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

Explanation

Two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical meutrality. Hence, some cation vacancies are created. For example, if in the ionic solid, `Na^(+)Cl^(-)`, impurity of `Sr^(2+)` is added (as `SrCl_(2)`), two `Na^(+)` lattice sites will become vacant and one of these will be occupied by `Sr^(2+)` ion and the other will remain vacant.