Editing Nitrogen (section)

===Allotropes===
{{see also|Solid nitrogen}}
[[File:N2MolecularDiagramCR.jpg|thumb|right|upright=1.8|[[Molecular orbital diagram]] of dinitrogen molecule, N<sub>2</sub>. There are five bonding orbitals and two antibonding orbitals (marked with an asterisk; orbitals involving the inner 1s electrons not shown), giving a total bond order of three.]]
Atomic nitrogen, also known as active nitrogen, is highly reactive, being a [[radical (chemistry)|triradical]] with three unpaired electrons. Free nitrogen atoms easily react with most elements to form nitrides, and even when two free nitrogen atoms collide to produce an excited N<sub>2</sub> molecule, they may release so much energy on collision with even such stable molecules as [[carbon dioxide]] and [[water]] to cause homolytic fission into radicals such as CO and O or OH and H. Atomic nitrogen is prepared by passing an electric discharge through nitrogen gas at 0.1–2&nbsp;mmHg, which produces atomic nitrogen along with a peach-yellow emission that fades slowly as an afterglow for several minutes even after the discharge terminates.<ref name="Greenwood412">Greenwood and Earnshaw, pp. 412–16</ref>

Given the great reactivity of atomic nitrogen, elemental nitrogen usually occurs as molecular N<sub>2</sub>, dinitrogen. This molecule is a colourless, odourless, and tasteless [[diamagnetic]] gas at standard conditions: it melts at −210&nbsp;°C and boils at −196&nbsp;°C.<ref name="Greenwood412" /> Dinitrogen is mostly unreactive at room temperature, but it will nevertheless react with [[lithium]] metal and some [[transition metal]] complexes. This is due to its bonding, which is unique among the diatomic elements at standard conditions in that it has an N≡N [[triple bond]]. Triple bonds have short bond lengths (in this case, 109.76&nbsp;pm) and high dissociation energies (in this case, 945.41&nbsp;kJ/mol), and are thus very strong, explaining dinitrogen's low level of chemical reactivity.<ref name="Greenwood412" /><ref>{{cite web | url=http://www.uigi.com/nitrogen.html | title=Universal Industrial Gases, Inc...Nitrogen N2 Properties, Uses, Applications - Gas and Liquid }}</ref>

Other nitrogen [[oligomers]] and polymers may be possible. If they could be synthesised, they may have potential applications as materials with a very high energy density, that could be used as powerful propellants or explosives.<ref name="Lewars">{{cite book |title=Modeling Marvels: Computational Anticipation of Novel molecules |last=Lewars |first=Errol G. |year=2008 |publisher=[[Springer Science+Business Media]] |isbn=978-1-4020-6972-7 |doi=10.1007/978-1-4020-6973-4 |pages=141–63 }}</ref>  Under extremely high pressures (1.1&nbsp;million&nbsp;[[Atmosphere (unit)|atm]]) and high temperatures (2000&nbsp;K), as produced in a [[diamond anvil cell]], nitrogen polymerises into the single-bonded [[cubic gauche]] crystal structure. This structure is similar to that of [[diamond]], and both have extremely strong [[covalent bond]]s, resulting in its nickname "nitrogen diamond".<ref>{{Cite news|url=http://www.physorg.com/news693.html|title=Polymeric nitrogen synthesized|publisher=physorg.com|date=5 August 2004|access-date=2009-06-22|archive-date=2012-01-24|archive-url=https://web.archive.org/web/20120124231419/http://www.physorg.com/news693.html|url-status=live}}</ref>

