JEE Main · 2006 · 30 Apr 2006 · Offline · Chemical Bonding and Molecular Structure

Filling the molecular orbitals in order settles the question of unpaired electrons. The peroxide ion ${\mathrm{O}}_2^{2-}$ has enough electrons to fill the antibonding ${\pi }_{2p}^{\ast }$ pair completely, leaving none unpaired, so it is diamagnetic. By contrast ${\mathrm{B}}_2$ places its last two electrons singly in the two ${\pi }_{2p}$ orbitals and so has two unpaired electrons, ${\mathrm{N}}_2^{+}$ is left with one unpaired electron, and ${\mathrm{O}}_2$ has two unpaired electrons in its ${\pi }_{2p}^{\ast }$ set. The only species with every electron paired is therefore ${\mathrm{O}}_2^{2-}$.