Question: The $71^{\text {st }}$ electron of an element $X$ with an atomic number of 71 enters into the orbital:
Correct Option: , 3
Solution:
${ }_{71} \mathrm{X}=[\mathrm{Xe}] 6 s^{2} 4 f^{14} 5 d^{1}$
$\therefore$ Orbital occupied by last $e^{-}$is $5 d$.
