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Q.
Which of the following is paramagnetic ?
NEETNEET 2013Chemical Bonding and Molecular Structure
Solution:
Paramagnetic species contains unpaired electrons in their molecular orbital electronic configuration.
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 so, it is diamagnetic).
$O_2^- (8+8+1=17)$
$ = \sigma 1s^2 . _{\sigma}^{*} 1s^2, \sigma 2s^2, _{\sigma}^{*} 2s^2 , \sigma 2 P_Z^2 , \pi 2P_X^2 \approx \pi 2P_Y^2, $
$_{\pi}^{*} 2 P_X^2 \approx _{\pi}^{*} 2 P_Y^1$
(It contains one unpaired electron so, it is paramagnetic.)
$CN^- $ (6+ 7+ 1= 14) = same as $CO$
$NO^+$ (7 + 8 - 1) = same as $CO$
Thus, among the given species only $O_2^-$ is paramagnetic.