Final answer:
The correct formula for the oxide ion is O2-. It gains two electrons to form a stable anion that pairs with cations to create neutral ionic compounds like Al2O3.
Step-by-step explanation:
The correct formula for the oxide ion is C) O2-. Oxygen typically forms an anion with a -2 charge because it gains two electrons to achieve a stable electron configuration, similar to the noble gases. In ionic compounds, this oxide ion must balance out with cations to create a neutral compound. A classic example is aluminum oxide, which consists of aluminum ions (Al3+) and oxide ions (O2-) in a ratio that balances their charges, resulting in the formula Al2O3. This compound is electrically neutral because two aluminum ions (+3 each, totaling +6) balance with three oxide ions (-2 each, totaling -6), demonstrating the principle of charge neutrality in ionic compounds.