1. In a face-centred cubic (FCC) arrangement, atoms touch each other along the:
A. Body diagonal
B. Edge
C. Face diagonal
D. None of these
Answer: C. Face diagonal
Explanation:
In FCC, atoms are at the corners and face centres. The face diagonal = 4r = √2a.
2. The packing efficiency of a body-centred cubic (BCC) structure is:
A. 52.4%
B. 68%
C. 74%
D. 60%
Answer: B. 68%
Explanation:
In BCC, 2 atoms/unit cell, √3a = 4r → Packing efficiency = 68%.
3. The number of atoms in a face-centred cubic unit cell is:
A. 2
B. 4
C. 6
D. 8
Answer: B. 4
Explanation:
8 corner atoms × 1/8 + 6 face atoms × 1/2 = 1 + 3 = 4.
4. The coordination number of an atom in a hexagonal close-packed (hcp) structure is:
A. 6
B. 8
C. 12
D. 4
Answer: C. 12
Explanation:
Each atom touches 6 in its own layer, 3 above, and 3 below.
5. The number of tetrahedral and octahedral voids per atom in a cubic close-packed structure is:
A. 1 and 1
B. 2 and 1
C. 1 and 2
D. 4 and 2
Answer: B. 2 and 1
Explanation:
For every atom, 2 tetrahedral voids and 1 octahedral void exist.
6. Radius ratio for octahedral void is:
A. 0.225
B. 0.414
C. 0.732
D. 1.0
Answer: B. 0.414
Explanation:
Radius ratio = r+/r– = 0.414 for octahedral geometry.
7. A solid has a cubic structure where A atoms are at corners and B atoms at body centre. The formula of the compound is:
A. AB
B. AB₂
C. A₂B
D. AB₃
Answer: A. AB
Explanation:
A: 8 corners × 1/8 = 1 atom;
B: 1 at body centre = 1 atom → Formula AB.
8. In a crystal lattice, the ratio of number of octahedral voids to tetrahedral voids is:
A. 1 : 1
B. 1 : 2
C. 2 : 1
D. 1 : 4
Answer: B. 1 : 2
Explanation:
Always 1 octahedral and 2 tetrahedral voids per atom.
9. The number of nearest neighbours of an atom in BCC is:
A. 4
B. 6
C. 8
D. 12
Answer: C. 8
Explanation:
Each atom in BCC touches 8 others — coordination number = 8.
10. In which type of crystal defect density decreases?
A. Schottky defect
B. Frenkel defect
C. Interstitial defect
D. Metal deficiency defect
Answer: A. Schottky defect
Explanation:
Both cation and anion are missing → reduces mass but not volume → density ↓.
11. Which of the following is a metallic crystal?
A. Diamond
B. NaCl
C. Graphite
D. Copper
Answer: D. Copper
Explanation:
Metallic solids have delocalised electrons → good conductors.
12. The number of atoms per unit cell in BCC structure is:
A. 1
B. 2
C. 4
D. 6
Answer: B. 2
Explanation:
8 corners × 1/8 + 1 centre = 1 + 1 = 2.
13. If ‘r’ is the radius of atom and ‘a’ is the edge length in FCC, then relation is
A. a = 2r
B. a = 4r
C. a = 2√2r
D. a = √2r
Answer: D. a = √2r
14. What fraction of volume is empty in simple cubic lattice?
A. 26%
B. 32%
C. 47.6%
D. 68%
Answer: C. 47.6%
Explanation:
Packing efficiency = 52.4%, so voids = 100 – 52.4 = 47.6%.
15. Which defect does NOT change density of crystal?
A. Schottky
B. Frenkel
C. Metal excess
D. Vacancy
Answer: B. Frenkel defect
Explanation:
Ion leaves normal site but occupies interstitial → mass and volume constant → density unchanged.
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