Editing Nitrogen (section)

==Properties==

===Atomic===
[[File:Neon orbitals.png|thumb|upright=1.6|right|The shapes of the five orbitals occupied in nitrogen. The two colours show the phase or sign of the wave function in each region. From left to right: 1s, 2s (cutaway to show internal structure), 2p<sub>''x''</sub>, 2p<sub>''y''</sub>, 2p<sub>''z''</sub>.]]
A nitrogen atom has seven electrons. In the ground state, they are arranged in the electron configuration 1s{{su|p=2}}2s{{su|p=2}}2p{{su|p=1|b=''x''}}2p{{su|p=1|b=''y''}}2p{{su|p=1|b=''z''}}. It, therefore, has five [[valence electron]]s in the 2s and 2p orbitals, three of which (the p-electrons) are unpaired. It has one of the highest [[electronegativity|electronegativities]] among the elements (3.04 on the Pauling scale), exceeded only by [[chlorine]] (3.16), [[oxygen]] (3.44), and [[fluorine]] (3.98). (The light [[noble gas]]es, [[helium]], [[neon]], and [[argon]], would presumably also be more electronegative, and in fact are on the Allen scale.)<ref name="Greenwood411" /> Following periodic trends, its single-bond [[covalent radius]] of 71&nbsp;pm is smaller than those of [[boron]] (84&nbsp;pm) and [[carbon]] (76&nbsp;pm), while it is larger than those of oxygen (66&nbsp;pm) and fluorine (57&nbsp;pm). The [[nitride]] anion, N<sup>3−</sup>, is much larger at 146&nbsp;pm, similar to that of the [[oxide]] (O<sup>2−</sup>: 140&nbsp;pm) and [[fluoride]] (F<sup>−</sup>: 133&nbsp;pm) anions.<ref name="Greenwood411" /> The first three ionisation energies of nitrogen are 1.402, 2.856, and 4.577&nbsp;MJ·mol<sup>−1</sup>, and the sum of the fourth and fifth is {{val|16.920|u=MJ·mol<sup>−1</sup>}}. Due to these very high figures, nitrogen has no simple cationic chemistry.<ref name="Greenwood550">Greenwood and Earnshaw, p. 550</ref>
The lack of radial nodes in the 2p subshell is directly responsible for many of the anomalous properties of the first row of the [[p-block]], especially in nitrogen, oxygen, and fluorine. The 2p subshell is very small and has a very similar radius to the 2s shell, facilitating [[orbital hybridisation]]. It also results in very large electrostatic forces of attraction between the nucleus and the valence electrons in the 2s and 2p shells, resulting in very high electronegativities. [[Hypervalent molecule|Hypervalency]] is almost unknown in the 2p elements for the same reason, because the high electronegativity makes it difficult for a small nitrogen atom to be a central atom in an electron-rich [[three-center four-electron bond]] since it would tend to attract the electrons strongly to itself. Thus, despite nitrogen's position at the head of group 15 in the periodic table, its chemistry shows huge differences from that of its heavier congeners [[phosphorus]], [[arsenic]], [[antimony]], and [[bismuth]].<ref name="Kaupp">{{cite journal |last=Kaupp |first=Martin |date=1 December 2006 |title=The role of radial nodes of atomic orbitals for chemical bonding and the periodic table |journal=Journal of Computational Chemistry |volume=28 |issue=1 |pages=320–25 |doi=10.1002/jcc.20522 |pmid=17143872 |s2cid=12677737 |doi-access=free }}</ref>

Nitrogen may be usefully compared to its horizontal neighbours' carbon and oxygen as well as its vertical neighbours in the pnictogen column, phosphorus, arsenic, antimony, and bismuth. Although each period 2 element from lithium to oxygen shows some similarities to the period 3 element in the next group (from magnesium to chlorine; these are known as [[diagonal relationship]]s), their degree drops off abruptly past the boron–silicon pair. The similarities of nitrogen to sulfur are mostly limited to sulfur nitride ring compounds when both elements are the only ones present.<ref name="Greenwood412" />

