(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: $\mathrm{He}_{2}^{+}<\mathrm{O}_{2}^{-}<\mathrm{NO}<\mathrm{C}^{2-}$