There are several known allotropy of oxygen. The most familiar is oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are:
Atomic oxygen
Atomic oxygen, denoted O or O
1, is very reactive, as the individual atoms of oxygen tend to quickly bond with nearby molecules. Its lowest-energy electronic state is a
Triplet state, designated by the
term symbol 3P. On Earth's surface, it exists naturally for a very short time. 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.
[ "Out of Thin Air" .NASA.gov. February 17, 2011.]
Atomic oxygen has been detected on Mars by Mariner program, Viking program, and the SOFIA observatory.
Dioxygen
The common allotrope of elemental oxygen on Earth, , 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, . As a major component (about 21% by volume) of Earth's atmosphere, elemental oxygen is most commonly encountered in the diatomic form.
Aerobic organism use atmospheric dioxygen as the terminal oxidant in cellular respiration in order to obtain
chemical energy. 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. The
Doublet state state requires an odd number of electrons, and so cannot occur in dioxygen without gaining or losing electrons, such as in the
superoxide ion () or the
dioxygenyl ion ().
The ground state of has a bond length of 121 Picometre and a bond energy of 498 kJ/mol.
Singlet oxygen
Singlet oxygen is the common name used for the two
of molecular
oxygen () with higher energy than the ground state
triplet oxygen. Because of the differences in their electron shells, singlet oxygen has different chemical and physical 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
Trioxidane in water or the reaction of hydrogen peroxide with
hypochlorite.
Ozone
Triatomic oxygen (ozone, ) is a very reactive allotrope of oxygen that is a pale blue gas at standard temperature and pressure. Liquid and solid have a deeper blue color than ordinary , and they are unstable and explosive.
[Mellor 1939][Cotton, F. Albert and Wilkinson, Geoffrey (1972). Advanced Inorganic Chemistry: A comprehensive Text. (3rd Edition). New York, London, Sydney, Toronto: Interscience Publications. .] In its gas phase, ozone is destructive to materials like
rubber and
fabric and is damaging to
lung tissue.
[Stwertka 1998, p.48] Traces of it can be detected as a pungent, chlorine-like smell,
[ Chemistry Tutorial : Allotropes from AUS-e-TUTE.com.au] coming from
electric motors,
, and
, as it is formed whenever air is subjected to an electrical discharge. It was named "ozon" in 1840 by Christian Friedrich Schönbein,
[Christian Friedrich Schönbein, Über die Erzeugung des Ozons auf chemischen Wege , p. 3, Basel: Schweighauser'sche Buchhandlung, 1844.] from ancient Greek ὄζειν (ozein: "to smell") plus the suffix
-on, commonly used at the time to designate a derived compound and anglicized as
-one.
["Ozone", Oxford English Dictionary online, retrieved 29 June 2020.]
Ozone is thermodynamically unstable and tends to react toward the more common dioxygen form. It is formed by reaction of intact with atomic oxygen produced when UV radiation in the upper atmosphere splits .[ Ozone absorbs strongly in the ultraviolet and in the stratosphere functions as a shield for the biosphere against and other damaging effects of sun UV radiation (see ozone layer).][ Tropospheric ozone is formed near the Earth's surface by the photochemical disintegration of nitrogen dioxide in the exhaust of .][Stwertka 1998, p.49] Ground-level ozone is an air pollution that is especially harmful for senior citizens (elderly), children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma.
Cyclic ozone
Cyclic ozone is a theoretically predicted molecule in which its three atoms of oxygen bond in an equilateral triangle instead of an open angle.
Tetraoxygen
Tetraoxygen had been suspected to exist since the early 1900s, when it was known as oxozone. It was identified in 2001 by a team led by Fulvio Cacace at the University of Rome.
Phases of solid oxygen
There are six known distinct phases of solid oxygen. One of them is a dark-red cluster. When oxygen is subjected to a pressure of 96 GPa, it becomes , in a similar manner to hydrogen,
Further reading
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Theoretical analysis of some and lead-ref for others:
