
| Hexagonal Close Pack (HCP) | 6 | 12 | 74.0 % | — |
Density Formula
ρ = (Z × M) / (NA × a³)
- Z = atoms per unit cell
- M = molar mass (g/mol)
- NA = Avogadro number = 6.022 × 10²³
- a = edge length (cm); cube it for volume
Express edge length in cm to get density in g/cm³ — common error is mixing pm and cm. 1 pm = 10⁻¹⁰ cm.
Voids in Close-Packed Structures
- Tetrahedral void: Volume = 0.225r per atom; 2 voids per atom
- Octahedral void: Volume = 0.414r per atom; 1 void per atom
- FCC: 4 atoms, 4 octahedral voids, 8 tetrahedral voids
- HCP: 6 atoms, 6 octahedral voids, 12 tetrahedral voids
Ionic Solid Structures
| Compound | Type | Cation:Anion | Coord. No. |
|---|---|---|---|
| NaCl (Rock salt) | FCC anion + cations in octahedral voids | 1:1 | 6:6 |
| CsCl | BCC arrangement | 1:1 | 8:8 |
| ZnS (Zinc blende) | FCC anion + cations in tetrahedral voids | 1:1 | 4:4 |
| CaF₂ (Fluorite) | FCC cation + anions in tetrahedral voids | 1:2 | 8:4 |
| Na₂O (Antifluorite) | FCC anion + cations in tetrahedral voids | 2:1 | 4:8 |
Point Defects
1. Stoichiometric Defects
- Schottky defect: Equal numbers of cations and anions missing. Density decreases. Example: NaCl, KCl, CsCl.
- Frenkel defect: Cation displaces to interstitial site. Density unchanged. Example: AgCl, AgBr, ZnS.
2. Non-stoichiometric Defects
- Metal excess (F-centres): Anion vacancy filled by an electron. Gives colour. Example: NaCl heated in Na vapour gives yellow colour.
- Metal deficiency: Cation vacancy compensated by higher oxidation state cation nearby. Example: Fe0.95O.
3. Impurity Defects
Doping AgCl with CdCl₂: 2 Ag⁺ replaced by 1 Cd²⁺, creating one cation vacancy.
Magnetic Properties
| Property | Cause | Example |
|---|---|---|
| Diamagnetic | All paired electrons | NaCl, H₂O |
| Paramagnetic | Unpaired electrons | O₂, Cu²⁺ |
| Ferromagnetic | Aligned dipoles | Fe, Co, Ni |
| Antiferromagnetic | Antiparallel alignment | MnO |
| Ferrimagnetic | Unequal antiparallel | Fe₃O₄, ferrites |
Semiconductors
- Intrinsic: Pure Si or Ge
- n-type: Si doped with P or As (group 15) — extra electron
- p-type: Si doped with B or Ga (group 13) — hole
35 Practice MCQs
- Crystalline solids show — (A) short-range order (B) long-range order (C) no order (D) random arrangement
- Number of Bravais lattices — (A) 7 (B) 12 (C) 14 (D) 16
- Number of atoms in BCC unit cell — (A) 1 (B) 2 (C) 4 (D) 8
- Number of atoms in FCC unit cell — (A) 1 (B) 2 (C) 4 (D) 8
- Coordination number in BCC — (A) 4 (B) 6 (C) 8 (D) 12
- Packing efficiency of FCC — (A) 52% (B) 68% (C) 74% (D) 95%
- Packing efficiency of BCC — (A) 52% (B) 68% (C) 74% (D) 95%
- Density formula uses — (A) ρ = ZM/NAa³ (B) ρ = M/V (C) ρ = NA/V (D) ρ = a³
- For 1 pm — (A) 10⁻⁹ cm (B) 10⁻¹⁰ cm (C) 10⁻¹² cm (D) 10⁻⁶ cm
- NaCl crystal type — (A) BCC (B) FCC (C) Fluorite (D) Antifluorite
- CsCl crystal type — (A) BCC (B) FCC (C) Zinc blende (D) Wurtzite
