Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 11

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Q: 23. Explain the following terms with suitable examples:

(i) Schottky defect

(ii) Frenkel defect

(iii) Interstitials and

(iv) F-centres

Ans:

(i) Schottky Defect:

Schottky defect is basically a vacancy defect shown by ionic solids. In this defect, an equal number of cations and anions are missing to maintain electrical neutrality. It decreases the density of a substance. Significant number of Schottky defects is present in ionic solids. For example, in , there are approximately Schottky pairs per at room temperature. Ionic substances containing similar-sized cations and anions show this type of defect. For example: , etc.

(ii) Frenkel Defect:

Ionic solids containing large differences in the sizes of ions show this type of defect. When the smaller ion (usually cation) is dislocated from its normal site to an interstitial site, Frenkel defect is created. It creates a vacancy defect as well as an interstitial defect. Frenkel defect is also known as dislocation defect. Ionic solids such as , and show this type of defect.

(iii) Interstitials:

Interstitial defect is shown by non-ionic solids. This type of defect is created when some constituent particles (atoms or molecules) occupy an interstitial site of the crystal. The density of a substance increases because of this defect.

(iv) F-centres:

When the anionic sites of a crystal are occupied by unpaired electrons, the ionic sites are called F-centres. These unpaired electrons impart colour to the crystals. For example, when crystals of are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The ions diffuse from the crystal to its surface and combine with Na atoms, forming . During this process, the atoms on the surface of the crystal lose electrons. These released electrons diffuse into the crystal and occupy the vacant anionic sites, creating F-centres.

Q: 24. Aluminium crystallises in a cubic close-packed structure. Its metallic is 125 pm.

(i) What is the length of the side of the unit cell

(ii) How many unit cells are there in aluminium?

Ans:

(i) For cubic close – packed structure:

(ii) Volume of one unit cell

Therefore, number of unit cells in