Allotropes

Allotropes
Main article: Allotropes of oxygen

Space-filling model representation of dioxygen (O2) molecule
The common allotrope of elemental oxygen on Earth is called dioxygen, O
2, the major part of the Earth’s atmospheric oxygen (see Occurrence). O2 has a bond length of 121 pm and a bond energy of 498 kJ/mol,[37] which is smaller than the energy of other double bonds or pairs of single bonds in the biosphere and responsible for the exothermic reaction of O2 with any organic molecule.[3][38] Due to its energy content, O2 is used by complex forms of life, such as animals, in cellular respiration. Other aspects of O
2 are covered in the remainder of this article.

Trioxygen (O
3) is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue.[39] Ozone is produced in the upper atmosphere when O
2 combines with atomic oxygen made by the splitting of O
2 by ultraviolet (UV) radiation.[18] Since ozone absorbs strongly in the UV region of the spectrum, the ozone layer of the upper atmosphere functions as a protective radiation shield for the planet.[18] Near the Earth’s surface, it is a pollutant formed as a by-product of automobile exhaust.[39] At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft.[40]

The metastable molecule tetraoxygen (O
4) was discovered in 2001,[41][42] and was assumed to exist in one of the six phases of solid oxygen. It was proven in 2006 that this phase, created by pressurizing O
2 to 20 GPa, is in fact a rhombohedral O
8 cluster.[43] This cluster has the potential to be a much more powerful oxidizer than either O
2 or O
3 and may therefore be used in rocket fuel.[41][42] A metallic phase was discovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa[44] and it was shown in 1998 that at very low temperatures, this phase becomes superconducting.[45]

Physical properties