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The electronic configurations of Eu (Atomic no. 63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are
A
$[Xe]4f^65d^16s^2$, $[Xe]4f^75d^16s^2$ and $[Xe]4f^96s^2$
B
$[Xe]4f^65d^16s^2$, $[Xe]4f^75d^16s^2$ and $[Xe]4f^85d^16s^2$
C
$[Xe]4f^76s^2$, $[Xe]4f^75d^16s^2$ and $[Xe]4f^96s^2$
D
$[Xe]4f^76s^2$, $[Xe]4f^86s^2$ and $[Xe]4f^85d^16s^2$
Explanation
Half-filled $4f^7$ is favoured in Eu and Gd.
Detailed Solution
$_{63}Eu = [Xe]4f^76s^2$
$_{64}Gd = [Xe]4f^75d^16s^2$
$_{65}Tb = [Xe]4f^96s^2$
$_{64}Gd = [Xe]4f^75d^16s^2$
$_{65}Tb = [Xe]4f^96s^2$