Which of the following diatomic molecular species has only $\pi$ bonds according to Molecular Orbital Theory?

Solution & Explanation

\textbf{Answer:} (C) $\mathrm{C}{}_2$ \textbf{Solution:} In $\mathrm{C}{}_2$ (MO config: $KK(\sigma 2s{)}^2({\sigma }^{\ast }2s{)}^2(\pi 2p_x{)}^2(\pi 2p_y{)}^2$), the $\sigma 2s$ and ${\sigma }^{\ast }2s$ cancel and no electrons occupy the $\sigma 2p_z$ bonding MO, so its bond order of $2$ arises entirely from two $\pi$ bonds. Hence $\mathrm{C}{}_2$ has only $\pi$ bonds.