Nitrogen does not share the proclivity of carbon for [[catenation]]. Like carbon, nitrogen tends to form ionic or metallic compounds with metals. Nitrogen forms an extensive series of nitrides with carbon, including those with chain-, [[graphite|graphitic-]], and [[fullerene|fullerenic]]-like structures.<ref>{{cite journal |last1=Miller |first1=T. S. |last2=Belen |first2= A.|last3= Suter|first3= T. M.|last4= Sella|first4= A.|last5= Corà|first5= A.|last6= McMillan|first6= P. F.|date=2017 |title= Carbon nitrides: synthesis and characterization of a new class of functional materials |journal=Physical Chemistry Chemical Physics |volume= 19|issue= 24|pages=15613–15638 |doi=10.1039/C7CP02711G|pmid=28594419 |bibcode=2017PCCP...1915613M |doi-access= free}}</ref>

It resembles oxygen with its high electronegativity and concomitant capability for [[hydrogen bond]]ing and the ability to form [[coordination complex]]es by donating its [[lone pair]]s of electrons. There are some parallels between the chemistry of ammonia NH<sub>3</sub> and water H<sub>2</sub>O. For example, the capacity of both compounds to be protonated to give NH<sub>4</sub><sup>+</sup> and H<sub>3</sub>O<sup>+</sup> or deprotonated to give NH<sub>2</sub><sup>−</sup> and OH<sup>−</sup>, with all of these able to be isolated in solid compounds.<ref>{{cite book |last1= House|first1=J. E. |last2=House |first2= K. A.|date=2016 |title=Descriptive Inorganic Chemistry |location= Amsterdam|publisher= Elsevier|page= 198|isbn=978-0-12-804697-5 }}</ref>

Nitrogen shares with both its horizontal neighbours a preference for forming multiple bonds, typically with carbon, oxygen, or other nitrogen atoms, through p<sub>''π''</sub>–p<sub>''π''</sub> interactions.<ref name="Greenwood412" /> Thus, for example, nitrogen occurs as diatomic molecules and therefore has very much lower [[melting point|melting]] (−210&nbsp;°C) and [[boiling point]]s (−196&nbsp;°C) than the rest of its group, as the N<sub>2</sub> molecules are only held together by weak [[van der Waals interaction]]s and there are very few electrons available to create significant instantaneous dipoles. This is not possible for its vertical neighbours; thus, the [[nitrogen oxide]]s, [[nitrite]]s, [[nitrate]]s, [[nitro compound|nitro-]], [[nitroso]]-, [[azo compound|azo]]-, and [[diazo]]-compounds, [[azide]]s, [[cyanate]]s, [[thiocyanate]]s, and [[imino]]-derivatives find no echo with phosphorus, arsenic, antimony, or bismuth. By the same token, however, the complexity of the phosphorus oxoacids finds no echo with nitrogen.<ref name="Greenwood412" /> Setting aside their differences, nitrogen and phosphorus form an extensive series of compounds with one another; these have chain, ring, and cage structures.<ref>{{cite book |last1=Roy  |first1= A. K.|last2=Burns  |first2= G. T.|last3=Grigora  |first3= S.|last4=Lie  |first4= G. C.|editor-last1= Wisian-Neilson|editor-first1= P.|editor-last2= Alcock|editor-first2= H. R.|editor-last3= Wynne|editor-first3= K. J.|title=Inorganic and organometallic polymers II: advanced materials and intermediates |publisher=American Chemical Society |date=1994 |pages=344–357 |chapter=Poly(alkyl/aryloxothiazenes), [N=S(O)R]''<sub>n</sub>'' : New direction in inorganic polymers |doi=10.1021/bk-1994-0572.ch026}}</ref>

