The ground-state electronic configuration of the oxygen molecule ($O_{2}$), which contains 16 electrons, is $(\sigma 1s)^{2} (\sigma^{*} 1s)^{2} (\sigma 2s)^{2} (\sigma^{*} 2s)^{2} (\sigma 2p_{z})^{2} (\pi 2p_{x}^{2} = \pi 2p_{y}^{2}) (\pi^{*} 2p_{x}^{1} = \pi^{*} 2p_{y}^{1})$ . In this configuration, the highest occupied molecular orbitals (HOMO) are the degenerate $\pi^{*}$ antibonding orbitals, each containing one unpaired electron . When $O_{2}$ is converted to the superoxide ion ($O_{2}^{-}$), it gains one additional electron. According to the Aufbau principle, this incoming electron will occupy the lowest available energy state, which is one of the partially filled $\pi^{*}$ antibonding orbitals .