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Among the following pairs of ions, identify the pair in which BOTH ions possess a half-filled 4f subshell and therefore exhibit a stable oxidation state.

1. Eu2+ and Tb4+
2. Ce3+ and Yb2+
3. Lu3+ and Gd2+
4. Pr5+ and Nd2+

Correct answer: Eu2+ and Tb4+

Solution

Eu2+ has configuration [Xe]4f⁷ and Tb4+ also has [Xe]4f⁷, both having a half-filled f subshell which imparts extra stability. All other pairs lack both ions with half-filled f shells simultaneously.

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