Noble gas 21140 226161751 2008-07-17T02:41:45Z Nihiltres 236191 Closing some tags for good form {| style="float: right; border: 1px solid #ccc; margin: 0.5em 0pt 0.8em 1.4em; padding: 3px !important; width: 75px;" ! [[Group (periodic table)|Group]]&nbsp;→!! 18 |- ! ↓&nbsp;[[Period (periodic table)|Period]] |- ! [[Period 1 element|1]] | {{element cell| 2|Helium|He| |Gas|Noble gases|Primordial}} |- ! [[Period 2 element|2]] | {{element cell|10|Neon|Ne| |Gas|Noble gases|Primordial}} |- ! [[Period 3 element|3]] | {{element cell|18|Argon|Ar| |Gas|Noble gases|Primordial}} |- ! [[Period 4 element|4]] | {{element cell|36|Krypton|Kr| |Gas|Noble gases|Primordial}} |- ! [[Period 5 element|5]] | {{element cell|54|Xenon|Xe| |Gas|Noble gases|Primordial}} |- ! [[Period 6 element|6]] | {{element cell|86|Radon|Rn| |Gas|Noble gases|Natural radio}} |- ! [[Period 7 element|7]] | {{element cell|118|Ununoctium|Uuo| |UnknownPhase|Unknown chemical properties|Synthetic|Ununoctium|#ffffff}} |- | |- | colspan="2"| ---- <center>Legend</center> {| | colspan="2" style="text-align: center; border: 1px solid #AAAAAA; background:{{Element color/Noble gases}};" | <small>''Noble gas''</small> |- | colspan="2" style="text-align: center; border: 1px solid #AAAAAA; color: {{element color/Gas}};" | <small>''Gas''</small> |- | colspan="2" style="text-align: center; border: {{element frame/Primordial}};" | <small>''[[Primordial element]]''</small> |- | colspan="2" style="text-align: center; border: {{element frame/Natural radio}};" | <small>''[[Trace radioisotope|From decay]]''</small> |- | colspan="2" style="text-align: center; border: {{element frame/Synthetic}};" | <small>''[[Synthetic elements|Synthetic]]''</small> |} |} The '''noble gases''' are the [[chemical element|elements]] in group&nbsp;18 (previously known as group&nbsp;0) of the [[periodic table]]. These elements are characterized as [[nonmetal]]lic and chemically [[inert]], and are all [[gas]]eous at [[standard conditions]]. The six noble gases that occur naturally are [[helium|helium (He)]], [[neon|neon (Ne)]], [[argon|argon (Ar)]], [[krypton|krypton (Kr)]], [[xenon|xenon (Xe)]], and the radioactive [[radon|radon (Rn)]]. So far, three atoms of the next member of the group, [[ununoctium|ununoctium (Uuo)]] have been [[synthetic element|synthesized]] in a [[supercollider]], but very little is known of its properties due to tiny amount produced and its short [[half-life]]. Chemically, the noble gases are very stable because they have the maximum number of [[valence electron]]s that their [[Electron shell|outer shell]] can hold, and as a result they rarely react with other elements. Under standard conditions, they are odorless, colorless, [[monatomic]] gases. The [[melting point|melting]] and [[boiling point]]s for each noble gas are close together, differing by less than {{convert|10|C}}; consequently, they are liquids only over a small temperature range. The noble gases show extremely low [[chemical reactivity]], and therefore only a few hundred noble gas [[Chemical compound|compounds]] have been formed as of 2008. Neon, argon, krypton, and xenon are obtained from air using the methods of [[liquefaction of gases]] and [[fractional distillation]]. Helium is typically separated from [[natural gas]], and radon is usually isolated from the [[radioactive decay]] of dissolved [[radium]] compounds. Noble gases have several important applications in industries such as lighting, welding, and space exploration. Helium is often used in scuba diving to replace part of the breathing mixture. After the risks caused by the flammability of [[hydrogen]] became apparent, it was replaced with helium in blimps and balloons. ==History== ''Noble gas'' is translated from the German noun {{lang|de|''Edelgas''}}, first used in 1898 by [[Hugo Erdmann]]<ref>{{cite journal|journal=[[Science (journal)|Science]]|date=[[1901-02-15]]|volume=13|pages=268–270|last=Renouf|first=Edward|title=Noble gases|doi=10.1126/science.13.320.268}}</ref> to refer to the extremely low level of reactivity that most of the elements in group&nbsp;18 of the [[periodic table]] exhibit under [[standard conditions]]. The noble gases have also been referred to as ''[[inert gas]]es'', but this is an inaccurate label because several of them participate in chemical reactions.