Explain how vacancies are introduced in a solid NaCl crystal when divalent cations are added to it.
A higher-valence cation is introduced as an impurity in an ionic solid. Less than 2 higher-valence ions replace two or more lower-valance cations or ions. Few locations are left empty in order to ensure electrical neutrality. As an example, when Sr2+Sr2+ is introduced to NaCl, two Na+Na+ ions are replaced by each Sr2+Sr2+ion In other words, Sr2+Sr2+ has the same charge as two ions of Na+Na+, therefore, only one of those ions leaves.
