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The Solid State

Short Answer Type

131.

In terms of band theory, what is the difference between a conductor and an insulator?

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132.

In terms of band theory, what is the difference between a conductor and a semi-conductor?

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133. Explain the following terms with suitable examples:
'Schottky defect'

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134. Explain the following term with suitable examples:
'Frenkel defect'

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135. Explain the following term with suitable examples:
'Interstitials'.

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136. Explain the following term with suitable examples:
'F-centres'.

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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?

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

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Long Answer Type

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 O^2–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 O^2– in MgO.

But the magnitude of these coulombic forces of attraction is much higher in MgO as compared to that in NaF. (Mg²⁺ is divalent while Na⁺is monovalent, similarly O^2– and F^–).

The electrostatic forces of attraction between Mg²⁺and O^2– 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.

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Short Answer Type

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

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