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Oxygen is a chemical element; it has symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other compounds. Dioxygen gas currently constitutes approximately 20.95% molar fraction of the Earth's atmosphere, though this has changed considerably over long periods of time in Earth's history.