Question:
In the option are given sequences of increasing bond order for the following species. The correct option is :
$ \text{O}_2, \text{N}_2, \text{O}_2^{+} \text{ and } \text{O}_2^{-} $
Solution:
\text{HINT: Bond order} = \frac{1}{2} (\text{N}_b - \text{N}_a) \text{, where } \text{N}_a \text{ is number of antibonding electrons and } \text{N}_b \text{ is number of bonding electrons.}
\text{Explanation:}
\text{The MOT diagram of given ions is as follows:}
\text{1.) Electronic configuration of } \text{O}_2 \text{ (16 electrons)} = \sigma_{1s}^{2}, \sigma_{1s}^{*2}, \sigma_{2s}^{2}, \sigma_{2s}^{*2}, \sigma_{2p_{z}}^{2}, \pi_{2p_{x}}^{2} = \pi_{2p_{y}}^{2}, \pi_{2p_{x}}^{*1} = \pi_{2p_{y}}^{*1}
\text{Bond order} = \frac{1}{2} (\text{N}_b - \text{N}_a) = \frac{1}{2} (10 - 6) = 2
\text{2.) Electronic configuration of } \text{O}_2^{+} \text{ (15 electrons)} = \sigma_{1s}^{2}, \sigma_{1s}^{*2}, \sigma_{2s}^{2}, \sigma_{2s}^{*2}, \sigma_{2p_{z}}^{2}, \pi_{2p_{x}}^{2} = \pi_{2p_{y}}^{2}, \pi_{2p_{x}}^{*1} = \pi_{2p_{y}}^{*0}
\text{Bond order} = \frac{1}{2} (\text{N}_b - \text{N}_a) = \frac{1}{2} (10 - 5) = 2.5
\text{3.) Electronic configuration of } \text{O}_2^{-} \text{ (17 electrons)} = \sigma_{1s}^{2}, \sigma_{1s}^{*2}, \sigma_{2s}^{2}, \sigma_{2s}^{*2}, \sigma_{2p_{z}}^{2}, \pi_{2p_{x}}^{2} = \pi_{2p_{y}}^{2}, \pi_{2p_{x}}^{*2} = \pi_{2p_{y}}^{*1}
\text{Bond order} = \frac{1}{2} (\text{N}_b - \text{N}_a) = \frac{1}{2} (10 - 7) = 1.5
\text{4.) Electronic configuration of } \text{N}_2 \text{ (14 electrons)} = \sigma_{1s}^{2}, \sigma_{1s}^{*2}, \sigma_{2s}^{2}, \sigma_{2s}^{*2}, \pi_{2p_{x}}^{2} = \pi_{2p_{y}}^{2}, \sigma_{2p_{z}}^{2}
\text{Bond order} = \frac{1}{2} (\text{N}_b - \text{N}_a) = \frac{1}{2} (10 - 4) = 3
\begin{array}{|c|c|c|c|c|}
\hline
\text{Molecule} & \text{O}_2^{-} & \text{O}_2 & \text{O}_2^{+} & \text{N}_2 \\
\hline
\text{Bond order} & 1.5 & 2.0 & 2.5 & 3.0 \\
\hline
\end{array}
\text{Arranging in increasing order of bond order: } \text{O}_2^{-} < \text{O}_2 < \text{O}_2^{+} < \text{N}_2
\text{Therefore, the correct option is 2.}