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Q.
The reason for the stability of $Gd^{3+}$ ion is
The d-and f-Block Elements
Solution:
Gd (Z = 64) : $[Xe]^{54}\, 4f^7 \,5d^1 \, 6s^2 $
$ Gd^{3+} : [Xe]^{54} \, 4f^7$
$ Gd^{3+}$ is stable due to the presence of exactly half filled $4f$-subshell.