Which of the following statements regarding the molecular orbital theory of ${\text{O}}_2$ and its ions is correct?

Solution

1. The electronic configuration of ${\text{O}}_2$ is $(\sigma 1s{)}^2({\sigma }^{\ast }1s{)}^2(\sigma 2s{)}^2({\sigma }^{\ast }2s{)}^2(\sigma 2p_z{)}^2(\pi 2p_x{)}^2(\pi 2p_y{)}^2({\pi }^{\ast }2p_x{)}^1({\pi }^{\ast }2p_y{)}^1$.
2. For ${\text{O}}_2^{-}$, an electron is added to the ${\pi }^{\ast }$ orbital, resulting in a bond order of $(10-7)/2=1.5$.
3. The presence of an unpaired electron in the ${\pi }^{\ast }$ orbital makes ${\text{O}}_2^{-}$ paramagnetic.
4. Comparing bond orders, ${\text{O}}_2$ is $2.0$, ${\text{O}}_2^{+}$ is $2.5$, and ${\text{O}}_2^{-}$ is $1.5$.
5. Hence the answer is (B).