Molecular orbital configuration of O2 ( 16 electrons) σ1s2,σ∗1s2,σ2s2,σ∗2s2,σ2pz2,π2px2=π2py2,π∗2px2=π∗2py1
When an electron is added, it enters the π∗2px orbital and the configuration becomes σ1s2,σ∗1s2,σ2s2,σ∗2s2,σ2pz2,π2px2=π2py2,π∗2px2=π∗2py1