Final answer:
Ions with similar sizes and stoichiometries can often substitute for one another and form a stable ionic crystal structure such as the simple cubic structure. Ionic crystals are characterized by alternating cations and anions, and while they are insulators in solid form, they conduct electricity when molten or dissolved.
Step-by-step explanation:
When ions have similar properties and are able to substitute for each other in a given crystal structure, this is usually a consequence of similar sizes and stoichiometries. This similarity allows them to fit within the same crystal lattice and maintain the overall crystal's stability.
A common example of such a structural arrangement is the simple cubic structure. Ionic compounds like CsCl, which have similarly-sized cations and anions, often crystallize into this arrangement. The structure can be visualized by imagining unit cells with either cations at the corners and anions in the center, or vice versa.
Ionic crystals are composed of alternating positively-charged cations and negatively-charged anions. These crystals are known for being hard, brittle, and having high melting points. While ionic compounds do not conduct electricity as solids due to the fixed positions of ions, they become conductive when in a molten state or dissolved in aqueous solution, as the ions are free to move.