Top 3 Frequently Tested Topics
1. Density Calculation (ρ = ZM/Nₐ):
- Always identify crystal type to get Z
- Always convert edge length to cm (1 Å = 10^{-8} cm; 1 pm = 10^{-10} cm)
- Common trap: using Z = 2 (BCC) for an FCC crystal → answer is halved
2. Schottky vs Frenkel Defect:
- Schottky → density decreases (ALWAYS)
- Frenkel → density unchanged (ALWAYS)
- AgBr → BOTH defects (most tested exception)
3. Void Occupancy (NaCl vs ZnS):
- NaCl: ALL octahedral voids filled → 1:1 (XY)
- ZnS: HALF tetrahedral voids filled → 1:1 (XY)
- : ALL tetrahedral voids filled → 1:2 ()
Common NEET Traps
| Trap | Correct answer |
|---|---|
| CsCl is BCC | FALSE — CsCl is NOT BCC; it's a simple cubic of with at body centre — different elements |
| Frenkel defect changes density | FALSE — density is UNCHANGED |
| ZnS fills all tetrahedral voids | FALSE — only HALF are filled |
| NaCl fills tetrahedral voids | FALSE — fills OCTAHEDRAL voids |
| p-type dopant is Group 15 | FALSE — Group 15 is n-type; Group 13 is p-type |
Marks Distribution
NEET typically tests solid state with:
- 1 question on density calculation (~4 marks)
- 1 question on defects or ionic structures (~4 marks)
- Occasionally 1 question on semiconductors or magnetic properties