Explain how vacancies are introduced in an ionic solid when a cation of higher
valence is added as an impurity in it.
When a cation of higher valence is added to an ionic solid as an impurity
to it, the cation of higher valence replaces more than one cation of lower
valence so as to keep the crystal electrically neutral. As a result, some sites
become vacant. For example, when Sr2+is added to NaCl, each Sr2+�ion
replaces two Na+�ions. However, one Sr2+�ion occupies the
site of one Na+ ion and the other site remains vacant. Hence,
vacancies are introduced.