oxygen

Oxygen is the chemical element with the symbol O and the atomic number 8. It is a member of the chalcogen group and is the third most abundant element in the universe by mass. On Earth, oxygen accounts for about 46 percent of the crust and about 21 percent of the atmosphere. The most common form in ambient conditions is diatomic oxygen, O2; ozone (O3) forms in the upper atmosphere and absorbs ultraviolet radiation.

Oxygen is a colorless, odorless, and tasteless gas at room temperature. It has a boiling point of about −183°C and a molar mass of roughly 32 g/mol. The diatomic molecule O2 is weakly paramagnetic due to its two unpaired electrons.

Oxygen is produced mainly by photosynthesis, carried out by plants, algae, and cyanobacteria, which release O2 as a byproduct. It is consumed by respiration, combustion, and weathering processes. In the crust, oxygen is the most abundant element in oxides. It is essential for aerobic respiration in many organisms and plays a crucial role in protecting life through the ozone layer in the stratosphere.

Uses of oxygen include industrial and medical applications. It is used in steelmaking and other high-temperature processes, in welding and cutting, and as a medical oxygen therapy. In rocketry and space missions, liquid oxygen (LOX) serves as an oxidizer.

Production and supply are dominated by fractional distillation of liquid air, with other methods such as pressure swing adsorption and membrane separation contributing to supply. Purified oxygen can also be produced by the electrolysis of water.

Historically, oxygen was identified independently by Carl Wilhelm Scheele and Joseph Priestley in the 1770s and was later named as an element by Antoine Lavoisier. The most common natural isotope is 16O; smaller amounts of 17O and 18O occur naturally.