Oxygen

Oxygen is the gas that makes burning and breathing possible. You cannot survive more than a few minutes without it, and almost every fire that has ever burned depended on it too. It is the third most abundant element in the universe and the most abundant element in the Earth's crust, yet almost all of it is locked up inside rocks.

• Atomic Number88 protons, 8 electrons
• Atomic Mass15.999 uAbout 16× heavier than hydrogen
• State at Room TempGas21% of the air you breathe
• Density0.001429 g/cm³Slightly denser than air
• Melting / Boiling-218.8°C / -182.9°CLiquid at −183°C
• Discovered1774Priestley & Scheele, 1774

What is oxygen?

Oxygen is a non-metal in Group 16 of the periodic table. At room temperature it forms a colourless, odourless diatomic molecule, O₂. Oxygen is highly reactive, it combines with almost every other element, often releasing energy as heat and light when it does so. The reaction of oxygen with other substances is called oxidation. When it happens fast enough to produce visible light and heat, we call it fire.

Oxygen was named by Antoine Lavoisier from the Greek words oxys (sharp or acid) and genes (maker). Lavoisier believed, incorrectly, that all acids contained oxygen, in fact many do not. Despite this mistake, the name stuck. The symbol O comes from the element's name. In French it is sometimes still called oxygène, and this is the root of the word in most European languages.

Where you find oxygen

In space

Oxygen is produced inside stars when carbon absorbs another helium nucleus during nuclear fusion. Mars has a thin atmosphere that is 95% carbon dioxide, and its red surface is covered in iron oxide, chemically the same as rust. Venus also has a thick carbon dioxide atmosphere. Jupiter's moon Europa has a very thin oxygen atmosphere produced by sunlight breaking apart water ice on its surface.

On Earth

Oxygen is by far the most abundant element in the Earth's crust, making up nearly half of it by mass, but almost all of it is locked inside minerals like quartz and feldspar.

• The atmosphere. About 21% of the air is oxygen gas (O₂). Photosynthesis by plants, algae and cyanobacteria constantly replenishes the oxygen that animals and fires consume.
• The oceans. Oxygen is bound up in every water molecule (H₂O). The oceans represent an enormous store of chemically combined oxygen.
• Rocks and minerals. Quartz, feldspar, calcite and most common rocks are oxides or silicates, compounds in which oxygen is bonded to other elements.

How we use oxygen

• Medical oxygen. Hospitals pipe pure oxygen to patients whose lungs cannot extract enough from normal air. Premature babies, patients with lung disease and people under general anaesthetic all need extra oxygen.
• Steel making. Pure oxygen is blasted into molten iron to burn off impurities such as carbon and sulfur, converting pig iron into steel.
• Rocket propellant. Liquid oxygen is the oxidiser in many rocket engines. Combined with liquid hydrogen or kerosene, it provides an enormously powerful thrust. The Saturn V Moon rocket used liquid oxygen and kerosene.
• Water treatment. Oxygen is bubbled into rivers, lakes and sewage treatment works to help bacteria break down waste and keep water clean for fish and people.

How it was discovered

Oxygen was discovered almost simultaneously by two scientists who did not know about each other's work. The Swedish chemist Carl Wilhelm Scheele produced oxygen by heating several compounds between 1771 and 1773, but did not publish promptly. Meanwhile, the English scientist Joseph Priestley heated mercury oxide in 1774 and collected the gas released, noticing that a candle burned far more brilliantly in it. He called it "dephlogisticated air". Antoine Lavoisier then recognised this gas as responsible for combustion and named it oxygen, establishing it as a true element.

Oxygen is the element that makes complex life possible, from your first breath to the last. Moving one step along the periodic table brings you to fluorine, the most reactive element of all.