Dioxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others include:
Atomic oxygen, denoted O(3P), O(3P) or O((3)P), is very reactive, as the single atoms of oxygen tend to quickly bond with nearby molecules. On Earth's surface, it does not exist naturally for very long, but in outer space, the presence of plenty of ultraviolet radiation results in a low Earth orbit atmosphere in which 96% of the oxygen occurs in atomic form.
The common allotrope of elemental oxygen on Earth, O
2, is generally known as oxygen, but may be called dioxygen or molecular oxygen to distinguish it from the element itself. Elemental oxygen is most commonly encountered in this form, as about 21% (by volume) of Earth's atmosphere. The ground state of dioxygen is known as triplet oxygen because it has two unpaired electrons. The first excited state, singlet oxygen, has no unpaired electrons and is metastable.
O
2 has a bond length of 121 pm and a bond energy of 498 kJ/mol. It is a colourless gas with a boiling point of −183 °C (90 K; −297 °F). It can be condensed from air by cooling with liquid nitrogen, which has a boiling point of −196 °C (77 K; −321 °F). Liquid oxygen is pale blue in colour, and is quite markedly paramagnetic—liquid oxygen contained in a flask suspended by a string is attracted to a magnet.
Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O2) with higher energy than the ground state triplet oxygen. Because of the differences in their electron shells, singlet oxygen has different chemical properties than triplet oxygen, including absorbing and emitting light at different wavelengths. It can be generated in a photosensitized process by energy transfer from dye molecules such as rose bengal, methylene blue or porphyrins, or by chemical processes such as spontaneous decomposition of hydrogen trioxide in water or the reaction of hydrogen peroxide with hypochlorite.
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