There are several knownallotropes ofoxygen. The most familiar ismolecular oxygen (O2), present at significant levels inEarth's atmosphere and also known as dioxygen ortriplet oxygen. Another is the highly reactiveozone (O3). Others are:
Atomic oxygen, denoted O or O1, is very reactive, as the individual atoms of oxygen tend to quickly bond with nearby molecules. Its lowest-energy electronic state is aspin triplet, designated by theterm symbol3P. On Earth's surface, it exists naturally for a very short time. Inouter space, the presence of ampleultraviolet radiation results in alow Earth orbit atmosphere in which 96% of the oxygen occurs in atomic form.[1]
Atomic oxygen has been detected onMars byMariner,Viking, and theSOFIA observatory.[2]
The common allotrope of elemental oxygen on Earth,O2, is generally known as oxygen, but may be calleddioxygen,diatomic oxygen,molecular oxygen,dioxidene oroxygen gas to distinguish it from the element itself and from the triatomic allotropeozone,O3. As a major component (about 21% by volume) of Earth'satmosphere, elemental oxygen is most commonly encountered in the diatomic form.Aerobic organisms use atmospheric dioxygen as the terminal oxidant incellular respiration in order to obtainchemical energy. Theground state of dioxygen is known astriplet oxygen,3[O2], because it has two unpaired electrons. The first excited state,singlet oxygen,1[O2], has no unpaired electrons and ismetastable. Thedoublet state requires an odd number of electrons, and so cannot occur in dioxygen without gaining or losing electrons, such as in thesuperoxide ion (O−2) or thedioxygenyl ion (O+2).
The ground state ofO2 has a bond length of 121 pm and a bond energy of 498 kJ/mol.[3] It is a colourless gas with a boiling point of −183 °C (90 K; −297 °F).[4] 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 markedlyparamagnetic due to the unpaired electrons; liquid oxygen contained in a flask suspended by a string is attracted to a magnet.
Singlet oxygen is the common name used for the twometastable states of molecularoxygen (O2) with higher energy than the ground statetriplet 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 asrose bengal,methylene blue orporphyrins, or by chemical processes such as spontaneous decomposition ofhydrogen trioxide in water or the reaction ofhydrogen peroxide withhypochlorite.
Triatomic oxygen (ozone,O3) is a very reactive allotrope of oxygen that is a pale blue gas atstandard temperature and pressure. Liquid and solidO3 have a deeper blue color than ordinaryO2, and they are unstable and explosive.[5][6] In its gas phase, ozone is destructive to materials likerubber andfabric and is damaging tolung tissue.[7] Traces of it can be detected as a pungent, chlorine-like smell,[4] coming fromelectric motors,laser printers, andphotocopiers, as it is formed whenever air is subjected to an electrical discharge. It was named "ozon" in 1840 byChristian Friedrich Schönbein,[8] 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.[9]
Ozone isthermodynamically unstable and tends to react toward the more common dioxygen form. It is formed by reaction of intactO2 with atomic oxygen produced whenUV radiation in theupper atmosphere splitsO2.[5] Ozone absorbs strongly in the ultraviolet and in the stratosphere functions as a shield for thebiosphere againstmutagenic and other damaging effects ofsolarUV radiation (seeozone layer).[5] Tropospheric ozone is formed near the Earth's surface by the photochemical disintegration ofnitrogen dioxide in the exhaust ofautomobiles.[10]Ground-level ozone is anair pollutant that is especially harmful for senior citizens, children, and people with heart and lung conditions such asemphysema,bronchitis, andasthma.[11] Theimmune system produces ozone as an antimicrobial (see below).[12]
Cyclic ozone is a theoretically predictedO3 molecule in which its three atoms of oxygen bond in an equilateral triangle instead of an open angle.
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.[13] The moleculeO4 was thought to be in one of the phases ofsolid oxygen later identified asO8. Cacace's team suggested thatO4 probably consists of two dumbbell-likeO2 molecules loosely held together by induced dipole dispersion forces.
There are six known distinct phases of solid oxygen. One of them is a dark-redO8 cluster. When oxygen is subjected to a pressure of 96 GPa, it becomesmetallic, in a similar manner tohydrogen,[14] and becomes more similar to the heavierchalcogens, such asselenium (exhibiting a pink-red color in its elemental state),tellurium andpolonium, both of which show significant metallic character. At very low temperatures, this phase also becomessuperconducting.