<ref>{{harvnb|Ozima|2002|p=30}}</ref> ''Rare gases'' is another term that was used,<ref>{{harvnb|Ozima|2002|p=4}}</ref> but this is also inaccurate because [[argon]] forms a fairly considerable part (0.94% by volume, 1.3% by mass) of the [[Earth's atmosphere]].<ref>{{cite encyclopedia|encyclopedia=[[Encyclopædia Britannica]]|date=2008|title=argon|url=http://www.britannica.com/eb/article-9009382/argon}}</ref> [[Image:Helium spectrum.jpg|thumb|left|300px|Helium was first detected in the Sun due to its characteristic [[spectral line]]s.]] [[Pierre Janssen]] and [[Joseph Norman Lockyer]] were the first to discover a noble gas on [[August 18]], [[1868]] while looking at the [[chromosphere]] of the [[Sun]], and named it [[helium]] after the Greek name for the Sun, [[Helios]] ({{Polytonic|''ἥλιος''}}).<ref>''Oxford English Dictionary'' (1989), s.v. "helium". Retrieved [[December 16]], [[2006]], from Oxford English Dictionary Online. Also, from quotation there: Thomson, W. (1872). ''Rep. Brit. Assoc.'' xcix: "Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium."</ref> Before them, in 1784, the English chemist and physicist [[Henry Cavendish]] had discovered that air contains a small proportion of a substance less reactive than [[nitrogen]].<ref name="ozima 1">{{harvnb|Ozima|2002|p=1}}</ref> A century later, in 1895, [[John Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] discovered that samples of nitrogen from the air were of a different [[density]] than nitrogen resulting from [[chemical reaction]]s. Along with scientist [[William Ramsay]], Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that successfully isolated a new element, argon, named after the Greek word for inactive ({{Polytonic|''αργό(ν)''}}).<ref name="ozima 1">{{harvnb|Ozima|2002|p=1}}</ref> With this discovery, they realized an entire class of [[gas]]es was missing from the periodic table. During his search for argon, Ramsay also managed to isolate helium for the first time while heating [[cleveite]], a mineral. In 1902, having accepted the evidence for the elements helium and argon, [[Dmitri Mendeleev]] included these noble gases as group&nbsp;0 in his arrangement of the elements, which would later become the periodic table.<ref>{{harvnb|Mendeleev|1903|p=497}}</ref> Ramsay continued to search for these gases using the method of [[fractional distillation]] to separate [[liquid air]] into several components. In 1898, he discovered the elements [[krypton]], [[neon]], and [[xenon]], and named them after the Greek words {{Polytonic|''κρυπτός''}} (''kryptos'', hidden), {{Polytonic|''νέος''}} (''neos'', new), and {{Polytonic|''ξένος''}} (''xenos'', stranger). [[Radon]] was first identified in 1898 by [[Friedrich Ernst Dorn]],<ref>{{cite journal|title=Discovery of Radon|last=Partington|first=J. R.|journal=[[Nature (journal)|Nature]]|volume=179|issue=4566|pages=912|date=May 1957|doi=10.1038/179912a0}}</ref> and was named ''[[radium]] emanation'', but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases.<ref name="brit">{{cite encyclopedia|encyclopedia=[[Encyclopædia Britannica]]|date=2008|title=Noble Gas|url=http://www.britannica.com/eb/article-9110613/noble-gas}}</ref> Rayleigh and Ramsay received the 1904 [[Nobel Prize]]s in Physics and in Chemistry, respectively, for their discovery of the noble gases;<ref>{{cite web |title=The Nobel Prize in Physics 1904 Presentation Speech |author=Cederblom, J. E. |year=1904 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1904/press.html}}</ref><ref name=nobelchem>{{cite web |title=The Nobel Prize in Chemistry 1904 Presentation Speech |author=Cederblom, J. E. |year=1904 |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1904/press.html}}</ref> in the words of J. E. Cederblom, then president of the [[Royal Swedish Academy of Sciences]], "the discovery of an entirely new group of elements, of which no single representative had been known with any certainty, is something utterly unique in the history of chemistry, being intrinsically an advance in science of peculiar significance".