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

• A. Both ions have the same ionic radius
• B. Cannot be determined from the information given
• C. O^{2-}
• D. Mg^{2+}

Answer: (C)

Verified step by step guidance

1. Understand that ionic radius depends on the number of protons (nuclear charge) and the number of electrons in the ion. Both Mg^{2+} and O^{2-} have the same number of electrons (10), making them isoelectronic.
2. Recognize that Mg^{2+} has 12 protons attracting 10 electrons, while O^{2-} has 8 protons attracting the same 10 electrons. The greater the nuclear charge, the stronger the pull on the electrons, resulting in a smaller ionic radius.
3. Since Mg^{2+} has more protons than O^{2-}, it exerts a stronger effective nuclear charge on the electrons, pulling them closer and decreasing its ionic radius.
4. Conversely, O^{2-} has fewer protons, so the electrons experience less attraction and spread out more, leading to a larger ionic radius.
5. Therefore, by comparing the effective nuclear charge and electron count, you conclude that O^{2-} has a larger ionic radius than Mg^{2+}.