Nitrogen is a chemical element, symbolized by N, whose individual atoms comprise a nucleus of seven protons, i.e., it has an atomic number Z = 7, meaning the nucleus has a positive electrical charge designated 7 e, with e representing the unit elementary charge. Seven electrons in two surrounding cloud-like energy shells charge-balance the seven protons, rendering the atom electrically neutral. The element occupies the seventh position in the periodic table, in virtue of its atomic number. Its nucleus naturally contains either seven neutrons, the isotope designated 14N (natural abundance 99.632%), or eight neutrons, the isotope designated 15N (natural abundance 0.368%). Other isotopes of nitrogen lack stability, decaying in a few minutes or fractions of a second. 14N has an atomic mass (formerly called atomic weight) of 14.0030740052 u, where u is the unified atomic mass unit; 15N has an atomic mass of 15.0001088984 u. The averaged atomic mass (also known as standard atomic weight) is 14.0067 u.
Nitrogen has three unpaired electrons in its outermost shell and therefore can form three single covalent bonds with three other atoms, sharing one electron with each, as in NH3, ammonia; a single covalent bond with one atom and one double covalent bond with another, as in azo dyes, -N=N-; and, one triple covalent bond, as in N2. No other element has more oxidation states than nitrogen, which can have oxidation states of -3, -2, -1, 0, +1, +2, +3, +4, and +5.
Covalently bonded with itself in a triple bond (sharing six electrons in total), the diatomic gaseous nitrogen molecule, (N2), makes up about 78% by volume (75% by mass) of dry air. At the Earth's surface normal temperature and pressure N2 exists as a gas with no taste, color or odor. The nitrogen molecule is chemically rather inert, it does not burn, does not support combustion and is only slightly soluble in water. At high temperature it combines with oxygen to form NOx, a generic designation for the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide) which are formed during many combustion processes.
Liquid nitrogen, which boils at -195.8 C, is used as a cryogenic liquid to preserve organic material such as bacterial strains or sperm samples. It is also used, usually with liquid helium, to cool superconducting magnetic materials for a variety of instruments including mass spectrometers and nuclear magnetic resonance spectrometers.
Nitrogen is important for living organisms because it is an important element of amino acids, nucleic acids (DNA & RNA) and many other nitrogenous organic compounds. Synthetic fertilizers contain nitrogen obtained from air by the Haber-Bosch process, a so-called nitrogen fixation process.
Ammonia and ammonium salts
Ammonia production on an industrial-scale uses the Haber-Bosch process, in which nitrogen and hydrogen gasses react, in the presence of iron, heat and pressure, to produce ammonia. Ammonia can also be produced by reacting ammonium salts with excess alkali, as shown in the following equation:
NH4+ + OH- → NH3 + H2O
Ammonia has a very pungent odor. Because the molecule has a pyramidal structure, it is very polar and can form hydrogen bonds with water so that saturated aqueous ammonia is 15 M.