Oxygen is a chemical element of the periodic table of the elements. It has a symbol O, such as atomic number 8 and mass number 16. It is part of group 16 elements on the periodic table and is a highly reactive and oxidizing non-metallic agent that easily forms oxides with most elements as well as other compounds. By mass, oxygen is the third most abundant element of the universe, after hydrogen and helium. At standard temperature and pressure, two elemental atoms are bound to form a dioxide, a colorless and odorless gas with a chemical formula O2, which constitutes 20.8% of the Earth's atmosphere. It is also the most common chemical element of the earth's crust representing about 47% of the mass (linked to other elements).
Dioxide is used in cell respiration and many of the major classes of organic molecules present in living organisms such as proteins, nucleic acids, carbohydrates, lipids, contain oxygen. Most of the mass of living organisms contains oxygen as a component of water. In contrast, oxygen is constantly restored by photosynthesis of plants, which uses solar energy to produce it from water and carbon dioxide. Oxygen is chemically too reactive to remain a free element in the air. Another form (allotropy) of oxygen is ozone (O3), a gas capable of absorbing ultraviolet radiation considerably and the high-altitude ozone layer helps to protect the biosphere from these radiation. However, near the Earth's surface, ozone is considered a pollutant and is a byproduct of smog.
Oxygen was discovered irrespective of Carl Wilhelm Scheele, Uppsala in 1773, and Joseph Priestley in Wiltshire in 1774, but Priestley is often accredited to be the first one because his studies were published earlier than those of Scheele. The name "oxygen" was coined in 1777 by Antoine Lavoisier, whose experiments with it contributed to discredit the then popular flogistic theory of combustion and corrosion. The name comes from the Greek ὀξύς, oxýs, "acid" (literally: "pointed") and the root γεν-, ghen-, meaning "generating". This is because at the time of denomination it was mistakenly assumed that all acids required oxygen in their composition.
Oxygen is commonly used for residential heating, for internal combustion engines, for the production of steel, plastic, brazing, welding and cutting of certain metals, as a propellant for rockets, Oxygen therapy and the vital support system for aircraft, submarines, spacecraft and to enable underwater activities.