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(C) $Lanthanum$ $(La)$ has an atomic number of $57$. Its expected electronic configuration based on the $(n+l)$ rule would be $[Xe] 4f^1 6s^2$. Howeve

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$La$

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(C) $Lanthanum$ $(La)$ has an atomic number of $57$. Its expected electronic configuration based on the $(n+l)$ rule would be $[Xe] 4f^1 6s^2$. However,the actual observed electronic configuration is $[Xe] 5d^1 6s^2$. This occurs because the energy levels of the $4f$ and $5d$ orbitals are very close in energy,and the $5d$ orbital is slightly lower in energy than the $4f$ orbital for $La$,leading to a violation of the $Aufbau$ principle.

In the electronic configuration of which of the given elements,$Aufbau$ principle or $(n+l)$ rule is violated?

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