Question

Among the following, the species with one unpaired electron are:

• A. $O_2^{+}$
• B. $NO$
• C. $O_2^{-}$
• D. $B_2$

Answer 1

Answer: (A), (B), (C)

$O_2^{+}$
$NO$
$O_2^{-}$

(i) $O_2^{+}$ & NO both contain same no. of electrons and will have same electronic configuration. Total $e^{-}$ = 15 configuration: ${\sigma }_{1s}^2,{\sigma }_{1s}^{\ast 2},{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2},{\sigma }_{{2p}_z}^2,\left({{\Pi }_{2p_x}}^2{{\Pi }_{2py}}^2\right),\left({{\Pi }_{2p_x}}^{\ast 1}{{\Pi }_{2p_y}}^{\ast }\right)$ $\therefore$ $O_2^{+}$ & NO contain 1 unpaired electron. (ii) $O_2^{-}$ has a total of 17 $e^{-}$ configuration: ${\sigma }_{1s}^2,{\sigma }_{1s}^{\ast 2},{\sigma }_{2s}^2,{\sigma }_{2s}^{\ast 2},{\sigma }_{{2p}_z}^2,\left({{\Pi }_{2p_x}}^2{{\Pi }_{2py}}^2\right),\left({{\Pi }_{2p_x}}^{\ast 2}{{\Pi }_{2p_y}}^{\ast 1}\right)$ $\therefore$ $O_2^{-}$ contains 1 unpaired electron. (iii) $B_2$ contains even number of electrons so it cannot contain 1 unpaired electron. Either there will be no unpaired electrons or 2 unpaired electrons but cannot have 1 unpaired electron.