Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds.
It is a colourless, odourless gas.
Oxygen makes up 21% of the atmosphere by volume. This is halfway between 17% (below which breathing for unacclimatised people becomes difficult) and 25% (above which many organic compounds are highly flammable). The element and its compounds make up 49.2% by mass of the Earth’s crust, and about two-thirds of the human body.
Oxygen plays an important role in life on Earth. This role is carried out through the oxygen cycle which is the movement of oxygen between air, living things, and the Earth’s crust.
Oxygen found in the air is produced by photosynthesis – without plants there would be little oxygen in the air. Most oxygen on Earth comes from tiny ocean plants called phytoplankton.
Oxygen also exists as the allotrope, ozone. Ozone O3 is a different form of oxygen that combines three oxygen atoms together to create trioxygen. Ozone O3 forms the ozone layer in the Earth’s upper atmosphere. It protects the Earth from the sun’s harmful rays by filtering UV light.
Too much oxygen is however bad for us and causes a condition called the bends which is a particular problem for astronauts and scuba divers. This causes tiny bubbles in the blood which can be painful and sometimes deadly.
The green colour of the aurora borealis, or northern lights, is caused by solar wind particles colliding with oxygen atoms in the earth’s atmosphere.
The greatest commercial use of oxygen gas is in the steel industry. Large quantities are also used in the manufacture of a wide range of chemicals including nitric acid and hydrogen peroxide. It is also used to make epoxyethane (ethylene oxide), used as antifreeze and to make polyester, and chloroethene, the precursor to PVC.
Oxygen gas is used for oxy-acetylene welding and cutting of metals. A growing use is in the treatment of sewage and of effluent from industry.