81. Which type of defect involves the presence of extra atoms in the spaces between the regular lattice sites?
a) Frenkel defect
b) Schottky defect
c) Vacancy defect
d) Interstitial defect
Answer:
d) Interstitial defect
Explanation:
An interstitial defect occurs when extra atoms occupy the spaces (interstitial sites) between the regular lattice points in the crystal. This defect increases the density of the crystal since additional atoms are introduced without removing any.
82. The number of atoms in a hexagonal close-packed (hcp) unit cell is:
a) 2
b) 4
c) 6
d) 8
Answer:
c) 6
Explanation:
A hexagonal close-packed (hcp) unit cell contains 6 atoms: 2 atoms from the top and bottom hexagonal layers, and 3 atoms from the middle layer, along with 1 atom fully inside the unit cell.
83. Which of the following statements is true about metallic solids?
a) They are insulators
b) They are brittle
c) They are good conductors of heat and electricity
d) They are ionic in nature
Answer:
c) They are good conductors of heat and electricity
Explanation:
Metallic solids are characterized by a “sea of electrons” that are free to move throughout the structure, making them excellent conductors of both heat and electricity. They are also malleable and ductile, not brittle.
84. Which of the following elements forms a covalent network solid?
a) Sodium
b) Carbon (diamond)
c) Neon
d) Magnesium
Answer:
b) Carbon (diamond)
Explanation:
Diamond is an example of a covalent network solid, where carbon atoms are covalently bonded in a continuous, three-dimensional structure. This gives diamond its extreme hardness and high melting point.
85. In a crystal lattice, what is the term for the nearest neighboring particles surrounding a central particle?
a) Lattice point
b) Coordination number
c) Unit cell
d) Interstitial site
Answer:
b) Coordination number
Explanation:
The coordination number refers to the number of nearest neighboring particles surrounding a central particle in a crystal lattice. For example, in an FCC structure, the coordination number is 12.
86. Which of the following defects increases the electrical conductivity of a solid?
a) Schottky defect
b) Frenkel defect
c) Interstitial defect
d) Impurity defect
Answer:
d) Impurity defect
Explanation:
An impurity defect, where foreign atoms are introduced into the crystal lattice, can increase the electrical conductivity of a solid by providing additional charge carriers. This is the basis of semiconductor doping.
87. The coordination number of a simple cubic structure is:
a) 4
b) 6
c) 8
d) 12
Answer:
b) 6
Explanation:
In a simple cubic structure, each atom is surrounded by 6 nearest neighbors—4 in the same layer and 1 each above and below, giving a coordination number of 6.
88. Which type of crystal lattice has the highest packing efficiency?
a) Simple cubic
b) Body-centered cubic (BCC)
c) Face-centered cubic (FCC)
d) Tetragonal
Answer:
c) Face-centered cubic (FCC)
Explanation:
The face-centered cubic (FCC) lattice has the highest packing efficiency at 74%, which is greater than that of body-centered cubic (BCC) and simple cubic structures. This is because the atoms are packed more closely together in the FCC arrangement.
89. Which type of defect is created when an atom or ion is missing from its lattice site?
a) Frenkel defect
b) Schottky defect
c) Vacancy defect
d) Interstitial defect
Answer:
c) Vacancy defect
Explanation:
A vacancy defect occurs when an atom or ion is missing from its regular lattice site. This can affect the properties of the crystal, such as density and electrical conductivity.
90. Which of the following solids is likely to be a poor conductor of electricity?
a) Iron
b) Graphite
c) Diamond
d) Copper
Answer:
c) Diamond
Explanation:
Diamond is a covalent network solid where all the electrons are tightly bound in covalent bonds, leaving no free electrons to conduct electricity. This makes diamond an electrical insulator.
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