chemistry : class12 Crystal Defects : Crystalline solids : Solid State MCQ Questions and Answers
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Crystal Defects
MCQ questions and answers.
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MCQ Questions
1. In AgCl, Ag+ ions can leave their normal lattice sites and occupy interstitial positions without leaving the crystal. Which statement correctly describes this defect?
A.
It is a Schottky defect and significantly decreases the density
B.
It is a Frenkel defect and does not appreciably change the density
C.
It is an impurity defect caused by substitution of Ag+ by Cl−
D.
It is a metal-excess defect caused by anion vacancies
Correct Answer:
It is a Frenkel defect and does not appreciably change the density
Explanation:
A Frenkel defect occurs when a smaller ion, usually a cation, leaves its normal lattice site and occupies an interstitial site. Since no ion leaves the crystal, the mass and approximately the volume remain unchanged, so the density is essentially unaffected.
2. A crystal of NaCl contains equal numbers of cation vacancies and anion vacancies, with electrical neutrality maintained and no ions occupying interstitial sites. Which change is expected relative to a perfect NaCl crystal?
A.
Its stoichiometry remains unchanged, but its density decreases
B.
Its stoichiometry changes, but its density remains unchanged
C.
Its stoichiometry and density both remain unchanged
D.
Its electrical neutrality is destroyed because both types of vacancies are charged
Correct Answer:
Its stoichiometry remains unchanged, but its density decreases
Explanation:
This is a Schottky defect. Equal numbers of Na+ and Cl− ions are missing, so the stoichiometric ratio and electrical neutrality are preserved. However, the crystal loses mass while its lattice volume is approximately retained, causing a decrease in density.
3. When ZnO is heated, it becomes yellow and exhibits enhanced electrical conductivity. Which defect model best explains these observations?
A.
Oxygen vacancies produce electrons that occupy interstitial sites
B.
Zinc vacancies produce holes by removing Zn2+ ions from the lattice
C.
Zinc ions occupy interstitial sites and electrons are trapped in anion vacancies
D.
Equal numbers of zinc and oxygen vacancies produce a stoichiometric Schottky defect
Correct Answer:
Zinc ions occupy interstitial sites and electrons are trapped in anion vacancies
Explanation:
On heating, ZnO can lose oxygen, producing oxygen vacancies. To maintain electrical neutrality, some Zn2+ ions occupy interstitial positions and electrons are released. These electrons may become trapped at anion vacancies, forming colour centres and increasing conductivity.
4. FeO is commonly non-stoichiometric and is represented as Fe0.95O. Which description most appropriately accounts for this composition?
A.
Oxygen vacancies are balanced by Fe3+ ions occupying interstitial sites
B.
Some Fe2+ sites are vacant, and the remaining iron ions include Fe3+ to maintain charge neutrality
C.
Equal numbers of Fe2+ and O2− ions are absent, producing a Schottky defect
D.
O2− ions replace Fe2+ ions, producing a Frenkel defect
Correct Answer:
Some Fe2+ sites are vacant, and the remaining iron ions include Fe3+ to maintain charge neutrality
Explanation:
FeO commonly shows metal deficiency. Some iron sites remain vacant, while a fraction of Fe2+ ions is oxidized to Fe3+. The higher positive charge of Fe3+ compensates for the missing Fe2+ ions and preserves electrical neutrality.
5. A small amount of SrCl2 is added to a NaCl crystal. If Sr2+ ions substitute for Na+ ions, what is the defect relationship produced for every Sr2+ ion introduced?
A.
One Cl− vacancy is created
B.
One Na+ interstitial ion is created
C.
One Na+ vacancy is created
D.
Two Cl− interstitial ions are created
Correct Answer:
One Na+ vacancy is created
Explanation:
Each Sr2+ ion replacing Na+ contributes one extra positive charge. Charge neutrality is restored by creating one Na+ vacancy, which removes one positive charge. Thus, one cation vacancy is produced per Sr2+ ion incorporated.
6. An electron becomes trapped in an anion vacancy in an ionic crystal. What is the resulting defect commonly called, and what optical effect can it produce?
A.
A Frenkel defect that always increases density
B.
An F-centre that can impart colour to the crystal
C.
A Schottky defect that destroys electrical neutrality
D.
An impurity defect that necessarily produces a cation vacancy
Correct Answer:
An F-centre that can impart colour to the crystal
Explanation:
An anion vacancy containing a trapped electron is called an F-centre. The trapped electron absorbs specific wavelengths of visible light, so crystals containing F-centres may develop characteristic colours.
7. Which combination of properties most strongly favours the formation of a Frenkel defect rather than a Schottky defect in an ionic crystal?
A.
Nearly equal cation and anion sizes with a highly compact lattice
B.
A small cation, a large anion, and sufficient interstitial space
C.
A crystal composed only of neutral atoms with no ionic interactions
D.
A lattice in which removal of one ion necessarily removes an equal number of oppositely charged ions
Correct Answer:
A small cation, a large anion, and sufficient interstitial space
Explanation:
Frenkel defects are favoured when a relatively small cation can leave its normal site and fit into an interstitial position. The presence of a large anion and adequate interstitial space facilitates this displacement without destroying the overall stoichiometry.
8. A perfect ionic crystal has density d. A sample develops only Frenkel defects, with ions displaced from regular sites to interstitial sites. Assuming the macroscopic volume is unchanged, what is the approximate density of the defective crystal?
A.
d/2
B.
Less than d because ions leave regular lattice sites
C.
Approximately d
D.
Greater than d because interstitial sites contain extra ions
Correct Answer:
Approximately d
Explanation:
In a Frenkel defect, ions are displaced within the crystal rather than removed from it. Therefore, the total mass remains essentially constant, and with approximately unchanged volume, the density remains approximately equal to d.
9. A crystal originally contains N formula units. It develops n Schottky defect pairs, and the volume is assumed to remain constant. What is the approximate ratio of its new density to the original density?
A.
n/N
B.
(N + n)/N
C.
(N − n)/N
D.
N/(N − n)
Correct Answer:
(N − n)/N
Explanation:
Each Schottky pair removes one cation and one anion, corresponding to the loss of one formula unit from the crystal. Thus, the mass becomes proportional to N − n while the volume is assumed unchanged, giving a density ratio of (N − n)/N.
10. Which statement correctly distinguishes a non-stoichiometric defect from a stoichiometric point defect in an ionic crystal?
A.
A non-stoichiometric defect necessarily destroys electrical neutrality
B.
A non-stoichiometric defect changes the ideal ion ratio, while charge neutrality is maintained through vacancies, variable oxidation states, or electronic compensation
C.
A non-stoichiometric defect always involves only interstitial anions
D.
A non-stoichiometric defect cannot alter the electrical conductivity of the crystal
Correct Answer:
A non-stoichiometric defect changes the ideal ion ratio, while charge neutrality is maintained through vacancies, variable oxidation states, or electronic compensation
Explanation:
Non-stoichiometric defects cause the actual composition to differ from the ideal formula. Electrical neutrality is nevertheless preserved through mechanisms such as cation vacancies, changes in oxidation state, electron trapping, or the formation of interstitial ions.
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