Comparative Table: Schottky Defect vs Frenkel Defect with Examples
The difference between Schottky and Frenkel defect is a fundamental concept in JEE Main Chemistry, focusing on point defects found in crystalline ionic solids. Both are types of stoichiometric (intrinsic) defects but differ in how ions are positioned or missing within the crystal lattice, leading to distinct impacts on physical properties like density and electrical conductivity. Mastering these distinctions will help you solve related multiple-choice questions and numericals confidently, as well as understand applications in the solid state chapter.
Point defects are imperfections that affect individual lattice points in crystals. The two main types relevant for JEE—Schottky and Frenkel defects—are both found in ionic solids, but they originate and affect the lattice differently. These defects play a key role in the solid state properties of many compounds seen in textbooks and competitive exams.
Schottky Defect: Definition, Features, and Effects
Schottky defect occurs when equal numbers of cations and anions are missing from their lattice sites in an ionic solid. This produces vacancy sites and maintains electrical neutrality by the simultaneous loss of positive and negative ions. It mainly appears in crystals where the cations and anions have similar sizes and high coordination numbers, such as NaCl, KCl, and CsCl.
- Characterized as a vacancy defect (empty lattice points).
- Lowers the density of the crystal, since total mass decreases but volume is unchanged.
- No change in the chemical stoichiometry (schottky is a stoichiometric defect).
- More common in highly ionic solids with similar cation/anion radii.
- Leads to increased electrical conductivity at higher temperatures, due to ion mobility.
The number of Schottky defects, n, in a crystal varies with temperature, and is approximately given by:
n ∝ e−E/2kT, where E = energy required to create a defect, k = Boltzmann constant, T = temperature (K).
Frenkel Defect: Definition, Features, and Effects
Frenkel defect occurs when a cation (smaller ion) leaves its normal lattice position and occupies an interstitial site (space between lattice points) within the same crystal. The total number of ions remains the same, so electrical neutrality and chemical composition are preserved. This defect is found in ionic solids with a large difference in ion size, especially when cations are much smaller, such as AgBr, AgCl, ZnS.
- Known as a displacement or interstitial defect (ion moves from normal to interstitial position within the lattice).
- Does not affect crystal density—mass and volume remain unchanged.
- No change in chemical stoichiometry; still a stoichiometric defect.
- Occurs in crystals where there is a significant size difference between cation and anion.
- Enhances ionic conductivity, especially in silver halides (mobile cations).
Frenkel defects generally only involve cations, because anions are usually too large to fit into interstitial sites.
Difference Between Schottky and Frenkel Defect
Here is a comparative table highlighting the core difference between Schottky and Frenkel defect for fast revision:
| Property | Schottky Defect | Frenkel Defect |
|---|---|---|
| What happens | Ions missing from lattice sites (vacancy) | Ion displaced from lattice to interstitial site |
| Density | Decreases (mass lost, volume same) | Unchanged (mass/volume unchanged) |
| Common in | Ionic solids with similar sized ions (NaCl, KCl) | Ionic solids with dissimilar ions (AgCl, ZnS) |
| Types of ions | Both cations and anions lost | Usually cations displaced only |
| Electrical conductivity | Increases slightly at high temperature | Increases due to mobile cations |
| Stoichiometry | No change (stoichiometric defect) | No change (stoichiometric defect) |
Schottky vs Frenkel defects is a key test area for distinguishing between different point defects and understanding how they change physical properties of solids.
Examples and Applications in JEE Problems
Knowing compound examples makes recalling the difference between Schottky and Frenkel defect easier in the exam:
- Schottky defect is seen in: NaCl, KCl, CsCl, RbBr
- Frenkel defect is seen in: AgCl, AgBr, AgI, ZnS
- NaCl shows mainly vacancy defects; AgBr can exhibit both defects simultaneously.
- MCQ: "Which compound shows a decrease in density due to missing ions?" (Schottky example answer: NaCl.)
- Numerical: If a NaCl crystal has 2% Schottky defects, calculate the change in density—a loss of mass!
The knowledge of stoichiometric and non-stoichiometric defects helps further understand the context for these defects. Also, applications in solid-state ionic conductivity, density calculations, and even device material choice for electrical properties of solids are crucial for higher-level reasoning.
Quick Practice and Revision Notes
- Remember: Schottky = vacancy; Frenkel = displacement to interstitial.
- Density falls only in Schottky-type solids.
- Frenkel defects do not affect mass, just position.
- Schottky occurs in ionic solids with equal ion sizes; Frenkel in solids with small cations.
- Both are stoichiometric defects (chemical composition not altered).
- JEE MCQs often ask for effects on density and conductivity, or to match compounds to defect types.
