It can be condensed from air by cooling with liquid nitrogen, which has a boiling point of −196 ☌ (77 K −321 ☏).

It is a colourless gas with a boiling point of −183 ☌ (90 K −297 ☏). The ground state of O 2 has a bond length of 121 pm and a bond energy of 498 kJ/mol. 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. 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.

Solid oxygen, existing in six variously colored phases, of which one is O 8 (red oxygen) and another one metallic (ζ-oxygen).Ītomic oxygen, denoted O( 3P) or O(3P), is very reactive, as the single atoms of oxygen tend to quickly bond with nearby molecules.

Tetraoxygen ( O 4), another metastable form.

Others are:Ģ), one of two metastable states of molecular oxygen. Another is the highly reactive ozone ( O 3). The most familiar is molecular oxygen ( O 2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. There are several known allotropes of oxygen. Different forms of the 8th element of Periodic Table

Ozone 8 advanced