Characteristics of oxygen - Allotropes
The common allotrope of elemental Oxygen on Earth is called dioxygen, . It has a bond length of 121 pm and a bond energy of 498 kJ·mol-1. () This is the form that is used by complex forms of life, such as animals, in cellular respiration (see Biological role) and is the form that is a major part of the Earth's atmosphere (see Occurrence). Other aspects of are covered in the remainder of this article.Trioxygen (O³01³) is usually known as ozone and is a very reactive allotrope of Oxygen that is damaging to lung tissue. (Stwertka 1998, p.48) Ozone is produced in the upper atmosphere when O2 combines with atomic Oxygen made by the splitting of O³45³ by ultraviolet (UV) radiation. Since ozone absorbs strongly in the UV region of the spectrum, it functions as a protective radiation shield for the planet (see ozone layer). Near the earth's surface, however, it is a pollutant formed as a by-product of automobile exhaust. (Stwertka 1998, p.49)
The metastable molecule tetraoxygen (O³67³) was discovered in 2001, (Cacace 2001, 4062) () and was assumed to exist in one of the six phases of solid Oxygen. It was proven in 2006 that that phase, created by pressurizing O³89³ to 20 GPa, is in fact a rhombohedral O8 cluster. (Lundegaard 2006, 201–04) This cluster has the potential to be a much more powerful oxidizer than either O³1213³ or O³1415³ and may therefore be used in rocket fuel. A metallic phase was discovered in 1990 when solid Oxygen is subjected to a pressure of above 96 GPa (Desgreniers 1990, 1117–22) and it was shown in 1998 that at very low temperatures, this phase becomes superconducting. (Shimizu 1998, 767–69)
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