Summary: The difference between Schottky and Frenkel defect lies in the type of imperfection created (vacancy versus displacement), effect on density, and occurrence pattern in different ionic crystals—core knowledge for mastering the solid state chapter in the JEE Main Chemistry syllabus.
For more on similar topics, explore these JEE links: Difference Between Strong Acid and Weak Acid, Atomic Weight vs Atomic Mass, Compound vs Mixture, and Cell vs Battery—all crucial for solid revision. Content reviewed by Vedantu Chemistry experts for JEE Main accuracy and clarity.
FAQs on Schottky vs Frenkel Defect: Key Differences, Examples & Effects
1. What is the difference between Schottky defect and Frenkel defect?
Schottky defect and Frenkel defect are both types of point defects in ionic solids but differ in how ions are displaced.
- In Schottky defect, equal numbers of cations and anions are missing from their lattice sites, creating vacancies and leading to a decrease in the solid's density.
- In Frenkel defect, a smaller ion (typically a cation) leaves its regular lattice site and occupies an interstitial site, creating a vacancy and an interstitial defect without changing the overall density.
These defects impact physical properties such as density and ionic conductivity and are important for board and entrance exams.
2. What is the Schottky defect?
Schottky defect is a type of stoichiometric point defect where equal numbers of cations and anions are missing from the lattice sites of an ionic solid.
- Seen in ionic crystals with similar size of cations and anions (e.g., NaCl, KCl)
- It creates vacant lattice sites, lowering the solid's density
- Maintains electrical neutrality
- Increases ionic conductivity slightly due to movable ions
This defect is commonly tested in competitive and board exams for its definition and impact on crystal density.
3. What is the Frenkel defect in simple terms?
Frenkel defect occurs when a smaller ion, usually a cation, moves from its normal lattice site to a position between lattice points (interstitial site), creating a vacancy and an interstitial defect, but keeping the number of ions the same.
- Found in crystals where there is a large difference in size between cations and anions (e.g., AgCl, ZnS)
- Does not change the density of the substance
- Maintains electrical neutrality
- Frequently asked in class 12 Chemistry and competitive exams as a short note or MCQ.
4. What are the key differences between Schottky and Frenkel defects?
The key differences are:
- Schottky defect involves loss of equal numbers of cations and anions from the crystal lattice, lowering density.
- Frenkel defect involves a smaller ion shifting to an interstitial site, with no change in overall density.
- Schottky is seen in crystals with similar-sized ions (e.g., NaCl), while Frenkel appears in those with large size differences (e.g., AgCl).
- Both maintain electrical neutrality, but their impact on density and ionic arrangement is distinct.
These points are often tested in comparative table and 'distinguish between' questions.
5. How do Schottky and Frenkel defects affect density of ionic solids?
Schottky defect decreases the density of the crystal because ions are missing from the lattice, reducing mass without changing volume.
Frenkel defect does not alter the density since no ions are actually lost; the total number of particles remains the same, only their position changes within the crystal.
This is a key difference for JEE/NEET and CBSE exams.
6. Give examples of compounds showing Schottky and Frenkel defects.
Common examples include:
- Schottky defect: NaCl, KCl, CsCl, AgBr
- Frenkel defect: AgCl, ZnS, AgBr, AgI
Some crystals like AgBr can show both defects under specific conditions. These examples are frequently asked in board and entrance exams.
7. Can both Schottky and Frenkel defects occur simultaneously in the same crystal?
Yes, certain substances such as silver bromide (AgBr) can exhibit both Schottky and Frenkel defects at the same time due to their unique crystal structure and ion sizes. This combined defect alters both vacancy and interstitial site populations and may moderately affect both density and conductivity.
8. Does Schottky defect affect electrical conductivity? How?
Schottky defect can slightly increase the electrical conductivity of an ionic solid by creating vacancies which allow ions to move more freely through the crystal lattice.
This enhanced mobility of ions assists in ionic conduction, especially at higher temperatures.
9. Why does the Frenkel defect not change the density of the crystal?
Frenkel defect does not change the crystal's density because no ions are lost or added; a small ion just moves from its regular position to an interstitial site. Therefore, the mass and overall volume of the crystal remains unchanged, maintaining the same density before and after the defect.
10. Which defect is more likely in crystals with large differences in cation and anion size?
In crystals where there is a large size difference between cation and anion (with the cation being much smaller), the Frenkel defect is more likely to occur. Small cations can easily shift to interstitial sites without disturbing the larger anions, as seen in AgCl, ZnS, and AgI.
11. Are Schottky and Frenkel defects only seen in ionic solids?
Yes, both Schottky and Frenkel defects are typically observed in ionic solids, as their mechanism of ion vacancies and movements relies on distinct cations and anions present in the lattice. Covalent and metallic solids do not generally show these specific point defects.
