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Oxygen, symbol O and atomic number 8, is a chemical element in the periodic table. It belongs to group 16, the chalcogens, and is a non-metal. Oxygen is the third-most abundant element in the universe by mass and makes up about 21 percent of Earth's atmosphere; on Earth it is the most common element by mass in the crust. In nature it occurs mainly as diatomic molecules, O2, though ozone (O3) is an important allotrope formed in the upper atmosphere and by electrical discharges.

Physical properties: At standard conditions, oxygen is a colorless, odorless gas. It has a boiling point of −183.0 °C and a melting point of −218.8 °C. The molecule O2 is paramagnetic due to two unpaired electrons.

Isotopes: The three stable isotopes are O-16, O-17, and O-18. O-16 accounts for the majority of natural oxygen. Isotopic ratios are used in climate studies, geology, and biology to trace processes such as photosynthesis and past temperatures.

Occurrence and production: Oxygen is abundant in the Earth's crust and atmosphere. Industrial production relies on fractional distillation of liquid air and, in smaller scale, electrolysis of water. Oxygen is essential for respiration and is a key reactant in steelmaking, chemical synthesis, welding, and medical therapy. Liquid oxygen serves as an oxidizer in rocketry.

Safety: Oxygen supports combustion and can intensify fires; it is not flammable itself but can enable rapid burning. High concentrations in enclosed spaces can cause asphyxiation, and prolonged exposure at elevated pressures may lead to oxygen toxicity in biological systems.

History: Oxygen was independently isolated by Carl Wilhelm Scheele and Joseph Priestley in the 1770s. Antoine Lavoisier named the element oxygen in 1777, deriving its name from Greek roots meaning "acid producer," reflecting the then-held theory that acids require oxygen.