Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted 1Σ and 1Δg (the preceding superscript "1" indicates a singlet state). The singlet states of oxygen are 158 and 95 kilojoules per mole higher in energy than the triplet ground state of oxygen. Under most common laboratory conditions, the higher energy 1Σ singlet state rapidly converts to the more stable, lower energy 1Δg singlet state. This more stable of the two excited states has its two valence electrons spin-paired in one π* orbital while the second π* orbital is empty. This state is referred to by the title term, "singlet oxygen", commonly abbreviated 1O2, to distinguish it from the triplet ground state molecule, 3O2.