chemistry : class12 Ionic Crystal Structures : Ionic solids : Solid State MCQ Questions and Answers
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Ionic Crystal Structures
MCQ questions and answers.
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MCQ Questions
1. An ionic solid has a radius ratio r+/r− = 0.50. Assuming the radius-ratio rules apply, which coordination environment is most geometrically suitable for the cation?
A.
Linear coordination with coordination number 2
B.
Tetrahedral coordination with coordination number 4
C.
Octahedral coordination with coordination number 6
D.
Cubic coordination with coordination number 8
Correct Answer:
Octahedral coordination with coordination number 6
Explanation:
A radius ratio between approximately 0.414 and 0.732 supports octahedral coordination. Since 0.50 lies within this range, the cation is surrounded by six anions.
2. Which statement correctly describes the ideal CsCl crystal structure?
A.
Each ion has six oppositely charged nearest neighbours, and the unit cell contains four CsCl units
B.
Each ion has eight oppositely charged nearest neighbours, and the unit cell contains one CsCl unit
C.
Each ion has four oppositely charged nearest neighbours, and the unit cell contains two CsCl units
D.
Each ion has twelve oppositely charged nearest neighbours, and the unit cell contains one CsCl unit
Correct Answer:
Each ion has eight oppositely charged nearest neighbours, and the unit cell contains one CsCl unit
Explanation:
In CsCl, one ion is at the body centre and the other ions occupy the eight corners. The body-centre ion is surrounded by eight corner ions, and the effective contribution of the corner ions is one ion per unit cell, giving one CsCl formula unit.
3. In the rock-salt structure of NaCl, chloride ions form a cubic close-packed array. Which combination is correct for the conventional unit cell?
A.
Four NaCl units and octahedral coordination of both ions
B.
Four NaCl units and tetrahedral coordination of both ions
C.
One NaCl unit and octahedral coordination of both ions
D.
Eight NaCl units and cubic coordination of both ions
Correct Answer:
Four NaCl units and octahedral coordination of both ions
Explanation:
The NaCl unit cell contains four Na+ ions and four Cl− ions, corresponding to four formula units. Each ion is surrounded by six ions of opposite charge, giving 6:6 octahedral coordination.
4. Which description correctly represents the fluorite structure of CaF2?
A.
Ca2+ ions form an fcc lattice and F− ions occupy all tetrahedral holes; Ca2+ has coordination number 8
B.
F− ions form an fcc lattice and Ca2+ ions occupy all tetrahedral holes; Ca2+ has coordination number 4
C.
Ca2+ ions form a simple cubic lattice and F− ions occupy all cubic holes; Ca2+ has coordination number 6
D.
F− ions form a simple cubic lattice and Ca2+ ions occupy half the octahedral holes; Ca2+ has coordination number 6
Correct Answer:
Ca2+ ions form an fcc lattice and F− ions occupy all tetrahedral holes; Ca2+ has coordination number 8
Explanation:
In the fluorite structure, Ca2+ ions occupy the fcc positions, while F− ions fill all eight tetrahedral sites associated with the four Ca2+ ions in the unit cell. Each Ca2+ is coordinated by eight F− ions, whereas each F− is coordinated by four Ca2+ ions.
5. The anti-fluorite structure of Li2O can be described by which arrangement?
A.
O2− ions form an fcc lattice and Li+ ions occupy all tetrahedral holes
B.
Li+ ions form an fcc lattice and O2− ions occupy all tetrahedral holes
C.
O2− ions form an fcc lattice and Li+ ions occupy half the octahedral holes
D.
Li+ ions form a simple cubic lattice and O2− ions occupy body-centre sites
Correct Answer:
O2− ions form an fcc lattice and Li+ ions occupy all tetrahedral holes
Explanation:
In an anti-fluorite structure, the positions of the cations and anions relative to fluorite are reversed. The oxide ions form an fcc array, and the two Li+ ions per O2− occupy all tetrahedral holes. Thus Li+ has tetrahedral coordination and O2− has coordination number 8.
6. In a zinc-blende-type AB crystal, the anions form a cubic close-packed array and the cations occupy exactly half of the tetrahedral holes. What is the coordination number of each ion?
A.
A: 4 and B: 4
B.
A: 6 and B: 6
C.
A: 8 and B: 4
D.
A: 12 and B: 6
Correct Answer:
A: 4 and B: 4
Explanation:
A cation in a tetrahedral hole is surrounded by four anions. Because only half the tetrahedral holes are occupied, each anion is also bonded to four cations, producing 4:4 coordination characteristic of zinc blende.
7. For an ionic crystal in which the cation-to-anion radius ratio is 0.30, which coordination number is predicted by the radius-ratio rule?
A.
2
B.
4
C.
6
D.
8
Correct Answer:
4
Explanation:
The radius-ratio range from approximately 0.225 to 0.414 corresponds to tetrahedral coordination. Since 0.30 lies in this interval, the predicted coordination number is 4.
8. A crystal has anions in a cubic close-packed arrangement, with cations occupying all the octahedral holes. Which empirical formula and structure are expected?
A.
AB with the rock-salt structure
B.
AB2 with the fluorite structure
C.
A2B with the anti-fluorite structure
D.
AB with the zinc-blende structure
Correct Answer:
AB with the rock-salt structure
Explanation:
A close-packed array of N anions contains N octahedral holes. Filling all of them with cations gives equal numbers of cations and anions, hence AB. This arrangement is the rock-salt or NaCl-type structure with 6:6 coordination.
9. In a normal spinel AB2O4, oxide ions form an approximately close-packed array. Which distribution of the cations is characteristic of the normal spinel structure?
A.
A2+ occupies tetrahedral sites and B3+ occupies octahedral sites
B.
A2+ occupies octahedral sites and B3+ occupies tetrahedral sites
Explanation:
In a normal spinel, the divalent cation A2+ occupies one-eighth of the tetrahedral holes, while the trivalent cation B3+ occupies half of the octahedral holes in the close-packed oxide-ion framework.
10. Which comparison between CsCl-type and NaCl-type ionic structures is correct?
A.
Both have coordination number 6, but CsCl has four formula units per unit cell
B.
CsCl has 8:8 coordination and one formula unit per unit cell, whereas NaCl has 6:6 coordination and four formula units per unit cell
C.
CsCl has 4:4 coordination and two formula units per unit cell, whereas NaCl has 8:8 coordination and one formula unit per unit cell
D.
Both have 8:8 coordination and the same number of formula units per unit cell
Correct Answer:
CsCl has 8:8 coordination and one formula unit per unit cell, whereas NaCl has 6:6 coordination and four formula units per unit cell
Explanation:
The CsCl arrangement places one ion at the body centre relative to the other ion at the corners, producing eight nearest neighbours and one formula unit per unit cell. The NaCl arrangement has octahedral 6:6 coordination and four formula units in its conventional fcc-based unit cell.
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