Chemistry Question on Chemical bonding and molecular structure

Question

Which of the following is paramagnetic ?

Answer 1

Solution

Paramagnetic species contain unpaired electrons in their molecular orbital electronic configuration. A molecule with one or more unpaired electrons is paramagnetic or odd electron system is paramagnetic.

The molecular orbital configuration of the given species is as

CO (6+8 = 14 ) = $\sigma 1s^2,{\sigma}^{*} 1s^2,$$\sigma 2s^2 ,{\sigma}^{*} 2s^2, \pi 2P_X^2 \approx \pi 2 P_Y^2 ,\sigma 2 P_Z^2$

All the electrons are paired hence, the molecule is diamagnetic.

O2- (8+8+1=17)
$= \sigma 1s^2 . {\sigma} 1s^{*2}, \sigma 2s^2, {\sigma} 2s^{*2} , \sigma 2 P_Z^2 , \pi 2P_X^2 \pi 2P_Y^2,$${\pi}^{*} 2 P_Y^{*2}{\pi} 2 P_X^{*1}$

The molecule contains one unpaired electron so, it isparamagnetic.

CN- (6 + 7+ 1 = 14) = same as CO

NO+ (7 + 8 - 1 = 14) = same as CO

Hence, among the given species only O2- is paramagnetic.
$O2^{−} ⇒ σ1 s^2σ^∗1 s^2σ2 s^2σ^∗2 s^2σ2p_z^2(π2p_x^2 = π2p_y^2)$
$(π^∗2p_x^2=π^∗2p_y^1)$

One unpaired electron is present in π∗ ABMO (Anti-Bonding Molecular Orbitals).