Molecular orbital
Molecular orbital theory predicts the electronic ground state denoted by the molecular term symbol 3Σ, and two low-lying excited singlet states with term symbols 1Δg and 1Σ. These three electronic states differ only in the spin and the occupancy of oxygen's two antibonding πg-orbitals, which are degenerate (equal in energy). These two orbitals are classified as antibonding and are of higher energy. Following Hund's first rule, in the ground state, these electrons are unpaired and have like (same) spin. This open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (1Σ) ground states.