JEE Main · 2019 · 09 Apr 2019 · Shift 2 · Chemical Bonding and Molecular Structure

A diamagnetic species has every electron paired. In molecular-orbital terms $\mathrm{O}{}_2$ carries two unpaired electrons in its ${\pi }^{\ast }$ orbitals, $\mathrm{NO}$ has one unpaired electron, and $\mathrm{B}{}_2$ has two unpaired electrons in its $\pi$ orbitals, so all three are paramagnetic. Only $\mathrm{CO}$, with its fourteen electrons filling the molecular orbitals completely in pairs to give a bond order of three like $\mathrm{N}{}_2$, has no unpaired electron, so carbon monoxide is the diamagnetic molecule.