Discussion
Which of the following lanthanoid has the outer electronic configuration 4f76s2 in its ground state?
Answer: C
The lanthanoid with 4f76s2 is Europium (Eu, Z = 63).
Across the lanthanoids the 4f subshell fills one electron at a time: Nd [Xe]4f46s2, Pm 4f56s2, Sm 4f66s2, Eu 4f76s2. Europium reaches the stable half-filled 4f7 arrangement with no 5d electron.
Gadolinium (Z = 64) is the trap. It also has seven 4f electrons, but its extra electron goes into 5d rather than 4f, giving [Xe]4f75d16s2, which keeps the 4f set half-filled. So Gd does not have the configuration 4f76s2.
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