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Nitrogen (pronounced /'na?t????n/) is a chemical element that has the symbol N and atomic number 7 and atomic weight 14.00674µ. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The very strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in converting the N2 into useful compounds, and releasing large amounts of energy when these compounds burn or decay back into nitrogen gas. The element nitrogen was discovered by Daniel Rutherford, a Scottish physician in 1772. Nitrogen occurs in all living organisms—it is a constituent element of amino acids and thus of proteins, and of nucleic acids (DNA and RNA); resides in the chemical structure of almost all neurotransmitters; and is a defining component of alkaloids, biological molecules produced by many organisms. Nitrogen (Latin nitrogenium, where nitrum (from Greek nitron) means "saltpetre" (see nitre), and genes means "forming") is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. That there was a fraction of air that did not support combustion was well known to the late 18th century chemist. 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 phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephetic air" or azote, from the Greek word a??t?? meaning "lifeless". Animals died in it, and it was the principal component of air in which animals had suffocated and flames had burned to extinction. Lavoisier's name for nitrogen still remains in the common names of many compounds, such as hydrazine and compounds of the azide ion. Compounds of nitrogen were known in the Middle Ages. The alchemists knew nitric acid as aqua fortis (strong water). The mixture of nitric and hydrochloric acids was known as aqua regia (royal water), celebrated for its ability to dissolve gold (the king of metals). The earliest military, industrial and agricultural applications of nitrogen compounds involved uses of saltpeter (sodium nitrate or potassium nitrate), notably in gunpowder, and much later, as fertilizer.
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