The Solid State

Question 137

Examine the illustration of a portion of the defective crystal given below and answer the following questions:
(i) What are these types of vacancy defects called?
(ii) How is the density of a crystal affected by these defects?
(iii) Name one ionic compound which can show this type of defect in the crystalline state.
(iv) How is the stoichiometry of the compound effected?


(i) These type of vacancy defects are called Schottky defects.
(ii) This defect decreases the density of the crystal.
(iii) NaCl shows this type of defect in the crystalline state.
(iv) This is the point defect which does not disturb stoichiometry of the solid.

Question 138

Schottky defects generate an equal number of cation and anion vacancies while doping produces only cation vacancies and not anion vacancies. Why?

Schottky defect also exists in pair to maintain electroneutrality. Therefore, it generates equal number of cation and anion vacancies. In case of doping of NaCl by CdCl2, CI ions enter Cl ions site in NaCl while Cd2+ displace Na+. In doping so, one positive charge becomes extra. Therefore one Na+ is displaced so as to maintain electro-neutrality. That is doing only cation vacancies exist.
Question 139

The ions of NaF and MgO all have the same number of electrons, and the internuclear distance are about the same (235 pm and 215 pm). Why then are the melting points of NaF and MgO so different. (992°C and 2642°C)?

The crystals of NaF and MgO are formed by Na+ and F- in NaF and Mg2+ and O2–ions in MgO respectively arranged in cubic closed structures. There are strong electrovalent bond forces (strong coulombic forces, attraction between ions) between Na+and F in NaF and Mg2+ and O2– in MgO.

But the magnitude of these coulombic forces of attraction is much higher in MgO as compared to that in NaF. (Mg
2+ is divalent while Na+is monovalent, similarly O2– and F).

The electrostatic forces of attraction between Mg
2+and O2– is almost 4 times as compared to Na+ and F ions. Melting point of ionic solids is almost the index of inter-ionic attraction in crystal lattice as lot of energy is required to break these forces/overcome these forces before the substance melts.

It changes into paramagnetic at hight temperature due to randomization of spins.

            (b)        The ions in MgO carry two unit charges. In NaCl only one unit charge. Hence electrostatic forces of attraction in MgO are stronger.

Question 140

How would you account for the following:
Frenkel defects are not found in alkali metal halides.

The frenkel defect is favoured by a large diffrance in size between the positive and negtive ion the metal cation are generally smaller than the anion which occupy the vacant lattice site or hole.

Frenkel defects are not found in alkali metal halides because the ions cannot get into interstitial positions due to their large sizes.

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