JEE Main · 2019 · 10 Jan 2019 · Shift 1 · Chemical Bonding and Molecular Structure

Counting with molecular-orbital theory, the species that carries exactly two $\pi$ bonds plus half of a $\sigma$ bond has bond order $2.5$ with its odd electron in a $\sigma$ orbital. For $\mathrm{N}{}_2{}^{+}$ the thirteen electrons fill up through the $\pi$ levels and leave a single electron in the bonding ${\sigma }_{2p}$, which reads as two full $\pi$ bonds and one half $\sigma$ bond. In $\mathrm{O}{}_2{}^{+}$ the half-bond instead lies in the $\pi$ system, so the clean description of two $\pi$ and half a $\sigma$ belongs to $\mathrm{N}{}_2{}^{+}$.