A hydrated Eu³⁺ salt has experimental magnetic moment 3.4 BM at 27°C, significantly different from its calculated ground-state value. Why? Eu atomic number is 63.
Correct answer: (A) Thermal population of higher J levels.
Explanation
Eu³⁺ is 4f⁶, with a ⁷F₀ ground level whose J = 0 gives zero in a ground-level-only moment calculation. Nearby higher-J components of the ⁷F term can be thermally populated at room temperature and contribute magnetism. Thus A explains the discrepancy; spin–orbit coupling sets the levels but thermal population explains why the measured finite-temperature value differs from the J = 0 value.
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