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In the electronic configuration of which of the given elements, Aufbau principle or $(n+l)$ rule is violated?
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The correct answer is:
$\mathrm{La}$
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.
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