Oxygen is the element with atomic number 8. It is a member of the chalcogen group on the periodic table, and is a highly reactive nonmetallic 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, odorless, tasteless diatomic gas with the formula O
2. Oxygen is the third most abundant element in the universe by mass after hydrogen and helium and the most abundant element by mass in the Earth's crust. Oxygen constitutes 88.8% of the mass of water and 20.9% of the volume of air. All the 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. Another form (allotrope) of oxygen, ozone (O
3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog. Oxygen was independently discovered by Joseph Priestley and Carl Wilhelm Scheele in the 1770s, but Priestley is usually given priority because he published his findings first. (more...)
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