What is the value of $(n+l)$ for the unpaired $e^{-}$ in an atom of an element which is present in the 3rd period and seventeenth group of the periodic table.

The Aufbau Principle states that electrons enter orbitals in accordance with the rising energy of the orbitals. This is followed by the $n+l$ rule, where n is the main quantum number and $l$ is the azimuthal quantum number.

The element present in Group-17 and Period =3 will be bromine. The seventeenth group of the periodic table has a halogen family.

The bromine's electronic structure $(z=17),$

$1s^{2}2s^{2}2-p^{6}3s^{2}3p^5$

$\therefore$the last electron entered $3p$ orbitals where $n = 3$ and for $p-orbital$-orbital $\to l=1$

$\therefore$$n+l value =3+1$

$\therefore (n+l)=4$