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.