Oxygen
Oxygen is a chemical element with the symbol O and atomic number 8. In Earth's atmosphere it is mostly found as molecular oxygen, O2, a colorless gas essential for aerobic respiration, combustion, many chemical reactions, and the chemistry of water, rocks, living cells, oceans, and the ozone layer.
What oxygen is
Oxygen is a nonmetal chemical element. In everyday air, oxygen usually means molecular oxygen, O2, in which two oxygen atoms are bonded together. Oxygen atoms also occur in water, minerals, organic molecules, carbon dioxide, ozone, and countless other compounds. That difference matters: oxygen as an element is everywhere, while O2 gas has specific biological and atmospheric roles.
O2 and ozone
The familiar oxygen gas in air is O2. Ozone is O3, a different molecule made from three oxygen atoms. O2 supports respiration and combustion, while stratospheric ozone helps absorb harmful ultraviolet radiation. Ground-level ozone, however, is an air pollutant. The same element can therefore appear in molecules with very different behavior.
In the atmosphere
Earth's lower atmosphere is mostly nitrogen and oxygen, with O2 making up about one fifth of dry air. Atmospheric oxygen is maintained over long time scales by photosynthesis, burial of organic carbon, respiration, oxidation, weathering, and other Earth-system processes. The present oxygen-rich atmosphere is the result of biological and geological history, not a fixed starting condition.
Photosynthesis and respiration
Photosynthesis by plants, algae, and cyanobacteria releases oxygen when water molecules are split during light-driven reactions. Aerobic respiration uses oxygen as a final electron acceptor, allowing cells to extract energy efficiently from food molecules and releasing carbon dioxide and water. These linked processes connect oxygen to food webs, metabolism, and the carbon cycle.
Combustion and oxidation
Oxygen supports combustion because it reacts readily with many substances, often releasing heat and light. Rusting, fire, fuel burning, and many industrial reactions are forms of oxidation. Oxygen itself is not fuel, but it allows fuels to burn. This is why oxygen-rich environments can make fires more intense and why oxygen handling requires safety controls.
In water and oceans
Dissolved oxygen is essential for many aquatic organisms. It enters water from the atmosphere and from photosynthesis by aquatic plants and phytoplankton. Oxygen levels can fall when warm water holds less gas, when microbes consume oxygen during decomposition, or when pollution and nutrient runoff trigger algal blooms. Low oxygen can stress or kill fish and bottom-dwelling organisms.
Medical and industrial uses
Oxygen is used in medicine for patients who need respiratory support, in welding and cutting, in steelmaking, in water treatment, in chemical manufacturing, and as an oxidizer in some rocket systems. Medical oxygen must be produced, stored, delivered, and monitored carefully. Industrial oxygen systems must manage fire and pressure hazards.
Safety and limits
Too little oxygen can cause hypoxia, while too much oxygen under some conditions can damage tissues or increase fire risk. Oxygen is colorless and odorless, so oxygen deficiency or enrichment may not be obvious without instruments. Confined spaces, hospitals, laboratories, aircraft, submarines, diving, and industrial sites all need oxygen management matched to the setting.
Why it matters
Oxygen matters because it links chemistry to life, atmosphere, fire, oceans, medicine, and planetary history. It is familiar because people breathe it, but it is not simple. O2 gas, oxygen atoms in compounds, ozone, dissolved oxygen, liquid oxygen, and reactive oxygen species all behave differently. Understanding oxygen helps explain both life's energy systems and many environmental risks.