Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is Earth's most abundant element, and after hydrogen and helium, it is the third-most abundant element in the universe. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O ₂. Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. Oxygen makes up almost half of the Earth's crust in the form of oxides. Many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone.

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Earth’s First Known Mass Extinction Event Starved Life of Oxygen

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Earth’s oxygen levels over time could help us spot alien life

How fluctuating oxygen levels may have accelerated animal evolution

System for generating oxygen within cells

New abiotic pathway for the formation of oxygen

Study uncovers mechanisms of reactive oxygen species in stem cell function and inflammation prevention

A smartphone's camera and flash could help people measure blood oxygen levels at home