Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not correct consequences of Schottky defect? (a) stability of crystal decreases (b) density decreases (c) electrical conductivity increases (d) lattice energy of crystal increases

Step-by-Step Solution

Answer

The incorrect consequence of Schottky defect is (d) 'lattice energy of crystal increases'.

Step 1: Understand Schottky Defect

Schottky defects are a type of point defect in crystals where equal numbers of cations and anions are missing from the lattice sites, maintaining electrical neutrality of the crystal. This defect typically occurs in ionic crystals and affects their properties.

Step 2: Impact on Crystal Stability

Since Schottky defects involve the loss of ions from the lattice without replacement, the regular arrangement of ions is disrupted, thereby decreasing the overall stability of the crystal structure. Hence, the statement (a) 'stability of crystal decreases' is correct.

Step 3: Effect on Density

Loss of ions results in a decrease in the total mass of the crystal without affecting its volume. Therefore, the density of the crystal decreases. Thus, statement (b) 'density decreases' is correct.

Step 4: Influence on Electrical Conductivity

The creation of Schottky defects leaves vacancies that allow ions to move more easily through the lattice. This improved mobility enhances the electrical conductivity of the crystal. Therefore, statement (c) 'electrical conductivity increases' is correct.

Step 5: Analyzing Lattice Energy

Lattice energy is the energy required to assemble ions into a lattice. Schottky defects create voids in the lattice, reducing the number of electrostatic interactions among ions. Consequently, this decreases the overall lattice energy. Therefore, statement (d) 'lattice energy of crystal increases' is incorrect.

Key Concepts

Crystal Stability

Crystal stability is central to understanding the behavior of ionic crystals. When we talk about stability, we refer to how well the crystal can maintain its organized structure against deformations or defects. Schottky defects decrease crystal stability by creating vacancies where ions should be. This disruption makes the crystal less rigid and more susceptible to structural collapse under certain conditions. Such defects can alter the crystal's mechanical properties, making them significant in determining the material's robustness.

Density and Defects

In a crystal, density refers to the mass per unit volume. When Schottky defects are present, some ions in the lattice are missing. This removal of ions leads to a reduction in the mass of the crystal while its volume remains unchanged. This results in overall reduced density. In practical terms, lower density in materials can influence how they handle external stress and their thermal properties. Schottky defects illustrate how imperfections at the atomic level can have macroscopic effects.

Electrical Conductivity in Ionic Crystals

Electrical conductivity is a material's ability to conduct electric current. In ionic crystals, this primarily involves the movement of ions rather than electrons. Schottky defects create voids that facilitate ionic mobility. As ions move to fill vacancies, the ease of their movement enhances conductivity. This is an essential concept in electronics and materials science, as it helps in designing better semiconductors and insulators. Understanding how defects influence ion movement is key to developing new materials with desired electrical properties.

Lattice Energy

Lattice energy quantifies the strength of the forces holding the ions together in a crystal. It is the amount of energy needed to form the crystal from its component ions. Schottky defects reduce the number of ion-ion interactions, thereby lowering the lattice energy. A lower lattice energy means the bonds between ions are weaker, resulting in a less stable crystal. This concept is crucial for predicting the thermal stability of a crystal and its response to external forces.

Point Defects in Crystals

Point defects are imperfections localized at a single or a few lattice sites in a crystal structure. These include vacancies like Schottky defects, interstitials, and substitutional defects. These small-scale defects can significantly affect the macroscopic properties of the material, such as mechanical strength, electrical conductivity, and optical properties. By studying point defects, scientists can tailor the material characteristics by controlling the type and concentration of defects, paving the way for advanced materials with specific functionalities.