71. The type of defect in ionic solids that reduces the overall density of the solid is:
a) Schottky defect
b) Frenkel defect
c) Interstitial defect
d) Vacancy defect
Answer:
a) Schottky defect
Explanation:
A Schottky defect occurs when equal numbers of cations and anions are missing from their lattice positions, creating vacancies. This defect reduces the density of the solid because mass is removed from the crystal without reducing its volume.
72. Which of the following materials would likely have the highest packing efficiency?
a) Diamond
b) Graphite
c) Sodium chloride
d) Copper
Answer:
d) Copper
Explanation:
Copper has a face-centered cubic (FCC) structure, which has a high packing efficiency of 74%. This is higher than the packing efficiencies of diamond, graphite, and sodium chloride, which have different types of structures.
73. Which of the following is true for molecular solids?
a) They have high melting points
b) They conduct electricity
c) They are hard and brittle
d) They are generally soft with low melting points
Answer:
d) They are generally soft with low melting points
Explanation:
Molecular solids are held together by weak intermolecular forces, such as van der Waals forces, leading to low melting points and softness. They do not conduct electricity and are not hard or brittle.
74. Which of the following statements is true about ionic solids?
a) They are malleable
b) They have high electrical conductivity in the solid state
c) They have high melting and boiling points
d) They are soft and flexible
Answer:
c) They have high melting and boiling points
Explanation:
Ionic solids have high melting and boiling points due to the strong electrostatic forces of attraction between oppositely charged ions. In the solid state, they do not conduct electricity, but they do conduct electricity when molten.
75. Which type of crystal defect is observed when cations move from their normal lattice positions to interstitial sites?
a) Schottky defect
b) Frenkel defect
c) Interstitial defect
d) Vacancy defect
Answer:
b) Frenkel defect
Explanation:
A Frenkel defect occurs when cations move from their normal lattice positions to interstitial sites within the crystal. This type of defect does not affect the overall density of the solid because no atoms are lost from the crystal.
76. Which of the following solids exhibits metallic bonding?
a) Sodium chloride
b) Diamond
c) Iron
d) Water
Answer:
c) Iron
Explanation:
Iron is a metallic solid that exhibits metallic bonding. In metallic solids, positive metal ions are surrounded by a sea of delocalized electrons that allow for electrical conductivity and malleability.
77. Which of the following has the least packing efficiency?
a) Simple cubic
b) Body-centered cubic (BCC)
c) Face-centered cubic (FCC)
d) Hexagonal close-packed (hcp)
Answer:
a) Simple cubic
Explanation:
Simple cubic structures have the least packing efficiency at approximately 52%. This is because the atoms are only arranged at the corners of the cube, leaving a lot of empty space compared to BCC (68%) and FCC or hcp (74%).
78. In which type of solid is the lattice composed of molecules held together by van der Waals forces?
a) Ionic solids
b) Molecular solids
c) Metallic solids
d) Covalent network solids
Answer:
b) Molecular solids
Explanation:
In molecular solids, molecules are held together by weak van der Waals forces, dipole-dipole interactions, or hydrogen bonds. These solids generally have low melting points and are poor conductors of electricity.
79. The crystal structure of a metal with a coordination number of 8 is most likely:
a) Simple cubic
b) Face-centered cubic (FCC)
c) Body-centered cubic (BCC)
d) Hexagonal close-packed (hcp)
Answer:
c) Body-centered cubic (BCC)
Explanation:
A body-centered cubic (BCC) structure has a coordination number of 8. Each atom in a BCC structure is surrounded by 8 nearest neighbors. In contrast, face-centered cubic (FCC) and hexagonal close-packed (hcp) structures have a coordination number of 12, while simple cubic has a coordination number of 6.
80. Which of the following is true about ionic solids?
a) They conduct electricity in the solid state
b) They have low melting points
c) They are soluble in polar solvents
d) They are malleable
Answer:
c) They are soluble in polar solvents
Explanation:
Ionic solids tend to dissolve in polar solvents like water because the polar molecules can interact with the ions and overcome the ionic bonds. Ionic solids do not conduct electricity in the solid state, but they do when molten or dissolved in a solvent.
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