Atomic physicists identify the ground state of O atom, with the 2s² 2p_x² 2p_y¹ 2p_z¹ valence configuration, as the ³P or "triplet P" state of the atom. But O is can also exist in a state with a 2s² 2p_x² 2p_y² valence configuration, which is called the ¹D or "singlet D" state of the atom. Triplet means that it has two unpaired electron spins, while singlet means that it has no unpaired electron spins.