 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.Two less stable, higher energy excited states are readily accessible from this ground state, again in accordance with Hund's first rule; the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the 1Δg state (a term symbol, where the preceding superscripted "1" indicates it as a singlet state).