- Oxygen 46.1%
- Silicon 28.2%
- Aluminium 8.2%
- Iron 5.6%
- Calcium 4.2%
- Everything else 7.7%
Approximate shares; surveys differ by a few tenths of a per cent. Oxygen is bound in the silicates and oxides that make up nearly every rock. After CRC Handbook.
Two atoms, hungry for electrons
An oxygen atom has eight protons and six outer electrons, two short of a full shell, and it fills the gap by taking electrons from almost any other element; only fluorine pulls harder. In the gas two atoms share two pairs, giving O₂, held by a double bond of about 498 kJ per mole — strong enough that reaction with most things needs a spark or a flame, weak enough that once started it runs. The molecule has two unpaired electrons, so liquid oxygen clings to the poles of a magnet.
Ozone, O₃, is a second form, made when ultraviolet light or an electric discharge splits O₂ and the loose atoms attach to whole molecules; a thin layer of it in the stratosphere absorbs the ultraviolet that would otherwise reach the ground. What oxygen does to other substances is called oxidation. Rust, fire, the browning of a cut apple and the slow burning of sugar in a cell are the same chemistry at different speeds.
The commonest element on Earth
By mass, oxygen is nearly half the crust, bound in the silicates of almost every rock, in quartz, feldspar, clay and limestone. It is nine tenths of waterCompound: Water. Two hydrogens, one oxygen, and most of the living world by mass, and so most of the sea and about two thirds of the human body. Only in the air is it free, a fifth of the volume, and that fifth is not original: the early atmosphere had none. Oxygen began to accumulate about 2.4 billion years ago, the waste of photosynthesis in cyanobacteria, and plants and algae have replenished it ever since; without life the air would slowly lose it to the rocks.
Industry takes oxygen from the air, cooled until it liquefies, at about −190 °C, and distilled: nitrogen boils off first and oxygen is left. Smaller quantities are sieved from air with zeolites, or made by electrolysing water, which gives hydrogenElement: Hydrogen. The lightest element, and most of the universe at the same time.
Breath, fire and steel
Animals burn food. The oxygen taken in at the lungs is carried by the ironElement: Iron. The metal that built the modern world, and the core of the planet beneath it in haemoglobin to every cell, where it accepts the electrons stripped from sugar and fat and leaves as water and carbon dioxide; the energy released along the way is what the body runs on. A person can live weeks without food, days without water, minutes without oxygen.
Most of the oxygen industry makes goes into steel: blown through molten iron it burns off the excess carbon in minutes, the basic oxygen process, introduced in the 1950s, that has made most of the world's steel since the 1970s. It cuts and welds metal and, as a liquid, is the oxidiser of rockets — the Saturn V burnt kerosene with it in its first stage and hydrogen in the others.
Discovery and name
Carl Wilhelm Scheele, an apothecary in Uppsala, made oxygen from several compounds by about 1772 and called it “fire air”, but his book was not printed until 1777. Meanwhile, on 1 August 1774, Joseph Priestley focused sunlight through a lens on to the red calx of mercury and collected a gas in which a candle burnt with a fierce flame and a mouse stayed lively far longer than in common air. He called it “dephlogisticated air”, published it in 1775, and described it to Antoine Lavoisier in Paris that October.
Lavoisier saw what it meant: that burning is combination with this gas, that phlogiston was a fiction, and that air is a mixture, not an element. He named the gas oxygène in 1777, from the Greek oxys, sharp, and genes, forming, because he thought it the ingredient of every acid. Hydrochloric acid, which contains none, proved him wrong, but by then the name was settled.