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

In the electronic configuration of which of the given elements, Aufbau principle or $(n+l)$ rule is violated?
  1. $\mathrm{Mn}$
  2. Ga
  3. $\mathrm{La}$
  4. Pu

Solution

Lanthanum has one electron in $5 d$-orbital even through, it is before elements with electrons in $4 f$-orbitals. It is because in La, the $4 f$ and $5 d$ electrons are so close in energy. Therefore, it violates Aufbau principle.

Asked in: AP EAMCET 2021 (24 Aug Shift 2)

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