nīˈtrəjən, gaseous chemical element; symbol N; at. no. 7; at. wt. 14.0067; m.p. −209.86 degrees Celsius; b.p. −195.8 degrees Celsius; density 1.25 grams per liter at STP; valence principally −3, +3, or +5. Nitrogen is a colorless, odorless, tasteless diatomic gas. It is found in group Va of the periodic table. It does not burn, does not support combustion, and is only slightly soluble in water. It is relatively inactive chemically, but many of its compounds display marked reactivity. At high temperatures it reacts with some of the other elements to form nitrides.

Nitrogen has several oxides. Nitrous oxide, N₂O, is a gas used as an anesthetic; it is often called laughing gas. Nitric oxide, NO, is a gas used in the manufacture of sulfuric acid; in air it forms nitrogen dioxide, NO₂, a poisonous reddish brown gas. Nitrogen trioxide, N₂O₃, is unstable at ordinary temperatures. Nitrogen pentoxide, N₂O₅, forms nitric acid when dissolved in water. Important compounds of nitrogen include nitric acid, ammonia, many explosives, cyanides, fertilizers, and the proteins. Many organic compounds contain nitrogen.

Nitrogen for industrial use is produced largely by the fractional distillation of liquid air. Nitrogen is used to some extent for filling light bulbs, in thermometers, and generally anywhere a relatively inert atmosphere is needed, but the chief importance of the element lies in its compounds. The expression "nitrogen fixation" refers to the extraction of the element from the atmosphere by its combination with other elements to form compounds. This is accomplished commercially in several ways. In the Haber process, nitrogen is reacted with hydrogen to form ammonia; in the cyanamide process, nitrogen is reacted with calcium carbide at high temperatures to form calcium cyanamide; in the arc process, nitrogen is reacted with oxygen in an electric arc to form nitrogen oxides.

Nitrogen is abundant in the atmosphere; it is about 78% (by volume) of dry air. Nitrogen is present in living things; it and its compounds are necessary for the continuation of life (see nitrogen cycle). Nitrogen is found in foods and is important in the human diet.

Nitrogen was discovered by Daniel Rutherford in 1772, although K. W. Scheele and others were studying phlogisticated air (air from which the oxygen had been removed, usually by combustion). Lavoisier was the first to treat phlogisticated air as a separate element, which he called azote. The term nitrogen was first used by J. A. Chaptal in 1790. This early "nitrogen" was later shown by John Strutt, or Lord Rayleigh, and William Ramsay to contain argon; Henry Cavendish had shown in 1785 that there was an unreactive gas other than nitrogen present in air.

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