91. In which of the following structures do atoms occupy the octahedral voids?
a) Simple cubic
b) Face-centered cubic (FCC)
c) Body-centered cubic (BCC)
d) Hexagonal close-packed (hcp)
Answer:
b) Face-centered cubic (FCC)
Explanation:
In a face-centered cubic (FCC) structure, atoms or ions can occupy octahedral voids. These voids are located at the edge centers and at the body center of the unit cell.
92. Which of the following compounds forms an ionic crystal lattice?
a) Diamond
b) Silicon
c) Sodium chloride
d) Graphite
Answer:
c) Sodium chloride
Explanation:
Sodium chloride (NaCl) forms an ionic crystal lattice, where Na⁺ and Cl⁻ ions are held together by electrostatic forces in a repeating three-dimensional structure.
93. What is the fraction of the volume occupied by atoms in a face-centered cubic (FCC) unit cell?
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
c) 74%
Explanation:
The packing efficiency of a face-centered cubic (FCC) unit cell is 74%, meaning that 74% of the unit cell’s volume is occupied by atoms, while the remaining 26% is empty space.
94. Which type of solid typically has the highest melting point?
a) Molecular solids
b) Ionic solids
c) Metallic solids
d) Covalent network solids
Answer:
d) Covalent network solids
Explanation:
Covalent network solids, like diamond and quartz, have very high melting points because the atoms are bonded together in an extended network of strong covalent bonds.
95. Which of the following defects can lead to the creation of color centers (F-centers) in a crystal?
a) Schottky defect
b) Frenkel defect
c) Vacancy defect
d) Interstitial defect
Answer:
c) Vacancy defect
Explanation:
Color centers, or F-centers, are created when an electron occupies a vacancy left by a missing anion in the lattice. This defect can absorb certain wavelengths of light, giving the crystal a color.
96. Which of the following types of solids generally has the lowest density?
a) Ionic solids
b) Metallic solids
c) Molecular solids
d) Covalent network solids
Answer:
c) Molecular solids
Explanation:
Molecular solids generally have the lowest density because they are held together by weak intermolecular forces, and the molecules are not as tightly packed as in ionic, metallic, or covalent network solids.
97. In a crystal lattice, tetrahedral voids are typically found in which structure?
a) Simple cubic
b) Body-centered cubic (BCC)
c) Face-centered cubic (FCC)
d) Hexagonal close-packed (hcp)
Answer:
c) Face-centered cubic (FCC)
Explanation:
Tetrahedral voids are commonly found in both face-centered cubic (FCC) and hexagonal close-packed (hcp) structures. These voids are smaller than octahedral voids and are located between four atoms in the lattice.
98. Which of the following defects does not alter the overall electrical neutrality of a crystal?
a) Schottky defect
b) Frenkel defect
c) Impurity defect
d) Vacancy defect
Answer:
b) Frenkel defect
Explanation:
In a Frenkel defect, cations move from their regular lattice positions to interstitial sites, but no ions are lost from the lattice. Therefore, the overall electrical neutrality of the crystal remains unchanged.
99. In an ionic crystal, which type of defect results in the crystal becoming electrically conductive?
a) Schottky defect
b) Frenkel defect
c) Impurity defect
d) Vacancy defect
Answer:
c) Impurity defect
Explanation:
An impurity defect can introduce additional charge carriers into the crystal, which can increase the electrical conductivity. For example, doping an ionic crystal with ions of different valency can create free electrons or holes.
100. Which of the following statements is true for hexagonal close-packed (hcp) structures?
a) The coordination number is 8
b) The packing efficiency is lower than that of FCC
c) Each atom is surrounded by 12 neighbors
d) The structure has a simple cubic arrangement
Answer:
c) Each atom is surrounded by 12 neighbors
Explanation:
In a hexagonal close-packed (hcp) structure, each atom is surrounded by 12 nearest neighbors, giving it the same coordination number as in a face-centered cubic (FCC) structure. The packing efficiency is also 74%, the same as FCC.
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