JEE Main · 2021 · 01 Sep 2021 · Shift 2 · s-Block Elements

Simple ionic oxides such as $\mathrm{L}{\mathrm{i}}_2\mathrm{O}$, $\mathrm{CaO}$, $\mathrm{MgO}$ and ${\mathrm{K}}_2\mathrm{O}$ contain the closed-shell ${\mathrm{O}}^{2-}$ ion and are diamagnetic. The peroxide $\mathrm{N}{\mathrm{a}}_2{\mathrm{O}}_2$ contains ${\mathrm{O}}_2^{2-}$, in which every molecular orbital up to ${\pi }^{\ast }2p$ is doubly filled, so it is diamagnetic. The superoxide $\mathrm{K}{\mathrm{O}}_2$ contains ${\mathrm{O}}_2^{-}$, which carries one unpaired electron in a ${\pi }^{\ast }2p$ orbital and is paramagnetic. Exactly one of the listed oxides is paramagnetic.