Oxygen

Oxygen (chemical symbol: O) is a chemical element with atomic number 8 (the number of protons in an oxygen atom). In its normal state it exists as a gas with two atoms, O2, which is colorless, odorless, and tasteless. Oxygen is a major component of Earth's atmosphere, making up just under a quarter of its volume.

Oxygen is intimately connected to life: aerobic organisms use it to burn food and release energy (respiration). It is also a component of water (H2O) and many minerals in Earth's crust. In water composition, oxygen accounts for a large share by mass.

Physical and chemical properties

O2 is a gas at room temperature. When cooled sufficiently, it becomes a pale blue liquid. Oxygen is a strong oxidizer, meaning it reacts readily with other elements to form oxides. Combustion (fire) is the rapid reaction between oxygen and a combustible substance. Oxygen does not burn by itself, but it accelerates burning.

There is another form of oxygen with three atoms: ozone (O3). Ozone in the upper atmosphere blocks some of the sun's ultraviolet radiation. Lower atmosphere ozone, by contrast, is air pollution that harms respiration.

The oxygen cycle and photosynthesis

Plants, algae, and certain bacteria release oxygen by using sunlight to convert water and carbon dioxide (photosynthesis). Organisms that respire use oxygen and release carbon dioxide in return. This creates a steady cycle. The abundant oxygen in Earth's atmosphere is believed to have originated from the photosynthetic activity of organisms over a very long time.

Uses

Hospitals use pure oxygen for patients with difficulty breathing. Iron foundries use oxygen in furnaces and welding. It is also used in rockets, where liquid oxygen burns fuel in the absence of air. Water purification and wastewater treatment also rely on oxygen.