Which of the following species is paramagnetic?

Solution

1. The molecular orbital 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. The presence of two unpaired electrons in the ${\pi }^{\ast }$ orbitals makes the molecule paramagnetic.
3. Other listed molecules have all paired electrons.
4. Hence the answer is (A).