Therefore, further addition of electrons occurs in the lower energy $4 f$ - orbital till it is exactly half filled at $Eu \,(Z=63)$
Thus, the electronic configuration of $Eu$ is $[ Xe ] \,4 f ^7 \,6 s ^2$, thereafter, addition of next electron does not occur in the more stable exactly half filled $4 f^7$ shell but occurs in the little higher energy $5 d$ - orbital.
Thus, the electronic configuration of $Gd (Z=64)$ is $[X e] \,4 f^7\, 5 d^1 \,6 s^2$
$I. C{H_3}^ + $ $II. N{H_2}^ - $ $III. N{H_4}^ + $ $ IV. N{H_3}$
$(A)\, n=4,l=1$ $(B)\, n=4,l=0$
$(C) \,n=3,l=2$ $(D)\, n=3,l=1$
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