41. Which of the following compounds forms a crystalline solid with a covalent network?
a) Sodium chloride
b) Graphite
c) Carbon dioxide
d) Ammonia
Answer:
b) Graphite
Explanation:
Graphite is a covalent network solid, where carbon atoms are arranged in layers of hexagonal rings. Each carbon atom is covalently bonded to three others in a single layer, and the layers are held together by weak van der Waals forces.
42. The packing efficiency in a body-centered cubic (BCC) structure is:
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
b) 68%
Explanation:
The packing efficiency of a body-centered cubic (BCC) structure is 68%, meaning that 68% of the space in the unit cell is occupied by atoms, and the remaining 32% is empty space.
43. The electrical conductivity of semiconductors increases with:
a) Decrease in temperature
b) Increase in temperature
c) Decrease in pressure
d) Increase in pressure
Answer:
b) Increase in temperature
Explanation:
The electrical conductivity of semiconductors increases with an increase in temperature. This is because heating a semiconductor provides more energy for electrons to jump from the valence band to the conduction band, resulting in higher conductivity.
44. The coordination number of an atom in a hexagonal close-packed (hcp) structure is:
a) 4
b) 6
c) 8
d) 12
Answer:
d) 12
Explanation:
In a hexagonal close-packed (hcp) structure, each atom is surrounded by 12 nearest neighbors—6 atoms in the same layer, 3 in the layer above, and 3 in the layer below. This results in a coordination number of 12.
45. Which of the following solids is classified as a molecular solid?
a) Sodium chloride
b) Copper
c) Iodine (I₂)
d) Diamond
Answer:
c) Iodine (I₂)
Explanation:
Iodine (I₂) is a molecular solid where individual iodine molecules are held together by weak van der Waals forces. It has low melting and boiling points compared to ionic, metallic, and covalent network solids.
46. The fraction of the volume occupied by atoms in a body-centered cubic (BCC) unit cell is:
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
b) 68%
Explanation:
In a body-centered cubic (BCC) structure, the packing efficiency is 68%. This means that 68% of the total volume of the unit cell is occupied by atoms, while 32% is empty space.
47. What is the type of defect called when a cation leaves its lattice site and moves to an interstitial position in the crystal?
a) Schottky defect
b) Frenkel defect
c) Vacancy defect
d) Interstitial defect
Answer:
b) Frenkel defect
Explanation:
A Frenkel defect occurs when a smaller ion, usually a cation, moves from its regular lattice site to an interstitial site, creating a vacancy at its original position while maintaining electrical neutrality.
48. Which of the following statements is true for covalent network solids?
a) They have high electrical conductivity
b) They are soft and have low melting points
c) They are hard and have high melting points
d) They have ionic bonds between their particles
Answer:
c) They are hard and have high melting points
Explanation:
Covalent network solids, such as diamond and quartz, consist of atoms covalently bonded in a continuous network. This strong bonding results in very high hardness and high melting points, making them different from molecular solids and ionic solids.
49. Which of the following is true about metallic solids?
a) They are brittle
b) They have high melting points and are malleable
c) They are poor conductors of electricity
d) They have sharp melting points
Answer:
b) They have high melting points and are malleable
Explanation:
Metallic solids have a characteristic structure where metal cations are surrounded by a “sea” of delocalized electrons. This gives them high melting points and makes them malleable and ductile, meaning they can be bent or shaped without breaking.
50. What is the number of atoms per unit cell in a hexagonal close-packed (hcp) structure?
a) 2
b) 4
c) 6
d) 8
Answer:
c) 6
Explanation:
A hexagonal close-packed (hcp) unit cell contains a total of 6 atoms. This is calculated based on contributions from the atoms at the corners, faces, and inside the unit cell.
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