- Nitrogen 78.08%
- Oxygen 20.95%
- Argon 0.93%
- Everything else 0.04%
Near the ground, water vapour aside; the rest is mostly carbon dioxide, ≈ 0.04 %, then neon, helium, methane and krypton. After CRC Handbook.
A molecule that will not break
A nitrogen atom has seven protons and five outer electrons, three short of a full shell. Two atoms complete each other by sharing three pairs, and the result is N₂, held by a triple bond that takes about 945 kJ per mole to break — one of the strongest in chemistry, and the whole character of the element. The gas is colourless, odourless and, at ordinary temperatures, almost inert: it does not burn and does not support burning. A candle goes out in it and a mouse suffocates, not because nitrogen is poisonous but because it does nothing.
Forced apart, the atoms are anything but placid. Nitrogen takes every oxidation state from −3 to +5, and many of its compounds hold energy in the strain of their bonds and give it back at once: nitroglycerine, TNT and ammonium nitrate owe their violence to nitrogen's hurry to return to N₂.
Air, soil and cells
Nitrogen is 78 % of the air by volume, yet the crust holds little: nitrates dissolve and wash to the sea, and the only large deposits were the saltpetre beds of the Atacama, shipped to Europe through the nineteenth century for fertiliser and gunpowder. Every living thing needs the element. It sits in every amino acid, so in every protein, and in the bases of DNA; an adult body is about 3 % nitrogen by mass.
The difficulty is getting it out of the air. Plants cannot use N₂; the bond is beyond them. What breaks it in nature is lightning, and the enzyme nitrogenase in a few kinds of bacteria, some living in the root nodules of peas, beans and clover. The whole nitrogen cycle turns on that narrow gate, and before 1900 the supply of fixed nitrogen set a ceiling on how much food the land could grow. Industry takes the gas from liquefied air, distilling off the more volatile nitrogen from the oxygen.
Bread from the air
In 1909 Fritz Haber showed that nitrogen and hydrogenElement: Hydrogen. The lightest element, and most of the universe combine into ammonia over a catalyst, osmium on his bench and iron in the works, at high pressure and a few hundred degrees; Carl Bosch of BASF turned the bench reaction into a plant at Oppau by 1913. The Haber–Bosch process now fixes well over a hundred million tonnes of ammonia a year, most of it for fertiliser, and by some estimates close to half the nitrogen in the world's human bodies has passed through it. The same ammonia, oxidised to nitric acid, made the explosives of both world wars.
The other uses are quieter. Nitrogen gas blankets whatever must be kept from oxygen — packaged food, electronics, the wine in a half-empty barrel. Liquid nitrogen, at −196 °C, freezes tissue, preserves cells and helps cool superconducting magnets. Nitrous oxide is an anaesthetic, and nitric oxide turned out in the 1980s to be a signal the body uses to relax its blood vessels.
Discovery and name
Several chemists met nitrogen in the early 1770s; Scheele, Cavendish and Priestley all had it in their vessels. The credit usually goes to Daniel Rutherford, a student of Joseph Black in Edinburgh, whose doctoral thesis of 1772 described what was left of the air after a candle had burnt in it and the fixed air had been absorbed by alkali. He called the residue “noxious” or “mephitic” air and noted that it supported neither flame nor life.
Lavoisier, who saw that it was an element, named it azote, from the Greek for lifeless, and the name survives in French and Romanian. Jean-Antoine Chaptal, objecting that other gases were unbreathable too, proposed nitrogène in 1790, after nitre, the saltpetre it forms; English took the name, and the symbol N comes from it.