Question

The diamagnetic species among the following is

• A. $O_2^{+}$
• B. $O_2^{-}$
• C. $O_2$
• D. $O_2^{2-}$

Answer 1

Answer: (D)

To be diamagnetic, no unpaired electron must be present.
Hint Write molecular orbital electronic configuration of each species.
For all options given,
${\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\sigma }_{2p_z}^2{\pi }_{2p_x}^2={\pi }_{2p_y}^2$ is common.
After that, $O_2^{+}:{\pi }_{2p_x}^{\ast 1}$
$O_2^{-}:{\pi }_{2p_x}^{\ast 2}={\pi }_{2p_y}^{{\ast }^1}$
and $O_2:{\pi }_{2p_x}^{\ast 1}={\pi }_{2p_y}^{\ast 1}$
all contain unpaired electron.
They are paramagnetic, but $O_2^{2-}:{\pi }_{2p_x}^{\ast 2}={\pi }_{2p_y}^{{\ast }^2}$ has no unpaired electrons.
$\therefore$ It is diamagnetic.