JEE Main · 2025 · 04 Apr 2025 · Shift 2 · Atomic Structure

The ground-state configuration of chromium is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$, the half-filled $3d^{5}4s^1$ arrangement being preferred for its extra stability. Electrons with azimuthal quantum number $\ell =1$ occupy the $p$ sub-shells, $2p^6$ and $3p^6$, giving $6+6=12$ electrons. Electrons with $\ell =2$ occupy the $d$ sub-shell, $3d^5$, giving $5$ electrons. Hence twelve electrons have $\ell =1$ and five electrons have $\ell =2$.