O2

O2, or molecular oxygen, is the diatomic molecule consisting of two oxygen atoms. It is colorless, odorless, and tasteless at room temperature and makes up about 21% of Earth's atmosphere. The molecule contains a double bond and a ground-state configuration with two unpaired electrons, which renders O2 paramagnetic. It is a gas under standard conditions, condensing to a pale blue liquid at low temperatures and forming a solid at very low temperatures.

Oxygen is highly reactive and participates in most oxidation reactions. It is essential for aerobic respiration in many organisms, where it acts as the final electron acceptor in cellular energy production. O2 is not only critical for life but also supports combustion; fires burn more vigorously in oxygen-rich environments.

Occurrence and production: O2 is produced primarily by photosynthesis and consumed by respiration and oxidation, making it a key component of the global carbon and energy cycles. Industrially, it is obtained from air through fractional distillation of liquefied air or cryogenic air separation. It can also be produced by the electrolysis of water. In the upper atmosphere, some O2 is converted to ozone (O3) by ultraviolet radiation, contributing to the ozone layer.

Uses: O2 is used in metalworking and welding, steelmaking, chemical synthesis, glassmaking, medical and diving applications, and as a rocket oxidizer. It is also employed in ecology, aquaculture, and various industrial processes requiring an oxidant.

Isotopes and safety: the most abundant isotope is 16O, with minor 17O and 18O. Safety considerations include its role in supporting combustion and the potential for oxygen toxicity at high partial pressures; handling and storage follow established guidelines for compressed gases or cryogenic liquids.