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Nitrogen

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Nitrogen is the chemical element in the periodic table that has the symbol N and atomic number 7. A common normally colorless, odorless, tasteless and mostly inert diatomic non-metal gas, nitrogen constitutes 78 percent of Earth's atmosphere and is a constituent of all living tissues. Nitrogen forms many important compounds such as ammonia, nitric acid, and cyanides.

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Notable characteristics

Nitrogen is a non-metal, with an electronegativity of 3.0. It has five electrons in its outer shell, so is trivalent in most compounds. Pure nitrogen is an unreactive colorless diatomic gas at room temperature, and comprises about 78.08% of the Earth's atmosphere. It condenses at 77 K and freezes at 63 K. Liquid nitrogen is a common cryogen.

Applications

The greatest single commercial use of nitrogen is as a component in the manufacture of ammonia via the Haber process. Ammonia is subsequently used for fertilizer production and to produce nitric acid. Nitrogen is used as an inert atmosphere in tanks of explosive liquids, during production of electronic parts such as transistors, diodes, and integrated circuits, and is used in the manufacture of stainless steel.

Liquid nitrogen (often referred to as LN2) is used as a coolant for the immersion freezing and transportation of food products, for the preservation of bodies and reproductive cells (sperm and egg), and for the stable storage of biological samples. It is also used in the study of cryogenics, and for demonstrations in science education. Liquid nitrogen is produced by distillation from liquid air.

The salts of nitric acid include some important compounds, for example potassium nitrate, or saltpeter, and ammonium nitrate. The former compound is a component of gunpowder, the latter important in fertilizer. Nitrated organic compounds, such as nitroglycerin and trinitrotoluene, are often explosives.

Nitric acid is used as an oxidizer in liquid fueled rockets. Hydrazine and hydrazine derivatives find use as rocket fuels.

Liquid nitrogen can also be used in the treatment of warts and of actinic keratosis (skin lesions that may progress to forms of skin cancer if untreated).

History

Nitrogen (Latin nitrum, Greek Nitron meaning "native soda", "genes", "forming") is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or phlogisticated 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 dephlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as azote, which stands for without life.

Compounds of nitrogen were known in the Middle Ages. The alchemists knew nitric acid as aqua fortis. The mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold.

Nitrogen is also used in filling automotive tires due to its relatively flat line of thermal expansion.

Occurrence

Nitrogen is the largest single component of the Earth's atmosphere (78.084% by volume, 75.5% by weight) and is acquired for industrial purposes by the fractional distillation of liquid air or by mechanical means of gaseous air (i.e. pressurised reverse osmosis membrane or PSA (Pressure Swing Adsorption). Compounds that contain this element have been observed in outer space. Nitrogen-14 is created as part of the fusion processes in stars. Nitrogen is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, and compounds of these nitrogenous products.

Molecular nitrogen has been known to occur in Titan's atmosphere for some time, and has now been detected in interstellar space by David Knauth and coworkers using the Far Ultraviolet Spectroscopic Explorer.

Compounds

The main hydride of nitrogen is ammonia (NH3) although hydrazine (N2H4) is also well known. Ammonia is somewhat more basic than water, and in solution forms ammonium ions (NH4+). Liquid ammonia is in fact slightly amphiprotic and forms ammonium and amide ions (NH2-); both amides and nitride (N3-) salts are known, but decompose in water. Singly and doubly substituted compounds of ammonia are called amines. Larger chains, rings and structures of nitrogen hydrides are also known but virtually unstable.

Other classes of nitrogen anions are azides (N3-), which are linear and isoelectronic to carbon dioxide. Another molecule of the same structure is dinitrogen monoxide (N2O), or laughing gas. This is one of a variety of oxides, the most prominent of which are nitrogen monoxide (NO) and nitrogen dioxide (NO2), which both contain an unpaired electron. The latter shows some tendency to dimerize and is an important component of smog.

The more standard oxides, dinitrogen trioxide (N2O3) and dinitrogen pentoxide (N2O5), are actually fairly unstable and explosive. The corresponding acids are nitrous (HNO2) and nitric acid (HNO3), with the corresponding salts called nitrites and nitrates. Nitric acid is one of the few acids stronger than hydronium.

Biological role

Nitrogen is an essential part of amino and nucleic acids which makes nitrogen vital to all life. Legumes like the soybean plant, can recover nitrogen directly from the atmosphere because their roots have nodules harboring microbes that do the actual conversion to ammonia in a process known as nitrogen fixation. The legume subsequently converts ammonia to nitrogen oxides and amino acids to form proteins.

Isotopes

There are two stable isotopes: N-14 and N-15. By far the most common is N-14 (99.634%), which is produced in the CNO cycle in stars. The rest is N-15. Of the ten isotopes produced synthetically, one has a half life of nine minutes and the remaining isotopes have half lives on the order of seconds or less. Biologically-mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil. These reactions almost always result in N-15 enrichment of the substrate and depletion of the product. Although precipitation often contains subequal quantities of ammonium and nitrate, because ammonium is preferentially retained by the canopy relative to atmospheric nitrate, most of the atmospheric nitrogen that reaches the soil surface is in the form of nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium.

Precautions

Nitrate fertilizer washoff is a major source of ground water and river pollution. Cyano (-CN) containing compounds form extremely poisonous salts and are deadly to many animals and all mammals.

See also

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References

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