The Solid State for NEET UG
Solids are made of particles packed in regular patterns. Unit cells, packing efficiency and defects explain density, hardness and even the colour of some crystals.
ChemistryClass 1225 MCQs2 lessons12 flashcards3 memory tricks
Syllabus check: This chapter was removed from NCERT's rationalised textbooks (2023–24) and may not be in the current NEET UG syllabus. It is included for completeness. Check the latest syllabus on the NTA NEET website before spending revision time on it.
What you'll learn
- Types of solids and unit cells
Classify solids and count the atoms in cubic unit cells. - Density from unit cell data, and defects
Calculate density from unit cell dimensions and explain point defects.
Key formulas
| Density of a cubic crystal | ρ = Z × M / (a³ × N_A) |
|---|
Memory tricks
Simple cubic: 8 × ⅛ = 1. Body-centred: 1 + 1 = 2. Face-centred: 1 + 6 × ½ = 4.
ccp/fcc and hcp: 74%. bcc: 68%. Simple cubic: 52.4%.
Schottky: equal numbers of cations and anions missing, so density falls (NaCl, KCl). Frenkel: the smaller ion moves to an interstitial site, so density is unchanged (AgCl, ZnS).
Try 3 NEET-style questions
Q1. The number of atoms per unit cell in a face-centred cubic (fcc) lattice is
- 1
- 2
- 4
- 8
Show answer
Answer: (3) 4
fcc has 8 corners × 1/8 + 6 faces × 1/2 = 4 atoms per unit cell.
Q2. A Frenkel defect
- lowers the density of the crystal
- does not change the density of the crystal
- increases the density by a large amount
- occurs only in covalent solids
Show answer
Answer: (2) does not change the density of the crystal
In a Frenkel defect an ion moves to an interstitial site, so no ions are lost and the density stays the same.
Q3. In the zinc blende (ZnS) structure, the coordination numbers of Zn²⁺ and S²⁻ are
- 4:4 (tetrahedral)
- 6:6
- 8:8
- 12:12
Show answer
Answer: (1) 4:4 (tetrahedral)
Each Zn²⁺ and S²⁻ is surrounded tetrahedrally by four ions of the opposite charge, so the coordination number is 4:4.
Practise all 25 questions Free · explanations for every option · see where you go wrong