Question

Which of the following species is not paramagnetic ?

• A. $O_2$
• B. $B_2$
• C. $NO$
• D. $CO$

Answer 1

Answer: (D)

CO has 14 electrons (even) $\therefore$ it is diamagnetic

NO has $15e^{-}$ (odd) $\therefore$ it is paramagnetic and has 1 unpaired electron in ${\pi }^{\ast }2p$ molecular orbital.

$B_2$ has $10e^{-}$ (even) but still paramagnetic and has two unpaired electrons in $\pi 2p_x$ and $\pi 2p_y$ (s-p mixing).

$O_2$ has $16e^{-}$ (even) but still paramagnetic and has two unpaired electrons in ${\pi }^{\star }2p_x$ and ${\pi }^{\star }2p_y$ molecular orbitals.