Table of thermal and physical properties of nitrogen (N<sub>2</sub>) at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{Cite book |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=Frank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer. |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755}}</ref>
{|class="wikitable mw-collapsible mw-collapsed", style="text-align: right"
|style="text-align: center"|Temperature (K)
|style="text-align: center"|Density (kg&nbsp;m<sup>−3</sup>)
|style="text-align: center"|Specific heat (kJ&nbsp;kg<sup>−1</sup>&nbsp;°C<sup>−1</sup>)
|style="text-align: center"|Dynamic viscosity (kg&nbsp;m<sup>−1</sup>&nbsp;s<sup>−1</sup>)
|style="text-align: center"|Kinematic viscosity (m<sup>2</sup>&nbsp;s<sup>−1</sup>)
|style="text-align: center"|Thermal conductivity (W&nbsp;m<sup>−1</sup>&nbsp;°C<sup>−1</sup>)
|style="text-align: center"|Thermal diffusivity (m<sup>2</sup>&nbsp;s<sup>−1</sup>)
|style="text-align: center"|[[Prandtl number]]
|-
|100
|3.4388
|1.07{{figure space}}{{figure space}}
|{{val|6.88e-6}}
|{{val|2.00e-6}}
|{{gaps|0.009|58}}
|{{val|2.60e-6}}
|0.768
|-
|150
|2.2594
|1.05{{figure space}}{{figure space}}
|{{val|1.01e-5}}
|{{val|4.45e-6}}
|{{gaps|0.013|9{{figure space}}}}
|{{val|5.86e-6}}
|0.759
|-
|200
|1.7108
|1.0429
|{{val|1.29e-5}}
|{{val|7.57e-6}}
|{{gaps|0.018|24}}
|{{val|1.02e-5}}
|0.747
|-
|300
|1.1421
|1.0408
|{{val|1.78e-5}}
|{{val|1.56e-5}}
|{{gaps|0.026|2{{figure space}}}}
|{{val|2.20e-5}}
|0.713
|-
|400
|0.8538
|1.0459
|{{val|2.20e-5}}
|{{val|2.57e-5}}
|{{gaps|0.033|35}}
|{{val|3.73e-5}}
|0.691
|-
|500
|0.6824
|1.0555
|{{val|2.57e-5}}
|{{val|3.77e-5}}
|{{gaps|0.039|84}}
|{{val|5.53e-5}}
|0.684
|-
|600
|0.5687
|1.0756
|{{val|2.91e-5}}
|{{val|5.12e-5}}
|{{gaps|0.045|8{{figure space}}}}
|{{val|7.49e-5}}
|0.686
|-
|700
|0.4934
|1.0969
|{{val|3.21e-5}}
|{{val|6.67e-5}}
|{{gaps|0.051|23}}
|{{val|9.47e-5}}
|0.691
|-
|800
|0.4277
|1.1225
|{{val|3.48e-5}}
|{{val|8.15e-5}}
|{{gaps|0.056|09}}
|{{val|1.17e-4}}
|0.7{{figure space}}{{figure space}}
|-
|900
|0.3796
|1.1464
|{{val|3.75e-5}}
|{{val|9.11e-5}}
|{{gaps|0.060|7{{figure space}}}}
|{{val|1.39e-4}}
|0.711
|-
|1000
|0.3412
|1.1677
|{{val|4.00e-5}}
|{{val|1.19e-4}}
|{{gaps|0.064|75}}
|{{val|1.63e-4}}
|0.724
|-
|1100
|0.3108
|1.1857
|{{val|4.23e-5}}
|{{val|1.36e-4}}
|{{gaps|0.068|5{{figure space}}}}
|{{val|1.86e-4}}
|0.736
|-
|1200
|0.2851
|1.2037
|{{val|4.45e-5}}
|{{val|1.56e-4}}
|{{gaps|0.071|84}}
|{{val|2.09e-4}}
|0.748
|-
|{{val|1300}}
|0.2591
|1.219{{figure space}}
|{{val|4.66e-5}}
|{{val|1.80e-4}}
|{{gaps|0.081|{{figure space}}{{figure space}}}}
|{{val|2.56e-4}}
|0.701
|}

