Final answer:
In ionic compounds, charge balance is maintained through crystal charge neutrality, ensuring the overall stable and electrically neutral state of the compound.
Step-by-step explanation:
The student's question pertains to the way in which charge balance is maintained in ionic compounds. When ions occupy sites that are usually vacant, it is to achieve crystal charge neutrality. This is important because ions are formed when atoms gain or lose electrons to achieve stable electron configurations, resulting in positive or negative charges, known as cations and anions, respectively. Ionic compounds must maintain a balance of these charges to remain stable and neutral overall.
For example, the ionic compound Na2O is formed when two sodium (Na) cations, each losing one electron, combine with one oxide (O) anion that has gained two electrons. The result is a compound that is electrically neutral. This is a manifestation of the principle of charge balance in ionic compounds.
The correct answer to the question is D. Crystal charge neutrality, which ensures that the overall charge is balanced within a crystal structure, maintaining the stability of the compound.