![]() The doublet state requires an odd number of electrons, and so cannot occur in dioxygen without gaining or losing electrons, such as in the superoxide ion ( O − 2) or the dioxygenyl ion ( O + 2). The first excited state, singlet oxygen, 1, has no unpaired electrons and is metastable. The ground state of dioxygen is known as triplet oxygen, 3, because it has two unpaired electrons. Aerobic organisms use atmospheric dioxygen as the terminal oxidant in cellular respiration in order to obtain chemical energy. As a major component (about 21% by volume) of Earth's atmosphere, elemental oxygen is most commonly encountered in the diatomic form. The common allotrope of elemental oxygen on Earth, O 2, is generally known as oxygen, but may be called dioxygen, diatomic oxygen, molecular oxygen, dioxidene or oxygen gas to distinguish it from the element itself and from the triatomic allotrope ozone, O 3. The unpaired electrons participate in three-electron bonding, shown here using dashed lines. See also: Dioxygen in biological reactions The most commonly encountered allotrope of elemental oxygen is triplet dioxygen, a diradical. Ītomic oxygen has been detected on Mars by Mariner, Viking, and the SOFIA observatory. ![]() In outer space, the presence of ample ultraviolet radiation results in a low Earth orbit atmosphere in which 96% of the oxygen occurs in atomic form. On Earth's surface, it exists naturally for a very short time. Its lowest-energy electronic state is a spin triplet, designated by the term symbol 3P.
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