MCQ
Electronic configuration of $Gd(64)$ is
  • A
    $[Xe]\, 4f^2,\,5d^9,\,6s^2$
  • $[Xe]\, 4f^7,\,5d^1,\,6s^2$
  • C
    $[Xe]\, 4f^6,\,5d^2,\,6s^2$
  • D
    $[Xe]\, 4f^7,\,5d^0,\,6s^2$

Answer

Correct option: B.
$[Xe]\, 4f^7,\,5d^1,\,6s^2$
b
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 $[ Xe ] 4 f ^7 6 s ^2$.

Therefore, addition of next electron doesn't occur in a more stable exactly halffilled $4 f ^7$ shell but occur in a little higher energy $5 d$-orbital. Thus, the electronic configuration of $Gd ( Z =64)$ is [Xe] $4 f ^7 5 d ^1 6 s ^2$.

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