Question: 1.19 - What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?
Answer: When a solid is heated a vacancy defect may arise. Because of vacancy defect the density of the solid decreases because of leaving of some of the constituent particles.
Question: 1.20 - What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr
Answer: (i) ZnS – Since there is large difference in the size of ions, thus it shows Frenkel defect.
(ii) AgBr – AgBr shows Frenkel defects and Schottky defects both.
Question: 1.21 - Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
Answer: In ionic solids, such as NaCl; during crystallization; a little amount of SrCl2 is also crystallized. In this process, Sr++ ions get the place of Na+ ions and create defects in the crystal of NaCl. In this defect, each of the Sr++ ion replaces two Na+ ions. Sr++ ion occupies one site of Na+ ion; leaving other site vacant. Hence it creates cationic vacancies equal number of Sr++ ions. CaCl2, AgCl, etc and is called vacancy defects. This is also known as impurities defects.
Question: 1.22 - Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
Answer: Metal excess defects are seen because of missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called F-centre and responsible for showing colour by the compound.
This defect is common in NaCl, KCl, LiCl, etc. Sodium atoms get deposited on the surface of crystal when sodium chloride is heated in an atmosphere of sodium vapour. In this process, the chloride ions get diffused with sodium ion to form sodium chloride and sodium atom releases electron to form sodium ion. This released electron gets diffused and occupies the anionic sites in the crystal of sodium chloride; creating anionic vacancies and resulting in the excess of sodium metal.
The anionic site occupied by unpaired electron is called F-centre. When visible light falls over the crystal of NaCl, the unpaired electron present gets excited because of absorption of energy and impart yellow colour. Because of similar defect if present, crystal of LiCl imparts pink colour and KCl imparts violet.
Question: 1.23 - A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?
Answer: Silicon and germanium, each has four valence electrons and they belong to 14th group of periodic table. Arsenic and phosphorous belong to 15th group of periodic table and they have valence electrons equal to 5. When silicon or germanium is doped with phosphorous or arsenic, four electrons of phosphorous or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon or germanium. Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged particle (electron), thus this is known as n-type of semiconductor.
The impurities belong to group 15 which is introduced with elements of group 14 to produce n-type of semiconductor.
Question: 1.24 - What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer.
Answer: Metal ions of ferromagnetic substances are randomly oriented in normal condition and substances do not act as a magnet. But when metal ions are grouped together in small regions, called domains, each domains act like a tiny magnet and produce strong magnetic field, in such condition ferromagnetic substance act like a magnet. When the ordering of domains in group persists even after removal of magnetic field a ferromagnetic substance becomes a permanent magnet. While domains are grouped in parallel and anti-parallel direction but in unequal number in ferromagnetic compounds.
Thus, Ferromagnetic substances, such as Ni, Co, Fe would make better permanent magnets rather than ferromagnetic substances.
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