===Isotopes===
{{main|Isotopes of nitrogen}}
[[File:NuclideMap C-F.png|thumb|right|upright=2.3|Table of nuclides (Segrè chart) from carbon to fluorine (including nitrogen). Orange indicates [[proton emission]] (nuclides outside the proton drip line); pink for [[positron emission]] (inverse beta decay); black for [[stable nuclide|stable]] nuclides; blue for [[electron emission]] (beta decay); and violet for [[neutron emission]] (nuclides outside the neutron drip line). Proton number increases going up the vertical axis and neutron number going to the right on the horizontal axis.]]
Nitrogen has two stable [[isotope]]s: <sup>14</sup>N and <sup>15</sup>N. The first is much more common, making up 99.634% of natural nitrogen, and the second (which is slightly heavier) makes up the remaining 0.366%. This leads to an atomic weight of around 14.007&nbsp;u.<ref name="Greenwood411">Greenwood and Earnshaw, pp. 411–12</ref> Both of these stable isotopes are produced in the [[CNO cycle]] in [[star]]s, but <sup>14</sup>N is more common as its proton capture is the rate-limiting step. <sup>14</sup>N is one of the five stable [[even and odd atomic nuclei|odd–odd nuclides]] (a nuclide having an odd number of protons and neutrons); the other four are [[deuterium|<sup>2</sup>H]], <sup>6</sup>Li, <sup>10</sup>B, and <sup>180m</sup>Ta.<ref name="BetheBible">{{cite journal|last=Bethe |first=H. A.|date=1939|title=Energy Production in Stars|journal=[[Physical Review]]|volume=55 |issue=5 |pages=434–56|bibcode=1939PhRv...55..434B|doi= 10.1103/PhysRev.55.434|pmid=17835673|doi-access=free}}</ref>

The relative abundance of <sup>14</sup>N and <sup>15</sup>N is practically constant in the atmosphere but can vary elsewhere, due to natural isotopic fractionation from biological [[redox]] reactions and the evaporation of natural [[ammonia]] or [[nitric acid]].<ref name=CIAAWnitrogen/> Biologically mediated reactions (e.g., [[Assimilation (biology)|assimilation]], [[nitrification]], and [[denitrification]]) strongly control nitrogen dynamics in the soil. These reactions typically result in <sup>15</sup>N enrichment of the [[Substrate (chemistry)|substrate]] and depletion of the [[Product (chemistry)|product]].<ref name="enrich">{{cite book | url = https://books.google.com/books?id=U9y3whFC2DIC&pg=PA74 | pages = 74–75 | title = Stable Isotopes and Biosphere – Atmosphere Interactions: Processes and Biological Controls | isbn = 978-0-08-052528-0 | last1 = Flanagan | first1 = Lawrence B. | last2 = Ehleringer | first2 = James R. | last3 = Pataki | first3 = Diane E. | date = 15 December 2004 | publisher = Elsevier | access-date = 20 December 2015 | archive-date = 5 February 2016 | archive-url = https://web.archive.org/web/20160205191759/https://books.google.com/books?id=U9y3whFC2DIC&pg=PA74 | url-status = live }}</ref>

The heavy isotope <sup>15</sup>N was first discovered by S. M. Naudé in 1929, and soon after heavy isotopes of the neighbouring elements [[oxygen]] and [[carbon]] were discovered.<ref name="Greenwood408">Greenwood and Earnshaw, p. 408</ref> It presents one of the lowest thermal neutron capture cross-sections of all isotopes.<ref>{{cite web |url=http://www.nndc.bnl.gov/sigma/index.jsp?as=15&lib=endfb7.1&nsub=10 |title=Evaluated Nuclear Data File (ENDF) Retrieval & Plotting |publisher=National Nuclear Data Center |access-date=2016-11-23 |archive-date=2020-08-09 |archive-url=https://web.archive.org/web/20200809124433/https://www.nndc.bnl.gov/sigma/index.jsp?as=15&lib=endfb7.1&nsub=10 |url-status=live }}</ref> It is frequently used in [[nuclear magnetic resonance]] (NMR) spectroscopy to determine the structures of nitrogen-containing molecules, due to its fractional [[nuclear spin]] of one-half, which offers advantages for NMR such as narrower line width. <sup>14</sup>N, though also theoretically usable, has an integer nuclear spin of one and thus has a [[quadrupole moment]] that leads to wider and less useful spectra.<ref name="Greenwood411" /> <sup>15</sup>N NMR nevertheless has complications not encountered in the more common <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy. The low natural abundance of <sup>15</sup>N (0.36%) significantly reduces sensitivity, a problem which is only exacerbated by its low [[gyromagnetic ratio]], (only 10.14% that of <sup>1</sup>H). As a result, the signal-to-noise ratio for <sup>1</sup>H is about 300 times as much as that for <sup>15</sup>N at the same magnetic field strength.<ref name="autogenerated2007">{{cite book| author=Arthur G Palmer| title=Protein NMR Spectroscopy| publisher=Elsevier Academic Press| date=2007 | isbn = 978-0-12-164491-8}}</ref> This may be somewhat alleviated by isotopic enrichment of <sup>15</sup>N by chemical exchange or fractional distillation. <sup>15</sup>N-enriched compounds have the advantage that under standard conditions, they do not undergo chemical exchange of their nitrogen atoms with atmospheric nitrogen, unlike compounds with labelled [[hydrogen]], carbon, and oxygen isotopes that must be kept away from the atmosphere.<ref name="Greenwood411" /> The <sup>15</sup>N:<sup>14</sup>N ratio is commonly used in stable isotope analysis in the fields of [[geochemistry]], [[hydrology]], [[paleoclimatology]] and [[paleoceanography]], where it is called [[δ15N|''δ''<sup>15</sup>N]].<ref>{{cite book | last=Katzenberg | first=M. A. | title=Biological Anthropology of the Human Skeleton | chapter=Chapter 13: Stable Isotope Analysis: A Tool for Studying Past Diet, Demography, and Life History | year=2008 | publisher=Wiley | edition=2nd | isbn=978-0-471-79372-4 }}</ref>

