Oxygen is a chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds.
Diatomic oxygen gas constitutes 20.95% of the Earth’s atmosphere. Oxygen was isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele in Uppsala in 1773. Schemee called it kvävegas (Swedish: “acid gas” or “sour air”), and he is credited with discovering the element. However, Priestley is generally given credit for its discovery.
The name “oxygen” was coined in 1777 by Antoine Lavoisier, who recognized it as a component of air (and correctly identified its role in combustion), and who distinguished it from azote (now known as nitrogen), which he wrongly supposed to be an elementary substance composed of two atoms of nitrogen.
Oxygen is used in cellular respiration and many major industrial processes, including steelmaking, plastics manufacture, and paper pulp production. Its importance to life on Earth makes it one of the most abundant elements in the universe; however, almost all free oxygen in the interstellar medium originates from photoionization of carbon monoxide by ultraviolet starlight rather than being produced directly by stellar nucleosynthesis—except very close to stars undergoing nuclear fusion on their surfaces (as opposed to inside their cores).
At standard temperature and pressure two atoms of oxygen form dioxygen molecules O2 comprising 21% elemental oxygen; this second allotrope exists under conditions of high temperature or low pressure but reverts back to O2 at ambient conditions due to its higher enthalpy of formation. Dioxygen reacts exothermically with most other elements except for helium and neon. At elevated temperatures ozone (O3) forms from dioxygen: this allotrope has much higher entropy than diooxygen due to greater kinetic energy imparted by three vibrating atoms rather than two.
Oxygen was first isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele in Uppsala in 1773. Schemee called it kvävegas (Swedish: “acid gas” or “sour air”),and he is credited with discovering the element. However, Joseph Priestley is generally given credit for its discovery. The name “oxygen” was coined in 1777 by Antoine Lavoisier, who recognized it as a component of air (and correctly identified its role in combustion),”distinguishing [it], moreover from those vapors which are called atmospheric air or common air… because they are capable neither of combining chemically with bodies nor burning.”:267–268 Nevertheless,”for want [of better terminology],”he retained part of Stahl’s original name nitro-aereus for what we now call nitric acid (“aer acidus nitricus”).