Question

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

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

Answer 1

Answer: (A)

The molecular orbital configuration of ${\mathrm{O}}_2^{2-}$ ion is $KK\sigma (2s{)}^2{\sigma }^{\ast }(2s{)}^2\sigma {\left(2p_z\right)}^2\pi {\left(2p_x\right)}^2\pi {\left(2p_y\right)}^2{\pi }^{\ast }{\left(2p_x\right)}^2{\pi }^{\ast }{\left(2p_y\right)}^2$
Here, $KK$ represents non-bonding molecular orbital of $1s$ orbital.
${\mathrm{O}}_2^{2-}$ contains no unpaired electrons. The molecular orbital configuration of ${\mathrm{B}}_2$ molecule is $KK\sigma (2s{)}^2{\sigma }^{\ast }(2s{)}^2\pi {\left(2p_x\right)}^1\pi {\left(2p_y\right)}^1$
It contains 2 unpaired electrons.
The molecular orbital configuration of ${\mathrm{N}}_2^{+}$ ion is $KK\sigma (2s{)}^2{\sigma }^{\ast }(2s{)}^2\sigma {\left(2p_z\right)}^2\pi {\left(2p_x\right)}^2\pi {\left(2p_y\right)}^2\sigma {\left(2p_z\right)}^1$
It contains one unpaired electron.
The molecular orbital configuration of ${\mathrm{O}}_2$ molecule is $KK\sigma (2s{)}^2{\sigma }^{\star }(2s{)}^2\pi {\left(2p_x\right)}^2\pi {\left(2p_y\right)}^2\sigma {\left(2p_z\right)}^2{\pi }^{\ast }{\left(2p_x\right)}^1{\pi }^{\ast }{\left(2p_y\right)}^1$
It contains 2 unpaired electrons.