JEE Main · 2021 · 25 Feb 2021 · Shift 1 · d and f Block Elements

A transition metal loses its $4s$ electrons before any $3d$ electron. Chromium is $[\mathrm{Ar}]3d^{5}4s^1$, so $\mathrm{C}{\mathrm{r}}^{+}$ is $[\mathrm{Ar}]3d^5$; manganese is $[\mathrm{Ar}]3d^{5}4s^2$, so $\mathrm{M}{\mathrm{n}}^{2+}$ is $[\mathrm{Ar}]3d^5$ as well. The other pairs differ: $\mathrm{N}{\mathrm{i}}^{2+}$ is $3d^8$ against $3d^{10}$ for $\mathrm{C}{\mathrm{u}}^{+}$, $\mathrm{F}{\mathrm{e}}^{2+}$ is $3d^6$ against $3d^8$ for $\mathrm{C}{\mathrm{o}}^{+}$, and ${\mathrm{V}}^{2+}$ is $3d^3$ against $3d^5$ for $\mathrm{C}{\mathrm{r}}^{+}$. The pair sharing an outermost configuration is $\mathrm{C}{\mathrm{r}}^{+}$ and $\mathrm{M}{\mathrm{n}}^{2+}$, both $[\mathrm{Ar}]3d^5$.