<ref name=nobelchem/> The discovery of the noble gases aided in the development of a general understanding of [[Atomic theory|atomic structure]]. In 1895, French chemist [[Henri Moissan]] attempted to form a reaction between [[fluorine]], the most [[electronegativity|electronegative]] element, and argon, one of the noble gases, but failed. Scientists were unable to prepare compounds of argon until the end of the 20th century, but these attempts helped to develop new theories of atomic structure. Learning from these experiments, Danish physicist [[Niels Bohr]] proposed in 1913 that the [[electron]]s in atoms are arranged in [[electron shell|shells]] surrounding the [[atomic nucleus|nucleus]], and that for all noble gases except helium the outermost shell always contains eight electrons.<ref name="brit" /> In 1916, [[Gilbert N. Lewis]] formulated the ''[[octet rule]]'', which concluded an octet of electrons in the outer shell was the most stable arrangement for any atom; this arrangement caused them to be unreactive with other elements since they did not require any more electrons to complete their outer shell.<ref>{{cite journal |author=Gillespie, R. J.; Robinson, E. A. |title=Gilbert N. Lewis and the chemical bond: the electron pair and the octet rule from 1916 to the present day |journal=J Comput Chem |volume=28 |issue=1 |pages=87–97 |year=2007 |month=January |pmid=17109437 |doi=10.1002/jcc.20545 |url=}}</ref> It was not until 1962 that [[Neil Bartlett]] discovered the first chemical compound of a noble gas, [[xenon hexafluoroplatinate]].<ref name="bartlett">{{cite journal|title=Xenon hexafluoroplatinate {{chem|Xe<sup>+</sup>[PtF|6|]<sup>–</sup>}}|last=Bartlett|first=N.|journal=[[Proceedings of the Chemical Society]]|issue=6|pages=218|year=1962|doi=10.1039/PS9620000197}}</ref> Compounds of other noble gases were discovered soon after: in 1962 for radon, [[radon fluoride]],<ref>{{cite journal|author=Fields, Paul R.; Stein, Lawrence; Zirin, Moshe H.|title=Radon Fluoride|journal=[[Journal of the American Chemical Society]]|year=1962|volume=84|issue=21|pages=4164–4165|doi=10.1021/ja00880a048}}</ref> and in 1963 for krypton, [[krypton difluoride]] ({{chem|Kr||F|2}}).<ref>{{cite journal|author=Grosse, A. V.; Kirschenbaum, A. D.; Streng, A. G.; Streng, L. V.|title=Krypton Tetrafluoride: Preparation and Some Properties|journal=Science|date=1963|volume=139|pages=1047–1048|doi=10.1126/science.139.3559.1047|pmid=17812982}}</ref> The first stable compound of argon was reported in 2000 when [[argon fluorohydride]] (HArF) was formed at a temperature of {{convert|40|k}}.<ref>{{cite journal|title=A stable argon compound|journal=[[Nature (journal)|Nature]]|issue=406|pages=874–876|date=[[2000-08-24]]|doi=10.1038/35022551|author= Khriachtchev, Leonid; Pettersson, Mika; Runeberg, Nino; Lundell, Jan; Räsänen, Markku|volume=406}}</ref> In December 1998, scientists at the [[Joint Institute for Nuclear Research]] working in [[Dubna]], [[Russia]] bombarded [[plutonium|plutonium (Pu)]] with [[calcium|calcium (Ca)]] to produce a single atom of element&nbsp;114,<ref>{{cite journal|doi=10.1103/PhysRevLett.83.3154|title=Synthesis of Superheavy Nuclei in the <sup>48</sup>Ca + <sup>244</sup>Pu Reaction |publisher=[[American Physical Society]]|year=1999|author=Oganessian, Yu. Ts.|journal=Physical Review Letters|volume=83|pages=3154}}</ref> which they temporarily named [[ununquadium|ununquadium (Uuq)]].<ref>{{cite web|accessdate=2008-06-26|url=http://www.post-gazette.com/healthscience/20030506element0506p4.asp|title=Chemical element No. 110 finally gets a name—darmstadtium |work=[[Pittsburgh Post-Gazette]]|date=2003-05-06|last=Woods|first=Michael}}</ref> Preliminary chemistry experiments have indicated this element may be the first [[superheavy element]] to show abnormal noble-gas-like properties, even though it is a member of [[group 14]] on the periodic table.