Test: Imperfections in Solids (Old NCERT) - NEET MCQ

In Frenkel defect, the smaller cation is dislocated from its normal position to an interstitial site.

In Frenkel defect, ions get displaced from their original position and move to interstitial sites. Hence, there is no change in the density of the crystal.

Frenkel defect is not shown by alkali metals because cations and anions have almost same size and cations cannot be accommodated in interstitial sites.

The stoichiometric defect shown by ZnS is Frenkel defect which is due to the large difference in the size of the ions, i.e. due to the small size of Zn²⁺.

In ionic crystals, the Schottky defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid.

Equal number of cations and anions are missing from the crystal.

Silver halides generally show both Frenkel and Schottky defects.

Each Sr²⁺ in the NaCl crystal replaces two Na⁺ ions. It occupies the site of one ion and the other remain vacant creating a cation vacancy.

F-centres are electron trapped anion site which are responsible for colour.

The anionic sites occupied by the unpaired electrons are called F-centres from the German word Farbenzenter for colour centre.

F-centres impart yellow colour to the crystal of NaCl when heated in an atmosphere of sodium vapour. Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl violet(or lilac)

Zn²⁺ ions and electrons moves to interstitial sites and F-centres are created which impart yellow colour to ZnO.

FeO is mostly found with a composition of  Fe_0.95O.(Ranges from Fe_0.93O - Fe_0.96O). In crystal of FeO, some Fe²⁺ cations are missing and the loss of positive charge is made up by the presence of the required number of Fe³⁺ ions.

Let the fraction o f metal which exists as M²⁺ be x. Then the fraction of metal as M²⁺ = (0.98 - x)
3x + 2( 0.98 - x) = 2
x + 1.96 = 2
x = 0.04
% of M³⁺ = 0.04/0.98 x 100
= 4.08%

Fe_{0 93}O is a metal deficiency defect. In FeO, some Fe²⁺ ions are missing and the loss of positive charge is made up by Fe³⁺ ions.