question_answer 21)

Explain how vacancies are introduced in an ionic solid when a cation of higher valence in added as an impurity in it. 

Answer:

When molten NaCl (Na+ is monovalent) containing a little amount of SrCl2 (Sr2+ is divalent) as impurity is crystallised, some of the sites of Na+ ions are occupied by Sr2+. Each Sr2+ replaces two Na+ ions. One site is occupied by Sr2+ and the other site remains vacant. The cationic vacancies thus produced are equal to the number of Sr2+ ions.