Question

Which of the following species is not paramagnetic?

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

Answer 1

Answer: (D)

$CO$

Answer 2

$CO$ has 14 electrons (even)
$\therefore$ it is diamagnetic
$NO$ has $15 e ^{-}$ (odd)
$\therefore$ it is paramagnetic and has 1 unpaired electron in $\pi^{*} 2 p$ molecular orbital.
$B _{2}$ has $10 e ^{-}$ (even) but still paramagnetic and has two unpaired electrons in $\pi 2 p_{x}$ and $\pi 2 p_{y}$ (s-p mixing).
$O _{2}$ has $16 e ^{-}$ (even) but still paramagnetic and has two unpaired electrons in $\pi^{\star} 2 p_{x}$ and $\pi^{\star} 2 p_{y}$ molecular orbitals.

So, the correct option is (D): $CO$.