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Question:

The electronic configuration of Eu (Atomic No. 63), Gd (Atomic No. 64), and Tb (Atomic No. 65) are:

[Xe]4f65d16s2,[Xe]4f75d16s2and[Xe]4f85d16s2

[Xe]4f75d16s2,[Xe]4f75d16s2and[Xe]4f96s2

[Xe]4f76s2,[Xe]4f75d16s2and[Xe]4f96s2

[Xe]4f76s2,[Xe]4f86s2and[Xe]4f85d16s2

Solution:

The electronic configuration of Eu (Atomic No. 63), Gd (Atomic No. 64), and Tb (Atomic No. 65) are: [Xe]4f76s2, [Xe]4f75d16s2, and [Xe]4f96s2 respectively. Gd possesses a single electron in the 5d subshell. In the case of other lanthanoids, the 5d orbital is empty. Hence, the correct option is C.