Final answer:
The ratio of ions in an ionic compound is determined by the need for electrical neutrality, reflecting the simplest whole-number ratio of ions that balances the total positive and negative charges.
Step-by-step explanation:
The ratio of ions in an ionic compound is determined by the requirement that the compound must be electrically neutral. This means that the total number of positive charges on the cations must equal the total number of negative charges on the anions. The formula of an ionic compound reflects the simplest whole-number ratio of ions that balances these charges. For example, in aluminum oxide, Al2O3, there are two aluminum cations (Al3+) for every three oxide anions (O2-), giving a neutral compound: (2 × +3) + (3 × -2) = 0. The relative sizes of the ions and the ratio of the numbers of cations to anions are essential in forming a stable ionic structure.