Question

Which of the following species is paramagnetic in nature

• A. $B_2$
• B. $O_2^{2-}$
• C. $N_2$
• D. $H{e}_2$

Answer 1

The correct option is A $B_2$

The molecular electronic configuration of $B_2$ is:
$\left({\sigma }_{1S}{)}^2\right({\sigma }_{1S}^{\ast }{)}^2\left({\sigma }_{2S}{)}^2\right({\sigma }_{2S}^{\ast }{)}^2\left({\pi }_{2p_x}{)}^1\right({\pi }_{2p_y}{)}^1$
Since it has unpaired electron thus it is paramagnetic in nature.

The molecular electronic configuration of $O_2^{2-}$,

$\left({\sigma }_{1S}{)}^2\right({\sigma }_{1S}^{\ast }{)}^2\left({\sigma }_{2S}{)}^2\right({\sigma }_{2S}^{\ast }{)}^2\left({\sigma }_{2p_z}{)}^2\right({\pi }_{2p_x}{)}^2\left({\pi }_{2p_y}{)}^2\right({\pi }_{2p_x}^{\ast }{)}^2({\pi }_{2p_y}^{\ast }{)}^2$
So all the electrons are paired. Hence it is diamagnetic in nature.

The molecular electronic configuration of $N_2$
$\left({\sigma }_{1S}{)}^2\right({\sigma }_{1S}^{\ast }{)}^2\left({\sigma }_{2S}{)}^2\right({\sigma }_{2S}^{\ast }{)}^2\left({\pi }_{2p_x}{)}^2\right({\pi }_{2p_y}{)}^2({\sigma }_{2p_z}{)}^2$
So number of unpaired electron is $0$ in $N_2$. Hence it is diamagnetic in nature.

The molecular electronic configuration of $H{e}_2$
$\left({\sigma }_{1S}{)}^2\right({\sigma }_{1S}^{\ast }{)}^2$
Hence it is also diamagnetic in nature.
(Although, bond order will be zero and molecule would not exist)