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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'

Schottky defects : When cations and anions both are missing from regular sites, the defect is called Schottky Defect. In Schottky Defects, the number of missing cations is equal to the number of missing anions in order to maintain the electrical neutrality of the ionic compound.

Schottky Defect is type of simple vacancy defect and shown by ionic solids having cations and anions; almost similar in size, such as NaCl, KCl, CsCl, etc. AgBr shows both types of defects, i.e. Schottky and Frenkel Defects.

Since, Schottky Defects arises because of mission of constituent particles, thus it decreases the density of ionic compound.

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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)?

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