Which of the following species is diamagnetic?
You may THINK O2 is diamagnetic if you drew a Lewis structure but it is in fact a paramagnetic species. Drawing a molecular orbital diagram, we know that O2 is a paramagnetic species because there is only 1 electron in each of the two 2p-π* molecular orbitals. Thus, to have these orbitals filled, we require 2 more electrons.