JEE Main · 2021 · 18 Mar 2021 · Shift 2 · d and f Block Elements

Magnetic behaviour requires unpaired electrons. Silicon dioxide is a covalent network in which every valence electron sits in a bond, magnesium oxide is built from the closed-shell ions $\mathrm{M}{\mathrm{g}}^{2+}$ and ${\mathrm{O}}^{2-}$, and the peroxide ion of $\mathrm{N}{\mathrm{a}}_2{\mathrm{O}}_2$ has all its electrons paired as well, so all three are diamagnetic. $\mathrm{M}{\mathrm{n}}_3{\mathrm{O}}_4$ is the mixed oxide $\mathrm{M}{\mathrm{n}}^{2+}\mathrm{M}{\mathrm{n}}_2^{3+}{\mathrm{O}}_4$, whose $d^5$ and $d^4$ centres carry unpaired electrons and couple in the spinel lattice. The oxide that shows magnetic property is $\mathrm{M}{\mathrm{n}}_3{\mathrm{O}}_4$.