question : Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it ?
key points to remember answer of this question : first of all Recall the definition of impurity defects occurred in solids with examples also , you have to keep in mind the definition of impurity defects and their examples and remember , a solid as a whole is neutral in nature.
solution : When a cation (positively charged ion) of higher valence is added as an impurity to an ionic solid, some vacancies are created in the solid. This phenomena of creating vacancies in any solid can be explained with the help of an example given below.
When strontium chloride (SrCl2) is added as an impurity to ionic solid sodium chloride (NaCl).
so we can say in short that –
impurity = SrCl2 (strontium chloride)
ionic solid = NaCl (sodium chloride)
due to addition of these impurities in ionic solid two vacant sites are created by removal of one Na+ ion. it means one Na+ ion will remove and two vacant sites (empty spaces) will create. from these two vacant One vacant site is replaced by Sr2+ ion , means one vacant will be fill by one Sr2+ ion but the other remains vacant means second vacant will be still empty or vacant. The reason is that the crystal as a whole is to remain electrically neutral. it means when the crystal suppose as a whole must be electrically neutral that means the positive and negative ions present in crystal must be equal so that the crystal contain overall zero charge on it. that is called electrically neutral crystal.
Note : [Cationic vacancies produced = No. of cations of higher valency X difference in valencies of the original cation and cation of higher valency]
remember this formula for your competitive exams , several times questions directly occurred from this equation.