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KeywordsWater Vapor Stable Isotope Oxygen Isotope Natural Abundance Electron Configuration
Oxygen is a chemical element with atomic number 8, represented by the symbol O. The name oxygen was coined in 1777 by Lavoisier. Its name derives from the Greek word “oxys” meaning “sharp,” referring to the sharp taste of acids, and “genēs” meaning “producer.” It is a chalcogen and is highly reactive, forming compounds such as oxides with most other elements. Oxygen is also the name of the molecular compound O2, also known as dioxygen, formed from two covalently linked oxygen atoms with a spin triplet electron configuration.
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. Dioxygen as a gas makes up 20.9 % of the volume of Earth’s atmosphere and is also present in the form of water vapor and CO2. Oxygen makes up 49.2 % of the Earth’s crust by mass, mostly in the form of silicates, carbonates, and metal oxides, and is the major component of seawater at 88.8 % by mass, in the form of water.
All major classes of biological molecules, including proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Oxygen was independently discovered by Scheele in 1773 (or possibly earlier) and Priestley in 1774.
Naturally occurring terrestrial oxygen includes three stable isotopes: 16O, 17O, and 18O. 16O is the most abundant, making up 99.762 % of the natural abundance. Most 16O is synthesized at the end of the helium fusion process in stars, but some is made by neon burning. 17O is primarily produced by the burning of hydrogen into helium during the CNO cycle. 18O is produced when 14N captures a 4He nucleus during stellar fusion.
Due to its electronegativity, oxygen forms chemical bonds with almost all other elements to give corresponding oxides. The oxidation state of atomic oxygen is −2 in almost all known oxygen-containing compounds, but in a few compounds such as peroxides, the oxidation state is −1.