JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech I Year ENGINEERING PHYSICS Previous Question Paper (July - 2021 - R13)

An ionic bond is formed by the electrostatic attraction between oppositely charged ions. It typically occurs between a metal (which loses electrons to form a cation) and a non-metal (which gains electrons to form an anion). The process involves three stages: ionization of the metal, electron capture by the non-metal, and the subsequent electrostatic attraction between the ions.

The cohesive energy (U) of an NaCl molecule is the energy required to dissociate the molecule into its constituent ions at infinite separation. It is calculated using the expression:

Where e is the electronic charge, ε₀ is the permittivity of free space, r₀ is the equilibrium interatomic distance, and n is the Born exponent (typically 8 for NaCl). Substituting the values, the cohesive energy represents the total potential energy at the equilibrium separation distance, accounting for both attractive Coulombic forces and repulsive forces due to Pauli exclusion.

Crystal imperfections, or defects, are deviations from the perfect periodic arrangement of atoms in a crystal lattice. They are classified based on their dimensionality:

• Point Defects (0-D): These involve single atoms or vacant lattice sites. Examples include vacancies (missing atoms), interstitials (extra atoms in non-lattice sites), and impurities (substitutional or interstitial foreign atoms). Schottky and Frenkel defects are specific ionic point defects.
• Line Defects (1-D): Also known as dislocations, these are lines along which atoms are misaligned. The two primary types are edge dislocations (an extra half-plane of atoms) and screw dislocations (a helical displacement of atoms).
• Surface/Planar Defects (2-D): These occur at the boundaries of crystals. Examples include grain boundaries (interfaces between crystals of different orientations), twin boundaries, and stacking faults.
• Volume Defects (3-D): These are macroscopic defects such as pores, cracks, foreign inclusions, or clusters of point defects that occupy a finite volume within the crystal.