[[File:Mountainous Shoreline of Sputnik Planum (PIA20198).png|thumb|right|upright=1.1|[[Solid nitrogen]] on the plains of [[Sputnik Planitia]] (on the bottom-right side of the image) on [[Pluto]] next to water ice mountains (on the up-left side of the image)]]
At [[atmospheric pressure]], molecular nitrogen [[condensation|condenses]] ([[liquid|liquefies]]) at 77&nbsp;[[Kelvin|K]] (−195.79&nbsp;°[[Celsius|C]]) and [[freezing|freezes]] at 63&nbsp;K (−210.01&nbsp;°C)<ref name="Gray">{{cite book|last=Gray|first=Theodore|title=The Elements: A Visual Exploration of Every Known Atom in the Universe|date=2009|publisher=Black Dog & Leventhal Publishers|location=New York|isbn=978-1-57912-814-2|url-access=registration|url=https://archive.org/details/elementsvisualex0000gray}}</ref> into the beta [[hexagonal close-packed]] crystal [[Allotropy|allotropic]] form. Below 35.4&nbsp;K (−237.6&nbsp;°C) nitrogen assumes the [[Cubic crystal system|cubic]] crystal allotropic form (called the alpha phase).<ref name="schu">{{cite journal|last1=Schuch|first1=A. F.|last2=Mills|first2=R. L.|title=Crystal Structures of the Three Modifications of Nitrogen 14 and Nitrogen 15 at High Pressure|journal=The Journal of Chemical Physics|date=1970|volume=52|issue=12|pages=6000–08|doi=10.1063/1.1672899|bibcode=1970JChPh..52.6000S}}</ref> [[Liquid nitrogen]], a colourless fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808&nbsp;g/mL), is a common [[cryogen]].<ref>{{Cite journal | last1 = Iancu | first1 = C. V. | last2 = Wright | first2 = E. R. | last3 = Heymann | first3 = J. B. | last4 = Jensen | first4 = G. J. | title = A comparison of liquid nitrogen and liquid helium as cryogens for electron cryotomography | doi = 10.1016/j.jsb.2005.12.004 | journal = Journal of Structural Biology | volume = 153 | issue = 3 | pages = 231–40 | year = 2006 | pmid = 16427786}}</ref> [[Solid nitrogen]] has many crystalline modifications. It forms a significant dynamic surface coverage on Pluto<ref>{{cite news|title=Flowing nitrogen ice glaciers seen on surface of Pluto after New Horizons flyby|url=http://www.abc.net.au/news/2015-07-25/flowing-nitrogen-ice-glaciers-seen-on-surface-of-pluto/6647636|newspaper=ABC News|access-date=6 October 2015|date=25 July 2015|archive-date=29 September 2015|archive-url=https://web.archive.org/web/20150929044226/http://www.abc.net.au/news/2015-07-25/flowing-nitrogen-ice-glaciers-seen-on-surface-of-pluto/6647636|url-status=live}}</ref> and outer moons of the Solar System such as [[Triton (moon)|Triton]].<ref>{{cite encyclopedia|title = Encyclopedia of the Solar System|chapter = Triton|last1 = McKinnon|first1 = William B.|last2 = Kirk|first2 = Randolph L.|publisher = [[Elsevier]]|date = 2014|editor1-first = Tilman|editor1-last = Spohn|editor2-first = Doris|editor2-last = Breuer|editor3-first = Torrence|editor3-last = Johnson|edition = 3rd|location = Amsterdam; Boston|isbn = 978-0-12-416034-7|pages = 861–82|chapter-url = https://books.google.com/books?id=0bEMAwAAQBAJ&pg=PA861|access-date = 2016-04-30|archive-date = 2016-09-03|archive-url = https://web.archive.org/web/20160903233037/https://books.google.com/books?id=0bEMAwAAQBAJ&pg=PA861|url-status = live}}</ref> Even at the low temperatures of solid nitrogen it is fairly volatile and can [[sublimation (phase transition)|sublime]] to form an atmosphere, or condense back into nitrogen frost. It is very weak and flows in the form of glaciers, and on Triton [[geyser]]s of nitrogen gas come from the polar ice cap region.<ref>{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Triton |publisher=[[NASA]] |access-date=September 21, 2007 |title=Neptune: Moons: Triton |archive-url=https://web.archive.org/web/20111015074425/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Triton |archive-date=October 15, 2011 |url-status=dead }}</ref>