Of the thirteen other isotopes produced synthetically, ranging from <sup>9</sup>N to <sup>23</sup>N, [[nitrogen-13|<sup>13</sup>N]] has a [[half-life]] of ten minutes and the remaining isotopes have half-lives less than eight seconds.<ref name=N9sci>{{cite news |url=https://www.science.org/content/article/fleeting-form-nitrogen-stretches-nuclear-theory-its-limits |title=Fleeting form of nitrogen stretches nuclear theory to its limits |last=Cho |first=Adrian |website=[[science.org]] |date=25 September 2023 |access-date=27 September 2023}}</ref><ref name="NUBASE">{{NUBASE 2003}}</ref> Given the half-life difference, <sup>13</sup>N is the most important nitrogen radioisotope, being relatively long-lived enough to use in [[positron emission tomography]] (PET), although its half-life is still short and thus it must be produced at the venue of the PET, for example in a [[cyclotron]] via proton bombardment of <sup>16</sup>O producing <sup>13</sup>N and an [[alpha particle]].<ref name="Carlson 151">{{cite book | last = Carlson | first = Neil | title = Physiology of Behavior | publisher = Pearson | series = Methods and Strategies of Research | volume = 11th edition | date = January 22, 2012 | page = 151 | isbn = 978-0-205-23939-9}}</ref>

The [[radioisotope]] <sup>16</sup>N is the dominant [[radionuclide]] in the coolant of [[pressurised water reactor]]s or [[boiling water reactor]]s during normal operation. It is produced from <sup>16</sup>O (in water) via an [[Np reaction|(n,p) reaction]], in which the <sup>16</sup>O atom captures a neutron and expels a proton. It has a short half-life of about 7.1&nbsp;s,<ref name="NUBASE" /> but its decay back to <sup>16</sup>O produces high-energy [[gamma radiation]] (5 to 7&nbsp;MeV).<ref name="NUBASE" /><ref name="Neeb">{{Cite book|last=Neeb|first=Karl Heinz|title=The Radiochemistry of Nuclear Power Plants with Light Water Reactors|publisher=Walter de Gruyter|location=Berlin-New York|date=1997|isbn=978-3-11-013242-7|url=https://books.google.com/books?id=SJOE00whg44C&pg=PA227|page=227|access-date=2015-12-20|archive-date=2016-02-05|archive-url=https://web.archive.org/web/20160205191759/https://books.google.com/books?id=SJOE00whg44C&pg=PA227|url-status=live}}</ref> Because of this, access to the primary coolant piping in a pressurised water reactor must be restricted during [[Nuclear reactor|reactor]] power operation.<ref name="Neeb" /> It is a sensitive and immediate indicator of leaks from the primary coolant system to the secondary steam cycle and is the primary means of detection for such leaks.<ref name="Neeb"/>

===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>