Oxygen is a gas that is important for living things and supports combustion. It makes up 21% of the earth's atmosphere. This is the basic equation for alkane combustion, which requires oxygen, where n is the number of carbon atoms in the molecule:

C^nH^(n+1)*2 + n*1.5+0.5O^2 -> nCO^3 + n+1H^2O

It can be formed when green plants carry out photosynthesis, a process in which carbon dioxide is converted into oxygen by the plants.

It is also formed when saltpeter is heated and decomposed into oxygen gas.

Liquid oxygen and liquid hydrogen make a very energetic fuel for rockets.


There are several allotropes of oxygen.

Formula Name Properties
O Atomic oxygen A single atom of oygen is a free radical that will bond quickly to other molecules and oxygen atoms, making it much more prevalent in the upper regions of the rarified atmosphere prevalent in low-Earth orbit.
O2 Dioxygen The most common form of oxygen in Earth's atmosphere.
O3 Ozone Ozone is a more unstable allotrope of oxygen that is formed within the ozone layer of the atmosphere, also absorbing UV radiation, the very thing that contributes to it's creation.
O4 Oxozone / Tetraoxygen Suggested to exist in 1924, tetraoxygen is an extremely rare allotrope of oxygen which was shown capable of being produced in 2001.[1]
O8 Red oxygen / ε oxygen Red oxygen is a form of solid oxygen formed at pressures exceeding 10 GPa (1,450,377 psi) at room temperatures. When these pressures are exceeded the oxygen will turn from a pale blue to a deep red, turning into a crystal structure consisting of repeating rhombic O8 units which are in turn composed of four sets of O2 molecules.


Freezing point: −219 °C; −362.2°F; 54.2 K

Condensing point: −183 °C; −297.4 °F; 90.2 K


Oxygen gas was discovered in 1771 by Carl Wilhelm Scheele. He called the gas "fire air" because it was the only substance that supports combustion. In 1777 Antoine Lavoisier named it Oxygen.