Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. It is a member of the halogen group on the periodic table, and is a highly reactive nonmetallic period 2 element that readily forms compounds (notably oxides) with almost all other elements. At standard temperature and pressure two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula O₂. Oxygen is the third most abundant element in the universe by mass after hydrogen and helium and the most abundant element by mass in the Earth's crust. Oxygen constitutes 88.8% of the mass of water and 20.9% of the volume of air. Oxygen can form a covalent bond with two hydrogen atoms to create the compound "water," an essential to many life forms. The Lewis pair structure for this is H:O:H where each colon represents two valence electrons (electrons shared).

Oxygen in the form of O₂ is produced from water by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all complex life. Oxygen is toxic to anaerobic organisms, which were the dominant form of early life on Earth until O₂ began to accumulate in the atmosphere 2.5 billion years ago. Fortunately, O₂ is the most stable form of the element, having an enthalpy of 0. Another form (allotrope) of oxygen, ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog.

Discovery
Oxygen was independently discovered by Joseph Priestley and Carl Wilhelm Scheele in the 1770s, but Priestley is usually given priority because he published his findings first. The name oxygen was coined in 1777 by Antoine Lavoisier,[5] whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites to remove carbon dioxide and nitrogen from air, electrolysis of water and other means. Uses of oxygen include the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support in aircraft, submarines, spaceflight and diving.

Covenant
Almost all Covenant species respire using oxygen. The vast majority of known Covenant worlds possess oxygenated atmospheres. The only known species whose homeworld is not predominantly oxygen atmosphere is the Unggoy. The Covenant symbol for oxygen is twin dots, surrounded by a triangle.

Forerunner
It is unknown whether or not the Forerunners breathed oxygen, though there is oxygen on all of the Halo Rings, Onyx, and the Ark.