Nitrogen N

Discovery

In the 1770s several chemists were studying the part of air that remains after something burns in it or an animal breathes it. The discovery is generally credited to the Scottish physician Daniel Rutherford, who described this 'noxious air' in his 1772 doctoral thesis: it could not support a flame or keep a mouse alive, and it was not absorbed by the alkali that removed 'fixed air' (carbon dioxide). Carl Wilhelm Scheele, Henry Cavendish and Joseph Priestley studied the same gas at about the same time.

Antoine Lavoisier recognised it as an element and called it azote. In the early twentieth century Fritz Haber and Carl Bosch developed a way to combine atmospheric nitrogen with hydrogen to make ammonia, one of the most consequential industrial inventions in history.

Origin of the name

The name comes from the French nitrogène, coined in 1790 by Jean-Antoine Chaptal from nitre (potassium nitrate, saltpetre) and the Greek -genes, 'forming', because the gas is a component of nitre. Lavoisier's name azote, from Greek for 'without life', survives in French and in the prefix azo- in chemistry. The symbol is N.

Main uses

• Fertilisers: most industrial nitrogen becomes ammonia, then urea, ammonium nitrate and other fertilisers.
• Chemicals and explosives: nitric acid, nylon, dyes and explosives such as TNT are nitrogen compounds.
• Inert atmospheres: nitrogen gas flushes food packaging, protects electronics manufacturing and blankets fuel tanks against fire.
• Cryogenics: liquid nitrogen freezes biological samples, cools equipment and shrink-fits machine parts, and is used in dermatology to remove warts.

Isotopes

Nitrogen has two stable isotopes: nitrogen-14, about 99.6 percent, and nitrogen-15, about 0.4 percent. Ecologists use the nitrogen-15 ratio in tissues to trace food webs and fertiliser sources, and labelled nitrogen-15 is used in NMR and metabolic studies. The radioactive nitrogen-13, with a half-life of about 10 minutes, is used in PET scans as labelled ammonia to image blood flow to the heart.

In everyday life

Although we breathe nitrogen constantly, our bodies cannot use the gas directly; we obtain nitrogen through proteins in food, and plants depend on soil bacteria or fertiliser to fix it. Divers who surface too quickly can suffer decompression sickness when dissolved nitrogen forms bubbles in the blood, and at depth nitrogen causes a drunk-like narcosis.

Nitrogen oxides from car engines and power plants contribute to smog and acid rain, and excess fertiliser runoff can cause algal blooms in rivers and coastal waters.