2. structure of atom — Chemistry STD 11 Science — Question

According to Molecular orbital theory, the electronic configuration for given molecules as follow:

$A]$ The electronic configuration of $NO$ :

$\sigma 1 s ^2 \,\sigma^* 1 s ^2 \,\sigma 2 s ^2 \,\sigma^* 2 s ^2 \,\sigma 2 p _{ z }^2\, \pi 2 p _{ x }^2 \,\pi 2 p _{ y }^2\, \pi^* 2 p _{ x }^1$

Therefore $NO$ molecule has one unpaired electron in $\pi^* 2 p _{ x }^1$ orbital. Therefore, option A is answer.

Explanation for incorrect options :

$B]$ The electronic configuration of $CO$ :

$\sigma 1 s ^2\,\sigma^* 1 s ^2 \,\sigma 2 s ^2 \,\sigma^* 2 s ^2\, \sigma 2 p _{ z }^2\, \pi 2 p _{ x }^2 \,\pi 2 p _{ y }^2$

All electrons in $CO$ molecule is paired.

$C]$ The electronic configuration of $CN ^{-}$:

$\sigma 1 s ^2\, \sigma^* 1 s ^2 \,\sigma 2 s ^2 \,\sigma^* 2 s ^2 \,\sigma 2 p _z^2 \,\pi 2 p _{ x }^2 \,\pi 2 p _{ y }^2$

All electrons in $CN ^{-}$molecule is paired.

$D$ ] The electronic configuration of $O _2$ :

$\sigma 1 s ^2\, \sigma^* 1 s ^2\, \sigma 2 s ^2 \,\sigma^* 2 s ^2\, \sigma 2 p _z^2 \,\pi 2 p _{ x }^2 \,\pi 2 p _{ y }^2\, \pi^* 2 p _{ x }^1 \,\pi^* 2 p _{ y }^1$

Therefore $O _2$ molecule has $1-1$ unpaired electron in $\pi^* 2 p _{ x }^1$ and $\pi^* 2 p _{ y }^1$ orbital. That mean, $O _2$ has two unpaired electrons.