<ref>{{cite web|accessdate=2008-05-31|url=http://lch.web.psi.ch/pdf/TexasA&M/TexasA&M.pdf|format=PDF|title=Gas Phase Chemistry of Superheavy Elements|publisher=[[Texas A&M University]]}}</ref> In October 2006, scientists from the Joint Institute for Nuclear Research and [[Lawrence Livermore National Laboratory]] successfully created synthetically [[ununoctium|ununoctium (Uuo)]], the seventh element in group&nbsp;18,<ref name=meaning>{{cite journal|journal=Electronic Journal of Literacy through Science|volume=4|issue=2|date=2005|title=Making Meaning in Chemistry Lessons|last=Wilson|first=Elaine}}</ref> by bombarding [[californium|californium (Cf)]] with calcium (Ca).<ref name="full">{{cite journal|last=Oganessian|first=Yu. Ts.|title=Synthesis of the isotopes of elements 118 and 116 in the {{SimpleNuclide|Californium|249}} and {{SimpleNuclide|Curium|245}} + {{SimpleNuclide|Calcium|48}} fusion reactions|journal=[[Physical Review]] C|volume=74|issue=4|pages=44602|date=[[2006-10-09]]|doi=10.1103/PhysRevC.74.044602}}</ref> ==Physical and atomic properties== <div style="float: right; padding-left: 10px;"> {| class="wikitable" style="text-align: center;" ! Property<ref name=brit /><ref name=greenwood891/>|| [[Helium]] || [[Neon]] || [[Argon]] || [[Krypton]] || [[Xenon]] || [[Radon]] |- |align="left" | [[Density]] (g/[[litre|dm³]]) || 0.1786 || 0.9002 || 1.7818 || 3.708 || 5.851 || 9.97 |- |align="left" | [[Boiling point]] (K) || 4.4 || 27.3 || 87.4 || 121.5 || 166.6 || 211.5 |- |align="left" | [[Melting point]] (K) || 0.95<ref>Under pressure of 25&nbsp;[[bar]]</ref> || 24.7 || 83.6 || 115.8 || 161.7 || 202.2 |- |align="left" | [[Enthalpy of vaporization]] (kJ/mol) || 0.08 || 1.74 || 6.52 || 9.05 || 12.65 || 18.1 |- |align="left" | [[Solubility]] in water at 20 °C (cm<sup>3</sup>/kg) || 8.61 || 10.5 || 33.6 || 59.4 || 108.1 || 230 |- |align="left"| [[Atomic number]] || 2 || 10 || 18 || 36 || 54 || 86 |- |align="left" | [[Atomic radius]] ([[picometer|pm]]) || 130 || 160 || 192 || 198 || 218 || &nbsp;– |- |align="left" | [[Ionization energy]] (kJ/mol) || 2372 || 2080 || 1520 || 1351 || 1170 || 1037 |} <span class="dablink" style="font-size:smaller; padding-left:0em;">For more data, see [[Noble gas (data page)]].</span></div> The noble gases have very weak [[interatomic force]], and consequently have very low [[melting point|melting]] and [[boiling point]]s. They are all [[monatomic]] [[gas]]es under [[Standard conditions for temperature and pressure|standard conditions]], including the elements with larger [[atomic mass]]es than many normally solid elements.<ref name="brit"/> Helium has several unique qualities when compared with other elements: its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit [[superfluidity]]; it is the only element that cannot be solidified by cooling under standard conditions—a [[Atmospheric pressure|pressure]] of {{convert|25|atm|lk=on}} must be applied at a temperature of {{convert|0.95|K}} to convert it to a solid.<ref>{{cite web |publisher=University of Alberta |title=Solid Helium |url=http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm |accessdate=2008-06-22}}</ref> The noble gases up to xenon have multiple stable [[isotope]]s. Radon has no [[stable isotope]]s; its longest-lived isotope, <sup>222</sup>Rn, has a [[half-life]] of 3.8&nbsp;days and decays to form helium and [[polonium]], which ultimately decays to [[lead]].<ref name="brit" /> <!-- This image is placed here because: straddling with the other images; it is next to the first paragraph that discusses ionization potential; it does not get bumped down by the Physical Properties table. I've tried to put the image lower because the text gets squeezed too much on a resolution of 1200x800-->[[Image:Ionization energies.png|left|thumb|300px|This is a plot of [[ionization potential]] versus atomic number. The noble gases, which are labeled, have the largest ionization potential for each period.]] The noble gas atoms, like atoms in most groups, increase steadily in [[atomic radius]] from one [[period (periodic table)|period]] to the next due to the increasing number of electrons. The size of the atom is related to several properties. For example, the [[ionization potential]] decreases with an increasing radius because the valence electrons in the larger noble gases are farther away from the [[atomic nucleus|nucleus]] and are therefore not held as tightly together by the atom. Noble gases have the largest ionization potential among the elements of each period, which reflects the stability of their electron configuration and is related to their relative lack of chemical reactivity.