Question

Which of the following molecules is paramagnetic?

• A. $O_2^{-}$
• B. $C{N}^{-}$
• C. $CO$
• D. $N{O}^{+}$

Answer 1

The correct option is A $O_2^{-}$

Electronic configuration of $CO,C{N}^{-}$ and $N{O}^{+}$ ($14$ electrons):
$\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_x^2=\pi 2p_y^2\sigma 2p_z^2$
As there are no unpaired electrons, they are diamagnetic in nature.
Electronic configuration of $O_2^{-}$:
$\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2\pi 2p_x^2=\pi 2p_y^2{\pi }^{\ast }2p_x^2={\pi }^{\ast }2p_y^1$
It contains $1$ unpaired electron and so it is paramagnetic in nature.