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. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen (O2), a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere.
The discovery of oxygen is often credited to Carl Wilhelm Scheele in 1771-1772, who called it "fire air" because it supported combustion. However, the publication of his findings was delayed, and Joseph Priestley independently discovered oxygen in 1774 by heating mercuric oxide and published his results first. Priestley called the gas "dephlogisticated air." Antoine Lavoisier later interpreted Priestley's experiment in terms of the combustion theory, naming the gas "oxygen" from the Greek roots ὀξύς (oxys) (acid, literally "sharp," from the taste of acids) and -γενής (-genēs) (producer, literally "begetter"). Lavoisier's work helped to establish the law of conservation of mass, which contributed to the modern understanding of chemical reactions.
Oxygen occurs mainly as a component of the atmosphere and in water. It is also found in numerous minerals, including oxides, silicates, and carbonates. The primary source of commercial oxygen is the fractional distillation of liquid air, where nitrogen is separated from oxygen.
Oxygen forms compounds with almost all other elements, except for the noble gases. Some common compounds include:
Britannica - Oxygen
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Jefferson Lab - It's Elemental - Oxygen