Oxygen is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly reactive non-metallic period 2 element that readily forms compounds (notably oxides) with almost all other elements. At standard temperature and pressure two atoms of the element bind to form dioxygen, a colorless, odourless, tasteless diatomic gas with the formula O2. Oxygen is the third most abundant element in the universe by mass after hydrogen and helium. All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Oxygen was independently discovered by Carl Wilhelm Scheele, in 1773 or earlier, and Joseph Priestley, in 1774, but Priestley is often given priority because his publication came out in print first. The name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites to remove carbon dioxide and nitrogen from air, electrolysis of water and other means.
At standard temperature and pressure, oxygen is a colorless, odourless gas with the molecular formula O2, in which the two oxygen atoms are chemically bonded to each other with a spin triplet electron configuration. This bond has a bond order of two, and is often simplified in description as a double bond or as a combination of one two-electron bond and two three-electron bonds. Triplet oxygen (not to be confused with ozone, O3) is the ground state of the O2 molecule. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals. These orbitals are classified as antibonding (weakening the bond order from three to two), so the diatomic oxygen bond is weaker than the diatomic nitrogen triple bond in which all bonding molecular orbitals are filled, but some antibonding orbitals are not. In normal triplet form, O2 molecules are paramagnetic—they form a magnet in the presence of a magnetic field—because of the spin magnetic moments of the unpaired electrons in the molecule, and the negative exchange energy between neighbouring O2 molecules. Liquid oxygen is attracted to a magnet to a sufficient extent that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet. Singlet oxygen, a name given to several higher-energy species of molecular O2 in which all the electron spins are paired, is much more reactive towards common organic molecules. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlight. Late in a massive star's life, 16-O concentrates in the O-shell, 17-O in the H-shell and 18-O in the He-shell Naturally occurring oxygen is composed of three stable isotopes, 16-O, 17-O, and 18-O, with 16-O being the most abundant (99.762% natural abundance). Most 16-O is synthesized at the end of the helium fusion process in stars but some is made in the neon burning process. 17-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 18-O is produced when 14-N (made abundant from CNO burning) captures a 4-He nucleus, making 18-O common in the helium-rich zones of stars. Fourteen radioisotopes have been characterized, the most stable being 15-O with a half-life of 122.24 seconds (s) and 14-O with a half-life of 70.606 s. All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds. The most common decay mode of the isotopes lighter than 16-O is β+ decay to yield nitrogen, and the most common mode for the isotopes heavier than 18-O is beta decay to yield fluorine..
The base value of each unit of ranges between 1 and 10Ð per unit, with up to 3 units being found at any one time.
Presence on Mars: Common
|Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6|
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