Oxygen · element 8
The greedy gas that makes Earth rust, rot and breathe, and makes up nearly half its crust
Key facts
Oxygen is the element that sets fire to wood, turns iron to rust and keeps us alive. It is a colourless gas that forms about a fifth of the air, and it is chemically greedy, grabbing electrons from almost any other element it meets. That greed is the basis of burning, of rusting and of respiration, which is a slow, controlled burning of food inside our cells.
Oxygen is also, by mass, the most abundant element in the Earth's crust and in the human body. It is in the water we drink, the rock beneath our feet and the air above. Yet for the first half of Earth's history there was almost no free oxygen at all. The air we breathe is the waste product of countless generations of photosynthesising organisms.
Where it is found
Roughly 46 per cent of the Earth's crust by mass is oxygen, bound into silicates, oxides and carbonates. The oceans are about 86 per cent oxygen by mass, since water is mostly oxygen. In the atmosphere it is about 21 per cent by volume, almost entirely as O2 molecules, with a trace of ozone, O3, in the upper atmosphere that absorbs harmful ultraviolet light.
Oxygen is the third most abundant element in the universe after hydrogen and helium. It is made inside massive stars and hurled out when they explode as supernovae. Free oxygen in a planet's atmosphere, however, is rare, and it is a signature that astronomers look for as a possible sign of life.
How it was discovered
Three people share the credit. Carl Wilhelm Scheele, a Swedish pharmacist, produced oxygen by heating compounds such as mercury oxide around 1771 and called it fire air, but his book was delayed until 1777. Joseph Priestley in England made the gas in 1774 by focusing sunlight on mercury oxide, and noticed that a candle burned brilliantly in it and a mouse stayed lively. He published promptly and told Antoine Lavoisier in Paris.
Lavoisier understood what the gas meant. He showed that burning and rusting are combinations with this gas, overturning the old idea that burning substances release something called phlogiston. He named it oxygen, meaning acid-former, because he wrongly believed all acids contained it. The name stuck even after the theory was corrected.
What we use it for
Industry uses more oxygen than any other purpose. Steelmaking blows pure oxygen through molten iron to burn off carbon and impurities, and it is the single largest consumer. Oxygen also cuts and welds steel, bleaches paper pulp and treats wastewater.
In medicine, oxygen is a drug. Hospitals pipe it to patients with lung disease, pneumonia and heart problems, and the pandemic of 2020 showed how much depends on a reliable supply. Liquid oxygen, combined with hydrogen or kerosene, is the oxidiser in most large rockets. Almost all industrial oxygen is separated from air by cooling it until it liquefies and then distilling it.
Reading the numbers
Oxygen boils at about minus 183 degrees Celsius and freezes at about minus 219. Because it boils at a higher temperature than nitrogen, the two can be separated when liquid air is warmed: nitrogen boils away first, leaving oxygen behind. As a gas its density is 1.43 grams per litre, about ten per cent more than air.
The configuration [He] 2s2 2p4 shows six outer electrons, two short of a full shell. Oxygen fills those two gaps by taking electrons outright, forming the O2- ion in oxides, or by sharing two pairs, as in water. Its electronegativity of 3.44 is second only to fluorine's, which is why it pulls electrons so strongly and why so many elements burn in it. Oxygen sits in group 16, at the head of the chalcogens, in period 2.
Safety and the environment
Oxygen does not burn, but it makes everything else burn faster and hotter. Oil and grease can ignite spontaneously in pure oxygen, and clothing soaked in oxygen-enriched air becomes a serious fire hazard. Hospitals and home oxygen users face strict rules about smoking and naked flames. Breathing pure oxygen at high pressure for long periods damages the lungs and can cause seizures, which limits how deep divers can go on oxygen-rich mixtures.
Ozone, a form of oxygen, protects us in the stratosphere but is a harmful pollutant at ground level. The oxygen content of the air is not in danger, but dissolved oxygen in lakes and seas can fall dangerously low when fertiliser pollution feeds algae, killing fish.
Did you know?
- Oxygen is the most abundant element in the Earth's crust, making up about 46 per cent of it by mass.
- About 65 per cent of the mass of the human body is oxygen, mostly as water.
- Liquid oxygen is pale blue and will hang between the poles of a strong magnet.
- Free oxygen only built up in Earth's atmosphere about 2.4 billion years ago, in what geologists call the Great Oxidation Event.
- Ozone, the gas that shields Earth from ultraviolet light, is simply three oxygen atoms bonded together.
- Roughly half of the oxygen produced by photosynthesis on Earth comes from microscopic plankton in the oceans.
More in the non-metals
Column-mates come first, since elements in one column have outer electrons arranged in a similar way and tend to behave alike; the rest are the nearest neighbours in the family.
See all 7 non-metals on one page, or browse all 118 elements.
Compare Oxygen with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is oxygen flammable?
Strictly no: oxygen itself does not burn. But it is what makes other things burn, and in oxygen-rich air fires start more easily and spread far faster. That is why oxygen cylinders and medical oxygen come with strict fire warnings.
How much of the air is oxygen?
About 21 per cent by volume. Almost all of the rest is nitrogen, with about 1 per cent argon and small traces of other gases including carbon dioxide.
Where does the oxygen we breathe come from?
From photosynthesis. Plants, algae and cyanobacteria split water using sunlight and release oxygen as a by-product. Land plants and ocean plankton each contribute roughly half of the total.
Why is oxygen so reactive?
An oxygen atom is two electrons short of a full outer shell and its small size lets the nucleus pull hard on any electrons nearby. It readily takes or shares electrons from almost every other element, releasing energy as it does so. That energy release is what we see as fire.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Oxygen reference article. Spotted an error? Tell us.