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Q.
The electronic configuration of gadolinium (atomic number $64$) is
Classification of Elements and Periodicity in Properties
Solution:
The electronic configuration of $La ( Z =57)$ is $[ Xe ] 5 d ^{1} 6 s ^{2}$.
Therefore, further addition of electrons occurs in a lower energy $4 f$ -orbital till it is exactly half filled at $Eu( Z =63)$.
Thus, the electronic configuration of $Eu$ is $[ X e ] 4 f ^{7} 6 s ^{2}$.
Therefore, addition of next electron doesn't occur in a more stable exactly half-filled $4 f ^{7}$ shell but occur in a little higher energy $5 d$-orbital.
Thus, the electronic configuration of $Gd(Z=64)$ is $[ X e ] 4 f ^{7} 5 d ^{1} 6 s ^{2}$