In which compound both removal or addition of electron leads to decrease in bond order ?
1)O2   2)F2   3)N2   4)CO

The solution is as follows:


$$\begin{gathered} O_2: \\ {{\sigma }_{1s}}^2{{{\sigma }^{*}}_{1s}}^2{{\sigma }_{2s}}^2{{{\sigma }^{*}}_{2s}}^2{{\sigma }_{2p_x}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^1{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}}}^1 \\ \mathrm{Bond} \mathrm{order} = \frac{{\mathrm{N}}_{\mathrm{b}}-\mathrm{Na}}{2}=\frac{8-4}{2}=2 \\ {{\mathrm{O}}_2}^{+}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^1{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}} \\ \mathrm{Bond} \mathrm{order} = \frac{8-3}{2}=2.5 \\ {{\mathrm{O}}_2}^{-}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}}}^1 \\ \mathrm{Bond} \mathrm{order} = \frac{8-5}{2}=1.5 \\ {\mathrm{F}}_2: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}}}^2 \\ \mathrm{Bond} \mathrm{order} = \frac{10-8}{2}=1 \\ {{\mathrm{F}}_2}^{+}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}}}^1 \\ \mathrm{Bond} \mathrm{order} =\frac{10-7}{2}=1.5 \\ {{\mathrm{F}}_2}^{-}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\pi }}^{*}}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{{\mathrm{\sigma }}^{*}}_{2{\mathrm{p}}_{\mathrm{x}}}}^1 \\ \mathrm{Bond} \mathrm{order} = \frac{10-9}{2}=0.5 \\ {\mathrm{N}}_2: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2 \\ \mathrm{Bond} \mathrm{order} = \frac{10-4}{2}=3 \\ {{\mathrm{N}}_2}^{+}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^1 \\ \mathrm{Bond} \mathrm{order} = \frac{9-4}{2}=2.5 \\ {{\mathrm{N}}_2}^{-}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^1 \\ \mathrm{Bond} \mathrm{order} = \frac{11-4}{2}=3.5 \\ \mathrm{CO}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2 \\ \mathrm{Bond} \mathrm{order} =\frac{10-4}{2}=3 \\ {\mathrm{CO}}^{+}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^1 \\ \mathrm{Bond} \mathrm{order} =\frac{10-3}{2}=3.5 \\ {\mathrm{CO}}^{-}: \\ {{\mathrm{\sigma }}_{1\mathrm{s}}}^2{{{\mathrm{\sigma }}^{*}}_{1\mathrm{s}}}^2{{\mathrm{\sigma }}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}}^2{{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{x}}}}^2{{{\mathrm{\sigma }}^{*}}_{2\mathrm{s}}}^2{{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}}^1 \\ \mathrm{Bond} \mathrm{order} =\frac{10-5}{2}=2.5 \end{gathered}$$
None of these molecules show decrease in bond order in both addition and removal of electrons.