21. In a hexagonal close-packed (hcp) structure, how many atoms are present in a single unit cell?
a) 4
b) 6
c) 8
d) 12
Answer:
b) 6
Explanation:
In a hexagonal close-packed (hcp) structure, each unit cell contains 6 atoms. These atoms are arranged in layers with a hexagonal arrangement, leading to efficient packing.
22. Which of the following solids is likely to dissolve in water and conduct electricity when molten?
a) Graphite
b) Sodium chloride (NaCl)
c) Sulfur
d) Diamond
Answer:
b) Sodium chloride (NaCl)
Explanation:
Sodium chloride (NaCl) is an ionic solid. It dissolves in water because the ions can separate and interact with water molecules. When molten, NaCl conducts electricity because the ions are free to move.
23. Which of the following best describes amorphous solids?
a) Have a well-defined geometric shape
b) Have long-range order
c) Have short-range order but lack long-range order
d) Are composed of ions arranged in a regular lattice
Answer:
c) Have short-range order but lack long-range order
Explanation:
Amorphous solids, such as glass, do not have a long-range ordered arrangement of their atoms. They may have short-range order, meaning that atoms or molecules are orderly only over short distances.
24. In an ionic solid, the magnitude of the lattice energy depends on:
a) The size of the ions
b) The charge on the ions
c) Both a and b
d) Neither a nor b
Answer:
c) Both a and b
Explanation:
The lattice energy of an ionic solid depends on both the charge of the ions and the size of the ions. A higher charge on the ions and smaller ionic radii result in a stronger electrostatic attraction and a higher lattice energy.
25. Which of the following has the highest packing efficiency?
a) Simple cubic
b) Body-centered cubic
c) Face-centered cubic
d) Tetragonal
Answer:
c) Face-centered cubic
Explanation:
The face-centered cubic (FCC) structure has a packing efficiency of 74%, which is higher than that of both simple cubic (52%) and body-centered cubic (68%) structures.
26. Which of the following statements is true about a crystal lattice?
a) A lattice point can be occupied by a single atom
b) The distance between two adjacent lattice points is called a unit cell
c) A unit cell is the smallest repeating unit in a crystal lattice
d) A crystal lattice is always amorphous
Answer:
c) A unit cell is the smallest repeating unit in a crystal lattice
Explanation:
The unit cell is the fundamental repeating unit in a crystal lattice. It defines the crystal structure by repeating in all three dimensions to form the entire lattice. Lattice points may contain atoms, ions, or molecules.
27. Which type of defect occurs when an atom or ion is missing from its regular lattice site in a crystal?
a) Interstitial defect
b) Vacancy defect
c) Frenkel defect
d) Schottky defect
Answer:
b) Vacancy defect
Explanation:
A vacancy defect occurs when an atom or ion is missing from its regular lattice position, creating a vacancy in the crystal structure. This type of defect decreases the density of the solid.
28. Which type of solid is characterized by a high melting point, hardness, and poor electrical conductivity in the solid state but good conductivity in the molten state?
a) Metallic solids
b) Ionic solids
c) Molecular solids
d) Covalent network solids
Answer:
b) Ionic solids
Explanation:
Ionic solids have high melting points and are hard due to the strong electrostatic forces between ions. In the solid state, they are poor conductors because the ions are fixed in place. However, in the molten state, they conduct electricity as the ions are free to move.
29. The effective number of atoms in a face-centered cubic (FCC) unit cell is:
a) 1
b) 2
c) 4
d) 6
Answer:
c) 4
30. The coordination number of a particle in a simple cubic structure is:
a) 4
b) 6
c) 8
d) 12
Answer:
b) 6
Explanation:
In a simple cubic unit cell, each particle is surrounded by 6 nearest neighbors: one on each of the six faces of the cube. Therefore, the coordination number is 6.
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