## Chemistry and Chemical Reactivity (9th Edition)

$O_2^-$ has 13 valence electrons: $[core]\ (\sigma_{2s})^2\ (\sigma_{2s}^*)^2\ (\sigma_{2p})^2\ (\sigma_{2p}^*)^2\ (\pi_{2p})^4\ (\pi_{2p}^*)^1$ HOMO: highest occupied molecular orbital, that's $\pi_{2p}^*$