Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{NaCl}\) shows Schottky defects and \(\mathrm{AgCl}\) Frenkel defects. Their electrical conductivity is due to (a) lower coorination number of \(\mathrm{NaCl}\) (b) higher coordination number of \(\mathrm{AgCl}\) (c) motion of ions and not the motion of electrons (d) motion of electrons and not the motion of ions

Step-by-Step Solution

Answer

(c) motion of ions and not the motion of electrons

Step 1: Understand the Types of Defects

Schottky and Frenkel defects are two types of point defects in crystalline solids. Schottky defects involve vacancies created by the missing cations and anions, whereas Frenkel defects involve a cation leaving its normal site to occupy an interstitial site within the crystal lattice.

Step 2: Role of Schottky Defects in NaCl

In crystals exhibiting Schottky defects, like NaCl, the missing ions (both anions and cations) create vacancies that allow neighboring ions to move into these vacant spots. This motion facilitates ionic conductivity, which refers to the movement of ions within the crystal lattice.

Step 3: Role of Frenkel Defects in AgCl

In the case of AgCl, Frenkel defects lead to a shift of the Ag+ ions from their normal position to interstitial positions. This movement provides pathways for other Ag+ ions to migrate, enhancing ionic conductivity in the crystal.

Step 4: Eliminating Incorrect Options

Option (a) and (b) relate to coordination numbers, which do not directly impact the mechanism of conductivity. Option (d) suggests electron movement, which is not the primary mechanism in ionic conductors. Hence, these options can be discounted.

Step 5: Conclusion on the Source of Conductivity

Both Schottky and Frenkel defects enhance ionic, and not electronic, movement within the crystals. Thus, the electrical conductivity in both NaCl and AgCl due to these defects is primarily because of the motion of ions.

Key Concepts

Schottky Defects

Schottky defects are fascinating types of point defects in crystalline solids. They occur when equal numbers of cations and anions are missing from their regular lattice sites, creating vacancies. This defect is named after the German physicist Walter Schottky, who made significant contributions to our understanding of solid-state physics.
In crystals like sodium chloride (NaCl), Schottky defects play an important role. These vacancies allow ions to move more easily throughout the crystalline lattice. As ions swap positions through these empty spaces, the material exhibits ionic conductivity. The overall effect is that the motion of ions increases, contributing significantly to the electrical conductivity of the crystal.
Schottky defects are critical in explaining the ionic conduction in certain solid electrolytes, providing pathways for ions to migrate and thus allowing the material to conduct electricity. This process is essential in various applications, like sensors and batteries.

Frenkel Defects

Frenkel defects offer another intriguing perspective on point defects within crystalline structures. They occur when a cation moves from its regular site to an interstitial site, which is a site not normally occupied in the crystal lattice. This type of defect is named after Yakov Frenkel, a physicist who studied atomic displacements in solids.
Silver chloride (AgCl) is an excellent example of a material exhibiting Frenkel defects. In AgCl, silver ions ( Ag^{+} ) leave their regular positions and occupy interstitial spaces within the lattice.
What makes Frenkel defects unique is that they do not affect the overall charge balance of the crystal because the displaced cation remains within the lattice structure. Instead, they create more pathways for ions like Ag+ to migrate. This movement enhances the ionic conductivity of the crystal tremendously.
Frenkel defects are particularly important in materials where maintaining a compact crystal structure is crucial while still allowing some degree of ionic movement. This is why materials with Frenkel defects often find use in solid-state batteries and other advanced technologies.

Crystalline Solids

Crystalline solids form the basis for many materials and have a well-organized, repeating pattern of atoms, ions, or molecules. These structures are characterized by their long-range order and regularity, allowing them to exhibit unique physical properties.
Understanding the role of crystalline solids is key to grasping how materials conduct electricity, especially through ionic movements. Their organized lattice structures can accommodate point defects like Schottky and Frenkel defects. These defects disrupt the perfect order of the crystal but enable the movement of ions, leading to ionic conductivity.
Some striking properties of crystalline solids include:

• Defined melting points delineating phase transitions
• Directional properties due to anisotropy
• Efficient packing that influences density and stability

Crystalline solids are indispensable in materials science because of their ability to control the flow of ions and electrons, making them significant in semiconductor technology and the production of various electronic devices.

Point Defects

Point defects are imperfections, often occurring at or around a single point within a crystal's structure. These defects are crucial in determining a material's electrical and mechanical properties.
There are different types of point defects, but Schottky and Frenkel defects are among the most studied due to their impact on the ionic conductivity of materials. Here’s why point defects are important:

• They alter the lattice dynamics, influencing how atoms or ions vibrate, which can affect thermal conductivity.
• They can introduce localized stress fields that impact mechanical stability and strength.
• Point defects play a pivotal role in doping processes, which tailor the electrical properties of semiconductors.

Understanding point defects helps in designing materials with specific desired properties, from conducting electricity to withstanding high temperatures and pressures. These defects allow for the constructive manipulation of materials, tailored to specific technological needs.