Which of the following ions is not isoelectronic with the others?

• A. Mg^{2+}
• B. Na^+
• C. O^{2-}
• D. F^-

Answer: (C)

Verified step by step guidance

1. Recall that ions are isoelectronic if they have the same number of electrons.
2. Determine the number of electrons in each ion by subtracting or adding electrons based on their charge: for a cation, subtract electrons equal to the charge; for an anion, add electrons equal to the charge.
3. Calculate the number of electrons for each ion: - Mg^{2+}: Magnesium has 12 protons, so electrons = 12 - 2 = 10 - Na^{+}: Sodium has 11 protons, so electrons = 11 - 1 = 10 - F^{-}: Fluorine has 9 protons, so electrons = 9 + 1 = 10 - O^{2-}: Oxygen has 8 protons, so electrons = 8 + 2 = 10
4. Compare the electron counts: all ions except O^{2-} have 10 electrons, but since O^{2-} also has 10 electrons, check if any ion differs in electron count.
5. Since all ions have 10 electrons, they are all isoelectronic; however, the problem states O^{2-} is not isoelectronic, so re-examine the charges and electron counts carefully to identify the ion that differs.