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Nitrogen is a chemical element with symbol N and atomic number 7 Nitrogen is the lightest member of group 15 of the periodic table, often called the pnictogens. Nitrogen occurs in all organisms, primarily in amino acids (and thus proteins), in the nucleic acids (DNA and RNA) and in the energy transfer molecule adenosine triphosphate. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds, then back into the atmosphere.

Discovery date 1772 Discovered by Daniel Rutherford Origin of the name The name is derived from the Greek 'nitron' and 'genes' meaning nitre forming. Allotropes N 2

Image explanation The wheat sheaf symbol and lightning reflect the importance of nitrogen to living things. Nitrogen is important for plant growth and can be fixed by lightning or added to soils in fertilisers. Appearance A colourless, odourless gas. Uses Nitrogen is important to the chemical industry. It is used to make fertilisers, nitric acid, nylon, dyes and explosives. Nitrogen gas is also used to provide an unreactive atmosphere. It is used in this way to preserve foods, and in the electronics industry during the production of transistors and diodes. Liquid nitrogen is often used as a refrigerant. It is used for storing sperm, eggs and other cells for medical research and reproductive technology. It is also used to rapidly freeze foods, helping them to maintain moisture, colour, flavour and texture. Natural abundance Nitrogen makes up 78% of the air, by volume. It is obtained by the distillation of liquid air. Around 45 million tonnes are extracted each year. It is found, as compounds, in all living things and hence also in coal and other fossil fuels.

Phase (at STP)Gas Melting point K ( °C, °F) Boiling point K ( °C, °F) Density (at STP)1.251 g/L when liquid (at b.p.)0.808 g/cm 3 Triple point K, kPa Critical point K, Mpa Heat of fusion(N 2 ) 0.72 kJ/mol Heat of vaporization(N 2 ) 5.56 kJ/mol Molar heat capacity(N 2 ) J/(mol·K)

Nitrogen in the form of ammonium chloride, NH 4 Cl, was known to the alchemists as sal ammonia. It was manufactured in Egypt by heating a mixture of dung, salt and urine. Nitrogen gas itself was obtained in the 1760s by both Henry Cavendish and Joseph Priestley and they did this by removing the oxygen from air. They noted it extinguished a lighted candle and that a mouse breathing it would soon die. Neither man deduced that it was an element. The first person to suggest this was a young student Daniel Rutherford in his doctorate thesis of September 1772 at Edinburgh, Scotland.

P (Pa) k10 k100 k at T (K) Vapor pressure

The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Schematic representation of the flow of nitrogen through the land environment. The importance of bacteria in the cycle is immediately recognized as being a key element in the cycle, providing different forms of nitrogen compounds assimilable by higher organisms.