Which ion has a larger ionic radius: Mg^{2+} or O^{2-}?

• A. O^{2-}
• B. Cannot be determined without additional information
• C. Mg^{2+}
• D. Both ions have the same ionic radius

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that ionic radius depends on the number of protons and electrons in the ion. Mg^{2+} has lost two electrons compared to neutral Mg, while O^{2-} has gained two electrons compared to neutral O.
2. Step 2: Recognize that Mg^{2+} has 12 protons and 10 electrons, resulting in a higher effective nuclear charge per electron, which pulls the electrons closer to the nucleus, decreasing the ionic radius.
3. Step 3: Recognize that O^{2-} has 8 protons and 10 electrons, resulting in a lower effective nuclear charge per electron, causing the electrons to be held less tightly and the ionic radius to increase.
4. Step 4: Compare the effective nuclear charge and electron configurations to conclude that O^{2-} will have a larger ionic radius than Mg^{2+} because it has more electron-electron repulsion and less nuclear pull per electron.
5. Step 5: Summarize that the ion with more electrons and less nuclear charge per electron (O^{2-}) generally has a larger ionic radius than the ion with fewer electrons and higher nuclear charge per electron (Mg^{2+}).