The common allotrope of elemental oxygen on Earth is called dioxygen, O. It has a bond length of pm and a bond energy of kJ·mol. 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 Earths atmosphere see Occurrence. Other aspects of O are covered in the remainder of this article. Trioxygen O is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue. Ozone is produced in the upper atmosphere when O combines with atomic oxygen made by the splitting of O 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 earths surface, however, it is a pollutant formed as a byproduct of automobile exhaust. The metastable molecule tetraoxygen O was discovered in , and was assumed to exist in one of the six phases of solid oxygen. It was proven in that that phase, created by pressurizing O to GPa, is in fact a rhombohedral O cluster. This cluster has the potential to be a much more powerful oxidizer than either O or O and may therefore be used in rocket fuel. A metallic phase was discovered in when solid oxygen is subjected to a pressure of above GPa and it was shown in that at very low temperatures, this phase becomes superconducting.
Physical properties
Oxygen is more soluble in water than nitrogen water contains approximately molecule of O for every molecules of N, compared to an atmospheric ratio of approximately . The solubility of oxygen in water is temperaturedependent, and about twice as much . mg·L- dissolves at °C than at °C . mg·L-. At °C and atm of air, freshwater contains about . milliliters mL of oxygen per liter, whereas seawater contains about . mL per liter. At °C the solubility increases to . mL more than at °C per liter for water and . mL more per liter for sea water. Oxygen condenses at . K -. °C, -. °F, and freezes at . K -. °C, -. °F. Both liquid and solid O are clear substances with a light skyblue color caused by absorption in the red in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light. Highpurity liquid O is usually obtained by the fractional distillation of liquefied air Liquid oxygen may also be produced by condensation out of air, using liquid nitrogen as a coolant. It is a highlyreactive substance and must be segregated from combustible materials.
Isotopes and stellar origin
Late in a massive stars life, O concentrates in the Oshell, O in the Hshell and O in the Heshell. Naturally occurring oxygen is composed of three stable isotopes, O, O, and O, with O being the most abundant . natural abundance. Oxygen isotopes range in mass number from to . Most O is synthesized at the end of the helium fusion process in stars but some is made in the neon burning process. O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most O is produced when N made abundant from CNO burning captures a He nucleus, making O common in the heliumrich zones of stars. Fourteen radioisotopes have been characterized, the most stable being O with a halflife of . seconds s and O with a halflife of . s. All of the remaining radioactive isotopes have halflives that are less than s and the majority of these have halflives that are less than milliseconds. The most common decay mode of the isotopes lighter than O is electron capture to yield nitrogen, and the most common mode for the isotopes heavier than O is beta decay to yield fluorine.
Occurrence
Oxygen is the most abundant chemical element, by mass, in our biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. About . of the Suns mass is oxygen. Oxygen constitutes . of the Earths crust by mass and is the major component of the worlds oceans . by mass. Oxygen gas is the second most common component of the Earths atmosphere, taking up . of its volume and . of its mass some tonnes. Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere Mars with . O by volume and Venus have far lower concentrations. However, the O surrounding these other planets is produced solely by ultraviolet radiation impacting oxygencontaining molecules such as carbon dioxide.
The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on Earth the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earths atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate of roughly th of the entire atmospheric oxygen per year.
Free oxygen also occurs in solution in the worlds water bodies. The increased solubility of O at lower temperatures see Physical properties has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content. Polluted water may have reduced amounts of O in it, depleted by decaying algae and other biomaterials see eutrophication. Scientists assess this aspect of water quality by measuring the waters biochemical oxygen demand, or the amount of O needed to restore it to a normal concentration.
Physical properties
Oxygen is more soluble in water than nitrogen water contains approximately molecule of O for every molecules of N, compared to an atmospheric ratio of approximately . The solubility of oxygen in water is temperaturedependent, and about twice as much . mg·L- dissolves at °C than at °C . mg·L-. At °C and atm of air, freshwater contains about . milliliters mL of oxygen per liter, whereas seawater contains about . mL per liter. At °C the solubility increases to . mL more than at °C per liter for water and . mL more per liter for sea water. Oxygen condenses at . K -. °C, -. °F, and freezes at . K -. °C, -. °F. Both liquid and solid O are clear substances with a light skyblue color caused by absorption in the red in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light. Highpurity liquid O is usually obtained by the fractional distillation of liquefied air Liquid oxygen may also be produced by condensation out of air, using liquid nitrogen as a coolant. It is a highlyreactive substance and must be segregated from combustible materials.
Isotopes and stellar origin
Late in a massive stars life, O concentrates in the Oshell, O in the Hshell and O in the Heshell. Naturally occurring oxygen is composed of three stable isotopes, O, O, and O, with O being the most abundant . natural abundance. Oxygen isotopes range in mass number from to . Most O is synthesized at the end of the helium fusion process in stars but some is made in the neon burning process. O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most O is produced when N made abundant from CNO burning captures a He nucleus, making O common in the heliumrich zones of stars. Fourteen radioisotopes have been characterized, the most stable being O with a halflife of . seconds s and O with a halflife of . s. All of the remaining radioactive isotopes have halflives that are less than s and the majority of these have halflives that are less than milliseconds. The most common decay mode of the isotopes lighter than O is electron capture to yield nitrogen, and the most common mode for the isotopes heavier than O is beta decay to yield fluorine.
Occurrence
Oxygen is the most abundant chemical element, by mass, in our biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. About . of the Suns mass is oxygen. Oxygen constitutes . of the Earths crust by mass and is the major component of the worlds oceans . by mass. Oxygen gas is the second most common component of the Earths atmosphere, taking up . of its volume and . of its mass some tonnes. Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere Mars with . O by volume and Venus have far lower concentrations. However, the O surrounding these other planets is produced solely by ultraviolet radiation impacting oxygencontaining molecules such as carbon dioxide.
The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on Earth the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earths atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate of roughly th of the entire atmospheric oxygen per year.
Free oxygen also occurs in solution in the worlds water bodies. The increased solubility of O at lower temperatures see Physical properties has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content. Polluted water may have reduced amounts of O in it, depleted by decaying algae and other biomaterials see eutrophication. Scientists assess this aspect of water quality by measuring the waters biochemical oxygen demand, or the amount of O needed to restore it to a normal concentration.
