JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Atomic Structure

Gallium has atomic number 31, so its ground state configuration is $[\mathrm{Ar}]3{\mathrm{d}}^{10}4{\mathrm{s}}^{2}4{\mathrm{p}}^1$. The electron lost in forming $\mathrm{G}{\mathrm{a}}^{+}$ is taken from the orbital of highest energy, the $4p$, which leaves $[\mathrm{Ar}]3{\mathrm{d}}^{10}4{\mathrm{s}}^2$. The valence shell of the ion is then $n=4$ and it holds only the $4s^2$ pair, the filled $3d$ subshell lying in an inner shell. For an $s$ orbital the azimuthal quantum number is $l=0$.