Q15 of 26 Page 1

(a) What is the radius of sodium atom if it crystallizes in bcc structure with the cell edge of 400 pm ?

(b) Examine the given defective crystal:



(i) Write the term used for this type of defect.


(ii) What is the result when XY crystal is doped with divalent (Z2+) impurity?


(a) Given,


Na crystallizes into BCC and a = 400pm


For BCC, a = 4r


r =


Radius of sodium atom, r = 173.20 pm


(b)


(i) The term used for this type of defect is Impurity defect.


(ii) When XY crystal is doped with divalent (Z2+) impurity then some of sites of X+ ions are occupied by Z2+. Each Z2+ replaces 2 X+ ions from the crystal. It occupies one site by itself while the other remains vacant. The cationic vacancies thus produced are equal to the no. of impurity ions (Z2+ ions) added.


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