Plus Two Notes Chemistry Chapter -01 Text Activities Part-03 

21. Gold (atomic radius = 0.144 nm) crystallises in a face-centred unit cell. What is the length of a side of the cell?  

For face centred cubic cell edge length  2 x 1.4142 x 0.144 mm = 0.407 nm 

22. In terms of band theory, what is the difference 

(i) between a conductor and an insulator 

(ii) between a conductor and a semiconductor? 

(i) A conductor may conduct electricity through movement of electrons or ions. Metallic conductors belong to the former category and electrolytes to the latter. If the gap between filled valence band and the next highe unoccupied band (conduction band) is large, electrons cannot jump to it and such a substance has very small conductivity and it behaves as an insulator. 

(ii) In case of semiconductors, the gap between the valence band and  conduction band is small Therefore,       some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called intrinsic semiconductors. 

23. Explain the following terms with suitable examples: 

(i) Schottky defect 

(ii) Frenkel defect 

(iii) Interstitials and 

(iv) F-centres. 

1. schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal Like simple vacancy defect, Schottky defect also decreases the density of the substance. Number of such defects in ionic solids is quite significant. For example, in NaCl there are approximately 106  Schottky pairs per cm3 at room temperature. In 1 cm3  there are about 1022 ions. Thus, there is one Schottky defect per 1016 ions. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. 

For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects. 

2. Frenkel Defect:This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl,ArgBr and AgI due to small size of Zn2+ and Ag+ ions. 

3. Interstitial Defect: When some constituent particles(atoms or molecules) occupy an interstitial site, the crystal is said to have interstitial defect. This defect increases the density of the substance. Vacancy and interstitial defects as explained above can be shown by non-ionic solids. Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel and Schottky defects.  

4. F centers- The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for color Centre). They impart yellow colors to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet (or lilac). 

24. Aluminum crystallises in a cubic close-packed structure. Its metallic radius is 125 pm.  
(i) What is the length of the side of the unit cell? 
(ii) How many unit cells are there in 1.00 cm3 of aluminum?

25. If NaCl is doped with 10–3 mol % of SrCl2, what is the concentration of cation vacancies? 

26. Explain the following with suitable examples: 
(i) Ferromagnetism 
(ii) Para magnetism 
(iii) Ferrimagnetism 
(iv) Antiferromagnetism 
(v) 12-16 and 13-15 group compounds 
1. Ferromagnetism: A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances. Besides strong attractions, these substances can be permanently magnetized. In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an unmagnetised piece of a ferromagnetic substance the 
domaind are randomly oriented and their magnetic moments get cancelled. 

2. Para magnetism: Paramagnetic substances are weakly attracted by a magnetic field. They are magnetized in a magnetic field in the same direction. They lose their magnetism in the absence of magnetic field. Para magnetism is due to presence of one or more unpaired electrons which are attracted by the magnetic field. O2, Cu2+, Fe3+, Cr3+ are some examples of such substances. 

3. Ferrimagnetism: Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers. They are weakly attracted by magnetic field as compared to ferromagnetic substances. Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4  are examples of such substances. These substances also lose ferrimagnetism on heating and become paramagnetic. 

4. Antiferromagnetism: Substances like MnO showingAntiferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other's magnetic moment.
5. 12 to 16 group  elements - combination of elements of group of elements 12 to 16 which yield some solid compounds. ZnS, CdS are examples . 13 to 15 group elements: solid state of elements are produced by combination of elements of group of 13 to 15 hence it is called as the 13 to 15 group of elements. Examples are InSb, AIP etc…