<ref name=greenwood891/> Some of the heavier noble gases, however, have ionization potentials small enough to be comparable to those of other elements and [[molecule]]s. It was the insight that xenon has an ionization potential similar to that of the [[dioxygen|oxygen molecule]] that led Bartlett to attempt oxidizing xenon using [[platinum hexafluoride]], an [[oxidizing agent]] known to be strong enough to react with oxygen.<ref name=bartlett/> Noble gases cannot accept an electron to form stable [[anion]]s; that is, they have a negative [[electron affinity]].<ref>{{cite journal |journal=Journal of Chemical Education |author=Wheeler, John C. |year=1997 |volume=74 |pages=123–127 |title=Electron Affinities of the Alkaline Earth Metals and the Sign Convention for Electron Affinity}}; {{cite journal |journal=Chemical Reviews |year=1994 |volume=94 |pages=2291–2318 |author= Kalcher, Josef; Sax, Alexander F.|title=Gas Phase Stabilities of Small Anions: Theory and Experiment in Cooperation |doi=10.1021/cr00032a004}}</ref> The [[macroscopic]] [[physical properties]] of the noble gases are dominated by the weak [[van der Waals forces]] between the atoms. The attractive force increases with the size of the atom as a result of the increase in [[polarizability]] and the decrease in ionization potential. This results in systematic group trends: as one goes down group&nbsp;18, the atomic radius, and with it the interatomic forces, increases, resulting in an increasing melting point, boiling point, [[enthalpy of vaporization]], and [[solubility]]. The increase in density is due to the increase in [[atomic mass]].<ref name=greenwood891>{{harvnb|Greenwood|1997|p=891}}</ref> The noble gases are nearly [[ideal gas]]es under standard conditions, but their deviations from the [[ideal gas law]] provided important clues for the study of [[intermolecular interactions]]. The [[Lennard-Jones potential]], often used to model intermolecular interactions, was deduced in 1924 by [[John Lennard-Jones]] from experimental data on argon before the development of [[quantum mechanics]] provided the tools for understanding intermolecular forces from [[first principles]].<ref>{{cite journal|title=John Edward Lennard-Jones. 1894-1954 |last=Mott|first=N. F.|journal=Biographical Memoirs of Fellows of the Royal Society|pages=175–184|volume=1|year=1955|doi=10.1098/rsbm.1955.0013}}</ref> The theoretical analysis of these interactions became tractable because the noble gases are monatomic and the atoms spherical, which means that the interaction between the atoms is independent of direction, or [[isotropic]]. ==Chemical properties== [[Image:Electron shell 010 Neon.svg|thumb|Neon, like all noble gases, has a full [[valence shell]]. Noble gases have eight electrons in the outermost shell, except in the case of helium, which has two.]] The noble gases make up group&nbsp;18 of the [[periodic table]]. The confirmed members are [[helium|helium (He)]], [[neon|neon (Ne)]], [[argon|argon (Ar)]], [[krypton|krypton (Kr)]], [[xenon|xenon (Xe)]], and [[radon|radon (Rn)]].<ref>{{harvnb|Ozima|2002|p=2}}</ref> These elements are colorless, odorless, tasteless, and nonflammable under standard conditions. They were once labeled ''group&nbsp;0'' in the periodic table because it was believed they had a [[valence (chemistry)|valence]] of zero, meaning their [[atom]]s cannot combine with those of other elements to form [[chemical compound|compounds]]. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse.<ref name="brit" /> Very little is known about the properties of the most recent member of group&nbsp;18, [[ununoctium|ununoctium (Uuo)]].