Oxygen is a chemical element with the symbol O and atomic number 8. It belongs to group 16 and period 2 of the periodic table. Under ordinary conditions, its common elemental form is dioxygen, O₂, a colorless, odorless gas consisting of two oxygen atoms per molecule. Oxygen constitutes approximately 21% of Earth’s atmosphere by volume and occurs extensively in compounds, especially water and minerals. Its chemical reactivity underlies combustion, numerous industrial processes, and the respiration of many organisms. (periodic-table.rsc.org)
Atomic structure and isotopes
A neutral oxygen atom contains eight protons and eight electrons. Its ground-state electron configuration is 1s² 2s² 2p⁴, placing six electrons in its outer shell. Its electronegativity is 3.44 on the Pauling scale, reflecting its strong tendency to attract electrons in chemical bonds. These properties help explain its widespread participation in compounds with metals and nonmetals. (periodic-table.rsc.org)
Natural oxygen has three stable isotopes: oxygen-16, oxygen-17, and oxygen-18. Oxygen-16 accounts for approximately 99.7% of oxygen atoms in normal terrestrial materials. The standard atomic weight is expressed as the interval [15.99903, 15.99977], because isotope proportions vary naturally among materials. Measurements of oxygen-isotope ratios are used to investigate the water cycle, biological processes, and past climates. Such measurements distinguish differences in isotope composition rather than differences in the element’s chemical identity. (ciaaw.org)
Molecular forms and physical properties
Dioxygen is gaseous at room temperature. At normal atmospheric pressure, it boils at approximately −182.96°C and melts at approximately −218.79°C. These low transition temperatures make cooling and liquefaction central to its large-scale separation from air. (periodic-table.rsc.org)
O₂ has two unpaired electrons and therefore exhibits paramagnetism: it is attracted by a magnetic field. This property is exploited in instruments that measure oxygen concentrations in gas mixtures. Its electronic structure is also important for understanding its chemical behavior; oxygen’s reactivity cannot be described fully by a simple picture in which all electrons are paired. (doi.org)
Ozone, O₃, is another molecular form of the element. In the stratosphere, ultraviolet radiation splits O₂ into individual oxygen atoms, which can combine with other O₂ molecules to form ozone. Ozone absorbs ultraviolet radiation, helping shield Earth’s surface. Ozone near the ground, by contrast, is an air pollutant formed through atmospheric reactions involving other pollutants and sunlight. (science.nasa.gov)
Chemical behavior
Oxygen is a strong oxidizing agent that combines with many elements to form oxides. Its reactions with organic materials and metals are particularly important in industrial applications. The distinction between oxygen and a fuel is fundamental: oxygen is not itself flammable, but it supports combustion. Materials that burn in ordinary air generally burn more vigorously in oxygen-rich environments. (airproducts.com)
Oxygen also participates in controlled oxidation–reduction reactions within living cells. In these processes, electrons pass through a sequence of carriers before reaching oxygen. Thus, the same broad chemical capacity to accept electrons is relevant both to industrial oxidation and to biological energy conversion, although the reaction pathways and operating conditions differ greatly. (ncbi.nlm.nih.gov)
Occurrence and environmental cycling
Free O₂ is a major component of the atmosphere of Earth, but much terrestrial oxygen is chemically bound. It is present in water and oxygen-bearing minerals; oxygen is the most abundant element in Earth’s crust by mass. Bound oxygen in these substances must be distinguished from molecular oxygen available for respiration. (periodic-table.rsc.org)
Photosynthesis supplies oxygen to the atmosphere by splitting water. Photosynthetic microorganisms, including cyanobacteria, played a central role in the long-term oxygenation of Earth. Atmospheric oxygen began accumulating persistently during the Great Oxidation Event, approximately 2.4 billion years ago. Geological evidence indicates that oxygenation of the atmosphere and oceans was a prolonged transition rather than an instantaneous change. (periodic-table.rsc.org)
Dissolved oxygen supports fish, shellfish, and other aquatic organisms. Water generally holds less dissolved oxygen as its temperature rises, making temperature an important influence on aquatic oxygen availability. Monitoring dissolved oxygen therefore provides information about conditions in rivers, estuaries, and coastal waters that atmospheric measurements alone cannot supply. (oceanservice.noaa.gov)
Biological role
In aerobic cellular respiration, oxygen serves as the terminal electron acceptor of the electron transport chain. In mitochondria, electrons ultimately pass to oxygen, which is reduced to water. Electron transfer is coupled to proton movement across a membrane, establishing a gradient that supports oxidative phosphorylation and the production of adenosine triphosphate (ATP). Oxygen enables this process but is not the source of the fuel molecules being oxidized. (ncbi.nlm.nih.gov)
Dependence on oxygen is not universal. Some microorganisms obtain energy through fermentation, while others use respiratory chains with terminal electron acceptors other than oxygen. The presence of oxygen therefore helps determine which metabolic pathways can operate in a particular environment. (ncbi.nlm.nih.gov)
Discovery and industrial production
Carl Wilhelm Scheele isolated oxygen in 1771, although his account was not published until 1777. Joseph Priestley independently produced it in 1774 by heating mercury oxide. Antoine Lavoisier interpreted combustion as combination with oxygen, helping replace the phlogiston theory. The name derives from Greek roots meaning “acid-forming,” reflecting his mistaken belief that oxygen was essential to all acids. (ciaaw.org)
Industrial oxygen is produced chiefly by separating air. Cryogenic distillation exploits differences in the boiling temperatures of oxygen, nitrogen, and argon. Adsorption-based methods provide an alternative, while electrolysis of water produces oxygen alongside hydrogen. Major applications include steelmaking, chemical manufacture, metal cutting and welding, and wastewater treatment. Oxygen is transported and stored as compressed gas or cryogenic liquid; its strong oxidizing action makes material compatibility and equipment cleanliness important engineering considerations. (airliquide.com)