- ZnS structure — (A) FCC + tet voids (B) FCC + oct voids (C) BCC (D) HCP
- CaF₂ structure — (A) Fluorite (B) Antifluorite (C) Wurtzite (D) Zinc blende
- Schottky defect — (A) anion + cation missing equally (B) cation interstitial (C) anion interstitial (D) impurity
- Frenkel defect example — (A) NaCl (B) KCl (C) AgCl (D) MgO
- Schottky defect changes — (A) density decreases (B) density unchanged (C) density increases (D) no change
- Frenkel defect changes density — (A) decreases (B) unchanged (C) increases (D) doubles
- F-centre is — (A) anion vacancy with electron (B) cation vacancy with electron (C) interstitial atom (D) substitution
- n-type semiconductor — (A) Si + B (B) Si + P (C) Si + Al (D) Si + Ga
- p-type semiconductor — (A) Si + As (B) Si + P (C) Si + Sb (D) Si + B
- Ferromagnetic example — (A) Fe (B) MnO (C) Cu²⁺ (D) NaCl
- Tetrahedral voids per atom in close packing — (A) 1 (B) 2 (C) 4 (D) 8
- Octahedral voids per atom — (A) 1 (B) 2 (C) 4 (D) 8
- Atoms in HCP unit cell — (A) 4 (B) 6 (C) 8 (D) 12
- The number of crystal systems is — (A) 5 (B) 7 (C) 14 (D) 30
- Diamond is — (A) FCC + half tet voids (B) BCC (C) HCP (D) SC
- Highest packing efficiency — (A) SC (B) BCC (C) FCC (D) all equal
- The Coordination number for fluorite (CaF₂) — (A) 8:4 (B) 6:6 (C) 4:4 (D) 8:8
- Edge length of FCC unit cell with r = 144 pm — (A) 288 pm (B) 408 pm (C) 332 pm (D) 144 pm
- 1 mole of NaCl has how many Schottky defects at 25 °C (approx) — (A) 10⁶ (B) 10⁷ (C) 10⁸ (D) negligible at room temp
- Wurtzite structure of ZnS is — (A) HCP variant (B) FCC variant (C) BCC (D) SC
- The most efficient packing is — (A) HCP and FCC (B) BCC (C) SC (D) Body-centred tetragonal
- The number of atoms per unit cell in simple cubic — (A) 1 (B) 2 (C) 4 (D) 8
- Anti-fluorite structure example — (A) Na₂O (B) CaF₂ (C) NaCl (D) ZnS
- The relation r = (√3/4)a applies to — (A) SC (B) BCC (C) FCC (D) HCP
Answer Key
1-B, 2-C, 3-B, 4-C, 5-C, 6-C, 7-B, 8-A, 9-B, 10-B, 11-A, 12-A, 13-A, 14-A, 15-C, 16-A, 17-B, 18-A, 19-B, 20-D, 21-A, 22-B, 23-A, 24-B, 25-B, 26-A, 27-C, 28-A, 29-B, 30-D, 31-A, 32-A, 33-A, 34-A, 35-B
FAQ
How many questions from Solid State in NEET 2027?
Typically 1–2 questions, with strong emphasis on numerical density calculation, packing efficiency, and ionic solid coordination numbers.
Which is the most asked numerical formula in Solid State?
Density: ρ = ZM / NAa³. Almost every NEET paper has at least one application of this formula across SC, BCC, or FCC.
Schottky vs Frenkel — which to remember first?
Schottky: missing pairs (NaCl-type), density drops. Frenkel: cation displaced to interstitial (AgCl-type), density unchanged. Remember: Frenkel = full house, Schottky = empty seats.
What is an F-centre?
An anion vacancy in a non-stoichiometric crystal that traps an electron — gives the crystal characteristic colour. Example: NaCl in Na vapour appears yellow.