Nitrogen

Nitrogen makes up 78 % of the atmosphere by volume, and most organisms cannot use any of it. The reason is the N₂ molecule: three bonds between the atoms make it one of the strongest in chemistry, and breaking it takes either a specialised enzyme or harsh industrial conditions.

That gap between abundance and availability shaped the history of agriculture, and one of the most consequential industrial reactions of the twentieth century.

The atmosphere is the main reservoir. There is little nitrogen in the crust, and almost all of it is concentrated in a few nitrate deposits; the largest lie in the Atacama Desert in Chile, where saltpetre survived because it never rains.

In living matter nitrogen is in amino acids, and therefore in every protein, and in the bases of the nucleic acids. Without nitrogen there is neither protein nor DNA.

In 1772 Daniel Rutherford removed the oxygen and carbon dioxide from air and described what was left as a gas in which a candle would not burn and a mouse would not survive. He called it "noxious air".

Lavoisier named the element azote, "lifeless". Most European languages settled on the other name, nitrogenium, "saltpetre former" — hence the symbol N.

Nitrogen shows oxidation states from −3 to +5, covering nearly the whole available range, and that accounts for the variety of its compounds: from ammonia, NH₃, to nitric acid, HNO₃.

Under ordinary conditions N₂ is so unreactive that it is used as a protective atmosphere — in welding, in food packaging, in aircraft tyres. Heated under pressure with a catalyst, however, it does combine with hydrogen.

That reaction — the Haber–Bosch process, developed in the early twentieth century — fixes atmospheric nitrogen as ammonia. Mineral fertiliser production begins there, and by some estimates about half the nitrogen in humanity's proteins has passed through it. The same reaction has another side: it supplied the feedstock for explosives.

Ammonia and nitric acid are the basis of fertiliser manufacture. Liquid nitrogen, boiling at 77 K, is a cheap refrigerant in medicine and in laboratories. The gas is used wherever an inert atmosphere is needed.

Excess nitrogen fertiliser creates an environmental problem of its own: nitrates washed into water bodies trigger algal blooms, after which the decaying algae strip the water of oxygen.

Nitrogen-fixing bacteria are the only organisms able to break the N₂ triple bond at ordinary temperatures. They do it with the enzyme nitrogenase; some live in nodules on the roots of legumes, which is the reason for the long-standing practice of rotating crops with peas and clover.

From there nitrogen moves along the chain: plants build amino acids from it, animals get those from food, and decomposing bacteria eventually return it to the atmosphere.