<ref name="heaviest yet">{{cite web|url=http://www.washingtonpost.com/wp-dyn/content/article/2006/10/16/AR2006101601083.html|title=Scientists Announce Creation of Atomic Element, the Heaviest Yet|accessdate=2008-06-26|date=2006-10-17|work=[[Washington Post]]}}</ref> The noble gases have full valence [[electron shells]]. [[Valence electron]]s are the outermost [[electron]]s of an atom and are normally the only electrons that participate in [[chemical bond]]ing. Atoms with full valence electron shells are extremely stable and therefore do not tend to form chemical bonds and have little tendency to gain or lose electrons.<ref>{{harvnb|Ozima|2002|p=35}}</ref> However, heavier noble gases such as radon are held less firmly together by [[electromagnetic force]] than lighter noble gases such as helium, making it easier to remove outer electrons from heavy noble gases. As a result of a full shell, the noble gases can be used in conjunction with the [[electron configuration]] notation to form the ''noble gas notation''. To do this, the nearest noble gas that precedes the element in question is written first, and then the electron configuration is continued from that point forward. For example, the electron notation of [[carbon]] is 1s²2s²2p², and the noble gas notation is [He]2s²2p². This notation makes it easier to identify elements, and is shorter and easier than writing out the full notation of [[atomic orbital]]s.<ref>{{harvnb|CliffsNotes|2007|p=15}}</ref> [[Image:Xenon-tetrafluoride-3D-vdW.png|thumb|left|Structure of {{chem|Xe||F|4}}, one of the first noble gas compounds to be discovered]] ===Compounds=== {{main|Noble gas compound}} The noble gases show extremely low chemical [[reactivity]]; consequently, only a few hundred [[noble gas compound]]s have been formed. Neutral [[chemical compound|compounds]] in which helium and neon are involved in [[chemical bond]]s have not been formed, while xenon, krypton, and argon have shown only minor reactivity.<ref name=Ngcomp>{{cite journal|last=Grochala|first=Wojciech|title=Atypical compounds of gases, which have been called noble|journal=[[Chemical Society Reviews]]|date=2007|issue=36|pages=1632–1655|doi=10.1039/b702109g|volume=36}}</ref> In 1933, [[Linus Pauling]] predicted that the heavier noble gases could form compounds with [[fluorine]] and [[oxygen]]. He predicted the existence of krypton hexafluoride ({{chem|KrF|6}}) and [[xenon hexafluoride]] ({{chem|XeF|6}}), speculated {{chem|Xe||F|8}} might exist as an unstable compound, and suggested [[xenic acid]] could form [[perxenate]] salts.<ref>{{cite journal|title=The Formulas of Antimonic Acid and the Antimonates|last=Pauling|first=Linus|journal=[[Journal of the American Chemical Society]]|volume=55|issue=5|pages=1895–1900|month=June|year=1933| doi=10.1021/ja01332a016}}</ref><ref name="Holloway">{{harvnb|Holloway|1968}}</ref> These predictions were shown to be generally accurate, except {{chem|Xe||F|8}} is now thought to be both [[thermodynamic stability|thermodynamically]] and [[kinetic stability|kinetically]] unstable.<ref>{{cite journal|last=Seppelt|first=Konrad|year=1979|month=June|title=Recent developments in the Chemistry of Some Electronegative Elements|journal=[[Accounts of Chemical Research]]|volume=12|pages=211–216|doi=10.1021/ar50138a004 }}</ref> Xenon compounds are the most numerous of the noble gas compounds that have been formed.<ref>{{cite journal|last=Moody|first=G. J.|title=A Decade of Xenon Chemistry|journal=Journal of Chemical Education|year=1974|month=October|issue=10|volume=51|pages=628–630| url=http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=EJ111480&ERICExtSearch_SearchType_0=no&accno=EJ111480|accessdate=2007-10-16}}</ref> Most of them have the xenon atom in the [[oxidation state]] of +2, +4, +6, or +8 bonded to highly [[electronegative]] atoms such as fluorine or oxygen, as in [[xenon difluoride]] ({{chem|Xe||F|2}}), [[xenon tetrafluoride]] ({{chem|Xe||F|4}}), [[xenon hexafluoride]] ({{chem|XeF|6}}), [[xenon tetroxide]] ({{chem|XeO|4}}), and [[sodium]] perxenate ({{chem|Na|4|XeO|6}}). Some of these compounds have found use in [[chemical synthesis]] as [[oxidizing agent]]s; {{chem|Xe||F|2}}, in particular, is commercially available and can be used as a [[fluorination|fluorinating]] agent.<ref>{{cite journal |title=Fluorination with XeF<sub>2</sub>. 