JEE Main · 2023 · 25 Jan 2023 · Shift 1 · d and f Block Elements

The spin-only magnetic moment, $\mu =\sqrt{n(n+2)}$, depends solely on the number of unpaired $d$ electrons, so ions with the same count of unpaired electrons share the same moment. Removing the appropriate electrons leaves vanadium(III) as $3d^2$ with two unpaired, chromium(III) as $3d^3$ with three unpaired, iron(II) as $3d^6$ with four unpaired, and nickel(III) as $3d^7$ with three unpaired. Only chromium(III) and nickel(III) agree, each with three unpaired electrons, so the number of ions sharing a common moment is two.