Nitrogen is a colorless, odorless gaseous element found in group VA of the periodic table. It was discovered by Daniel Rutherford in Edinburgh in 1772. Combined nitrogen occurs mainly as nitrates. Indeed, the name comes from the Greek nitron genes, which means "nitre forming"; nitre is potassium nitrate, commonly known as saltpeter.
Nitrogen is the most abundant gas in the Earth's atmosphere (78 percent by volume) and is obtained from it by fractional distillation of liquid air. The most common isotope is 14N (99.76%). Nitrogen is also the sixth most common element in the universe. It is formed during hydrogen-burning in main sequence stars and red giants, via the carbon-nitrogen-oxygen cycle.
|relative atomic mass||14.00674|
|atomic radius||71 pm|
|oxidation states||-3, 5|
|melting point||-209.9°C (-345.8°F)|
|boiling point||-195.8°C (-320.4°F)|
Chemistry of nitrogen
Nitrogen is almost insoluble in water. It does not burn, and if a burning substance is plunged into nitrogen the flames are quickly extinguished. Exceptionally, burning magnesium ribbon continues to burn feebly and magnesium nitride is formed.
Molecular nitrogen is inert because of the strong triple bond between the two atoms, but it will react with some elements, especially the alkaline earth metals to give nitrides; with oxygen (e.g., the electric-arc process); and with hydrogen (see Haber process; it also forms N2 ligand complexes with group VIII transition metals. Activated nitrogen, formed in an electric discharge, consists of nitrogen atoms and is much more reactive.
Although few nitrogen compounds can be formed by direct action between nitrogen and other elements, there are many nitrogen-containing compounds, including a large number of organic compounds, e.g., proteins and nucleic acids, which are essential to life.
Although nitrogen can be obtained from the air after oxygen and carbon dioxide have been removed, this residue also contains traces of the inert gases as impurity. A purer product may be made by gently warming a strong solution of ammonium chloride (NH4Cl) and sodium nitrite (NaNO2):
NH4Cl + NaNO2 → NaCl + 2H2O + N2
In a double decomposition reaction sodium chloride (NaCl) and ammonium nitrite are formed, but the latter compound splits up almost as soon as it is formed to yield nitrogen and water.
Compounds of nitrogen
Nitrogen forms mainly trivalent and pentavalent compounds. Nitric oxide (NO) is a colorless gas formed in the electric-arc process; it is readily oxidized further to nitrogen dioxide. The NO molecule is unusual in having an odd number of electrons, and so gives the nitrosyl ions NO+ and NO-. Melting point -164°C, boiling point -152°C.
Nitrites are salts (or esters) of nitrous acid (HNO2) and are mild reducing agents.
Nitrogen monoxide (N2O), also called nitrous oxide or laughing gas, is a colorless, soluble gas with a sweet odor. It is prepared by reducing nitric acid, by reacting sodium nitrite, sulfuric acid and iron (II) (ferrous) sulphate, or by heating ammonium nitrate. It reacts with oxygen to form nitrogen dioxide. Nitrogen monoxide is used as a weak anesthetic, sometimes producing mild hysteria, and also as an aerosol propellant. Melting point -91°C, boiling point -88°C.
Nitrogen dioxide (NO2), a pungent-smelling, red-brown toxic gas in equilibrium with its dimer (N2O4), is a constituent of automobile exhaust and smog. It is made by the action of concentrated nitric acid on copper, and dissolves in water to give a mixture of nitrous and nitric acids. It is also formed by the reaction of oxygen with nitrogen monoxide. A powerful oxidizing agent, it is used in the manufacture of sulfuric acid and in rocket fuels. It is also an intermediate in the manufacture of nitric acid. Its presence in the atmosphere contributes to the formation of acid rain and the depletion of the ozone layer. Melting point -11°C, boiling point 21°C.
Nitrogen and life
Nitrogen, along with hydrogen, carbon, and oxygen, is essential for life as we know it. Nitrogen is of central importance in the formation of amino acids, which are themselves essential components of proteins and nucleic acids. As an element, nitrogen is not particularly reactive because of its great affinity for itself. Nitrogen molecules are composed of two nitrogen atoms, linked by a triple bond that is difficult to break. However, the nitrogen in the atmosphere can be converted without the expenditure of too much energy into ammonia, which is the source of nitrogen in biological compounds.
Animals and plants can't use free nitrogen gas. Bacteria living in the roots of plants such as peas, take in atmospheric nitrogen, and, along with other microorganisms, convert the gas to ammonium salts and to nitrates (see nitrogen fixation). Plants get the nitrogen they need from the inorganic nitrogen compounds in the soil. Animals obtain the nitrogen they need from plants or from other animals.
The nitrogen content of soil, where the nitrogen cycle starts, is enriched and renewed by excretion and decay of animals and plants. Some of the trapped nitrogen is returned to the air as bacteria in soil decompose the nitrogen compounds and release the element back into its gaseous form.
Humans breathe nitrogen in and out of their lungs all the time, without any serious side effects. The nitrogen gas dissolves slightly in the blood and circulates around the body harmlessly. Under pressure however, such as when a person dives into deep water, the amount dissolved nitrogen increases. If the decompression is slow and careful, the dissolved nitrogen comes out of the body fluids and can be removed through the lungs, but, if decompression is too rapid, the 'bends' causes great pain and even death. This decompression sickness is caused by bubbles of nitrogen rapidly coming out of solution in the bloodstream.