44. Effect of Geometry and Heteroatom on the Regioselectivity of Fluorine Introduction into an Aromatic Ring |author=Zupan, Marko; Iskra, Jernej; Stavber, Stojan |journal=J. Org. Chem |year=1998 |volume=63 |issue=3 |pages=878–880 |doi=10.1021/jo971496e}}</ref> As of 2007, about half a thousand compounds of xenon bonded to other elements have been identified, including organoxenon compounds (those bonded to [[carbon]]), and xenon bonded to [[nitrogen]], [[chlorine]], [[gold]], [[mercury (element)|mercury]], and xenon itself.<ref name=Ngcomp/><ref>{{harvnb|Harding|2002|pp=90–99}}</ref> Compounds of xenon bound to boron, hydrogen, bromine, iodine, beryllium, sulphur, titanium, copper, and silver have also been observed but only at low temperatures in noble gas [[matrix isolation|matrices]], or in supersonic noble gas jets.<ref name=Ngcomp/> In theory, radon is more reactive than xenon, and therefore should form chemical bonds more easily than xenon does. However, in practice, due to the high radioactivity of the [[isotopes of radon|radon isotopes]], only a few [[fluoride]]s and [[oxide]]s of radon have been formed.<ref>{{cite journal|title=The Chemistry of Radon|volume=51|journal=Russian Chemical Review|year=1982|issue=1|pages=12–20|author=Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. | doi = 10.1070/RC1982v051n01ABEH002787}}</ref> Krypton, on the other hand, is less reactive than xenon, but several compounds have been reported with krypton in the oxidation state of +2 bonded to fluorine, but also some bonded to nitrogen and oxygen (but which are only stable below {{convert|-60|C}} and {{convert|-90|C}} respectively).<ref name=Ngcomp/> Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine, carbon), as well as some late [[transition metal]]s (copper, silver, gold) have been observed also, but only at low temperatures in noble gas matrices, or in supersonic noble gas jets.<ref name=Ngcomp/> Similar conditions were used to obtain the first few compounds of argon in 2000, such as [[argon fluorohydride]] (HArF), and some bound to the late transition metals copper, silver, and gold.<ref name=Ngcomp/> As of 2007, no stable neutral molecules involving covalently bound helium or neon are known.<ref name=Ngcomp/> The noble gases—including helium—can form stable [[molecular ion]]s in the gas phase. The simplest is the [[helium hydride molecular ion]], HeH<sup>+</sup>, discovered in 1925.<ref>{{cite journal |author=Hogness, T. R.; Lunn, E. G. |title=The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis |journal=Physical Review |year=1925 |volume=26 |pages=44–55 |doi=10.1103/PhysRev.26.44}}</ref> Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it is believed to occur naturally in the [[interstellar medium]], although it has not been detected yet.<ref>{{cite journal |author=Fernandez, J.; Martin, F. |title=Photoionization of the HeH<sub>2</sub><sup>+</sup> molecular ion |journal=J. Phys. B: At. Mol. Opt. Phys |year=2007 |volume=40 |pages=2471–2480 |doi=10.1088/0953-4075/40/12/020}}</ref> In addition to these ions, there are many known neutral [[excimer]]s of the noble gases. These are compounds such as ArF and KrF that are stable only when in an [[Excited state|excited electronic state]]; some of them find application in [[excimer laser]]s. In addition to the compounds where a noble gas atom is involved in a [[covalent bond]], noble gases also form [[non-covalent]] compounds. The first to be discovered were the [[clathrate]]s, where a noble gas atom is trapped within cavities of [[crystal lattice]]s of certain organic and inorganic substances. The essential condition for their formation is that the guest (noble gas) atoms must be of appropriate size to fit in the cavities of the host crystal lattice. For instance, argon, krypton, and xenon form clathrates with water and with [[hydroquinone]], but helium and neon do not because they are too small or insufficiently [[Polarizability|polarizable]] to be retained.