Gaseous chemical element, chemical symbol O, atomic number 8. It constitutes 21% (by volume) of air and more than 46% (by weight) of Earth's crust, where it is the most plentiful element. It is a colourless, odourless, tasteless gas, occurring as the diatomic molecule O. In respiration, it is taken up by animals and some bacteria (and by plants in the dark), which give off carbon dioxide (CO). In photosynthesis, green plants assimilate carbon dioxide in the presence of sunlight and give off oxygen. The small amount of oxygen that dissolves in water is essential for the respiration of fish and other aquatic life. Oxygen takes part in combustion and in corrosion but does not itself burn. It has valence 2 in compounds; the most important is water. It forms oxides and is part of many other molecules and functional groups, including nitrate, sulfate, phosphate, and carbonate; alcohols, aldehydes, carboxylic acids, and ketones; and peroxides. Obtained for industrial use by distillation of liquefied air, oxygen is used in steelmaking and other metallurgical processes and in the chemical industry. Medical uses include respiratory therapy, incubators, and inhaled anesthetics. Oxygen is part of all gas mixtures for manned spacecraft, scuba divers, workers in closed environments, and hyperbaric chambers. It is also used in rocket engines as an oxidizer (in liquefied form) and in water and waste treatment processes.