Final answer:
The Na⁺ ion is smaller than the O₂- ion because cations are always smaller than neutral atoms, while anions are always larger. The Na⁺ ion has a smaller radius than the neutral Na atom because it has lost an electron and achieved a closed shell electron configuration.
Step-by-step explanation:
The Na+ ion is smaller than the O2- ion. Cations are always smaller than the neutral atom they are derived from, while anions are always larger. The Na+ ion has a smaller radius than the neutral Na atom because it has lost an electron and achieved a closed shell electron configuration. On the other hand, the O2- ion is larger than the neutral O atom because it has gained two electrons, resulting in a larger valence electron configuration.