nitrogen nīˈtrəjən [key], gaseous chemical element; symbol N; at. no. 7; interval in which at. wt. ranges 14.00643–14.00728; m.p. −209.86℃; b.p. −195.8℃; 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 15 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, as in the production of electronic parts such as transistors, diodes, and integrated circuits, and in food storage packaging to prevent spoilage. It is used in the manufacture of stainless steel and as a coolant for the immersion freezing of food products, for the transportation of foods, for the preservation of bodies and reproductive cells (sperm and eggs), and for the storage of biological samples. However, the chief importance of the element lies in its compounds, among them ammonia, nitric acid, and cyanide.

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 also is found in foods and is important in the human diet.

Nitrogen compounds were known to alchemists as early as the Middle Ages, but nitrogen is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or phlogisticated air (air from which the oxygen had been removed, usually by combustion). Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or dephlogisticated air. It was well known to these late 18th century chemists that there was a fraction of air that did not support combustion. Antoine Lavoisier was the first to treat oxygenless air as a separate element, which he called azote, meaning without life. The term nitrogen was first used by J. A. Chaptal in 1790. This early “nitrogen” was later shown by John Strutt (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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