(where \(\mathrm{N}_{\mathrm{b}}=\) number of electrons in bonding molecular orbital \(\mathrm{N}_{\mathrm{a}}=\) number of electrons in anti - bonding molecular orbital)

\(\begin{array}{|c|c|}\hline \text { Diatomic species } & {\text { Bond order }} \\ \hline N 0 & {2.5} \\ {O_{2}^{-}} & {1.5} \\ {C_{2}^{2-}} & {3.0} \\ {H e_{2}^{+}} & {0.5} \\ \hline\end{array}\)

Thus, increasing order: \(\mathrm{He}_{2}^{+}<\mathrm{O}_{2}^{-}<\mathrm{NO}<\mathrm{C}^{2-}\)