Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. ĭioxygen provides the energy released in combustion and aerobic cellular respiration, and 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. Oxygen makes up almost half of the Earth's crust in the form of oxides.
Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula OĢ. Oxygen is Earth's most abundant element, and after hydrogen and helium, it is the third-most abundant element in the universe. 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 the chemical element with the symbol O and atomic number 8.
