Question

Which of the following molecules/ions does not contain unpaired electrons?

• A. $N_2^{+}$
• B. $O_2$
• C. $O_2^{2-}$
• D. $B_2$

Answer 1

The correct option is C $O_2^{2-}$

(a) Electronic Configuration of $N_2^{+}=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_x^2\pi 2p_y^2\sigma 2p_z^1.$
Hence, one unpaired electron.
(b) Electronic configuration of $O_2=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2\pi 2p_x^2\pi 2p_y^2{\pi }^{\ast }2p_x^1{\pi }^{\ast }2p_y^1$.
Number of unpaired elecrons $=2$.
(c) Electronic configuration of $O_2^{2-}=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2\pi 2p_x^2\pi 2p_y^2{\pi }^{\ast }2p_x^2{\pi }^{\ast }2p_y^2$.
In the molecular orbital diagram of $O_2^{-2},{\pi }^{\ast }2p_x$ and ${\pi }^{\ast }2p_y$ molecular orbitals are completely filled, which are partially filled in $O_2$. So, there are no unpaired electrons. So, $O_2^{-2}$ does not contain unpaired electrons.
(d) Electronic configuration of $B_2=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_x^1\pi 2p_y^1$.
Number of unpaired electrons $=2$.