    thumb|300px|upright|blue white glow from an oxygen discharge tube. oxygen is a chemical element with symbol&nbsp;o and atomic number&nbsp;8. it is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element and oxidizing agent that readily forms compounds (notably oxides) with most elements. by mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium.emsley 2001, p.297 at stp, two atoms of the element bind to form dioxygen, a diatomic gas that is colorless, odorless, and tasteless, with the formula . many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. most of the mass of living organisms is oxygen as it is a part of water, the major constituent of lifeforms (for example, about two-thirds of human body mass). elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on earth until  began to accumulate in the atmosphere. free elemental  only began to accumulate in the atmosphere about 2.5 billion years ago during the great oxygenation event, about a billion years after the first appearance of these organisms. diatomic oxygen gas constitutes 20.8% of the volume of air. oxyg
Biology_and_pharmacology_of_chemical_elements Breathing_gases Chalcogens Chemical_elements Diatomic_nonmetals Oxidizing_agents Oxygen