Solid State: What NEET Actually Tests

The three highest-yield areas in solid state for NEET 2026 are density calculations, defect identification, and ionic structures.

1. Density Calculations (Most Frequent)

The formula ρ = ZM/($a^{3}$Nₐ) requires you to know Z instantly:

• FCC → Z = 4; BCC → Z = 2; SC → Z = 1

The most common error is forgetting to convert the edge length. If a is given in Å, multiply by 10^{-8} to get cm. If in pm, multiply by 10^{-10}. The denominator $a^{3}$ is then in $cm^{3}$, giving density in g/$cm^{3}$.

NEET also tests back-calculation: given ρ and M, find a. Rearrange to $a^{3}$ = ZM/(ρNₐ), then take cube root.

2. Defect Identification (Second Most Frequent)

The single most tested question: "Which defect does NOT change density?" Answer: Frenkel (density is fixed — the ion merely moves within the crystal).

Schottky always decreases density. AgBr always shows both defects. F-centres make crystals coloured (yellow for NaCl, violet for KCl).

For doping: Group 15 (P, As, Sb) → n-type; Group 13 (B, Ga, In) → p-type.

3. Ionic Structures (Third Rank)

Know three facts cold:

• NaCl: $Na^{+}$ fills ALL octahedral voids → 6:6
• ZnS: $Zn^{2+}$ fills HALF tetrahedral voids → 4:4
• CsCl: CN = 8:8 (not BCC — different ions)

Trap: CsCl looks like BCC but is NOT — body centre and corners are different ions in CsCl, same element in BCC.

Packing Efficiency Order

SC (52.4%) < BCC (68%) < FCC = HCP (74%)

Key Examples for Magnetic Properties

Fe, Co, Ni → ferromagnetic | MnO → antiferromagnetic | $Fe_{3}O_{4}$ → ferrimagnetic | $O_{2}$ → paramagnetic | NaCl, $C_{6}H_{6}$ → diamagnetic.