a
\( \mathrm{O}_2(16 \mathrm{e}):\left(\sigma_{1 \mathrm{~s}}\right)^2\left(\sigma_{1 \mathrm{~s}}^*\right)^2\left(\sigma_{2 \mathrm{~s}}\right)^2\left(\sigma_{2 \mathrm{~s}}^*\right)^2 \)

\( \left(\sigma_{2 \mathrm{p}}\right)^2\left[\left(\pi_{2 \mathrm{p}}\right)^2=\left(\pi_{2 \mathrm{p}}\right)^2\right],\left[\left(\pi_{2 \mathrm{p}}^*\right)^1=\left(\pi_{2 \mathrm{p}}^*\right)^1\right]\)

Number of \(\mathrm{e}^{-}\)present in \(\left(\pi^*\right)\) of \(\mathrm{O}_2=2\)

Number of \(\mathrm{e}^{-}\)present in \(\left(\pi^*\right)\) of \(\mathrm{O}_2^{+}=1\)

Number of \(\mathrm{e}^{-}\)present in \(\left(\pi^*\right)\) of \(\mathrm{O}_2^{-}=3\)

So total \(\mathrm{e}^{-}\)in \(\left(\pi^*\right)=2+1+3=6\)