<ref>{{harvnb|Greenwood|1997|p=893}}</ref> [[Image:Endohedral fullerene.png|thumb|An endohedral fullerene compound containing a noble gas]] Noble gases can form [[Endohedral fullerenes|endohedral fullerene]] compounds, in which the noble gas atom is trapped inside a [[fullerene]] molecule. In 1993, it was discovered that when {{chem|C|60}}, a spherical molecule consisting of 60&nbsp;[[carbon]]&nbsp;atoms, is exposed to noble gases at high pressure, [[Complex (chemistry)|complex]]es such as {{chem|He@C|60}} can be formed (the ''@'' notation indicates He is contained inside {{chem|C|60}} but not covalently bound to it).<ref>{{cite journal|title=Stable compounds of helium and neon. He@C60 and Ne@C60|author=Saunders, M.; Jiménez-Vázquez, H. A.; Cross, R. J.; Poreda, R. J.|journal=[[Science (journal)|Science]]|year=1993|volume=259|pages=1428–1430|doi=10.1126/science.259.5100.1428|pmid=17801275}}</ref> As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been obtained.<ref>{{cite journal|title=Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure|author=Saunders, Martin; Jimenez-Vazquez, Hugo A.; Cross, R. James; Mroczkowski, Stanley; Gross, Michael L.; Giblin, Daryl E.; Poreda, Robert J. |journal=[[J. Am. Chem. Soc.]]|year=1994|volume=116|issue=5|pages=2193–2194|doi=10.1021/ja00084a089}}</ref> These compounds have found use in the study of the structure and reactivity of fullerenes by means of the [[nuclear magnetic resonance]] of the noble gas atom.<ref>{{cite journal|last=Frunzi|first=Michael|coauthors=Cross, R. James; Saunders, Martin|title=Effect of Xenon on Fullerene Reactions|journal=[[Journal of the American Chemical Society]]|year=2007|volume=129|doi=10.1021/ja075568n|pages=13343}}</ref> Noble gas compounds such as [[xenon difluoride]] ({{chem|XeF|2}}) are considered to be [[hypervalent]] because they violate the [[octet rule]]. Bonding in such compounds can be explained using a [[3-center-4-electron bond]] model.<ref>{{harvnb|Greenwood|1997|p=897}}</ref><ref>{{harvnb|Weinhold|2005|pp=275–306}}</ref> This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding in {{chem|XeF|2}} is described by a set of three [[molecular orbital]]s (MOs) derived from [[p-orbital]]s on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from each [[fluorine|F]] atom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty [[antibonding]] orbital. The [[highest occupied molecular orbital]] is localized on the two terminal atoms. This represents a localization of charge which is facilitated by the high electronegativity of fluorine.<ref>{{cite journal|last=Pimentel|first=G. C.|title= The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method|date=1951|issue=4|pages=446–448|doi=10.1063/1.1748245|journal=The Journal of Chemical Physics|volume=19}}</ref> {{clear}} {| align="center" |-valign="top" | [[Image:XeF2.png|300px|thumb|Bonding in {{chem|XeF|2}} according to the 3-center-4-electron bond model]] | [[Image:Xenon difluoride resonance structures.png|300px|thumb|Bonding in {{chem|XeF|2}} can also be represented using the [[resonance (chemistry)|resonant]] [[Lewis structure]]s. In this representation, the octet rule is not broken, the [[bond order]]s are 1/2, and there is increased electron density in the fluorine atoms.]] |} ==Occurrence and production== The abundances of the noble gases in the universe decrease as their [[atomic number]]s increase. Helium is the most common element in the [[universe]] after hydrogen, with a mass fraction of about 24%. Most of the helium in the universe was formed during [[Big Bang nucleosynthesis]], but the amount of helium is steadily increasing due to the fusion of hydrogen in [[stellar nucleosynthesis]].<ref>{{cite web|last=Weiss|first=Achim|title=Elements of the past: Big Bang Nucleosynthesis and observation|url=http://www.einstein-online.info/en/spotlights/BBN_obs/index.html|publisher=[[Max Planck Institute for Gravitational Physics]]|accessdate=2008-06-23}}</ref><ref>{{cite journal|author=Coc, A.; et al.|title=Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements|journal=[[Astrophysical Journal]]|volume=600|year=2004|pages=544|doi=10.1086/380121}}</ref> Abundances on Earth follow different trends; for example, helium is only the third most abundant noble gas in the atmosphere. The reason is that there is no [[primordial element|primordial]] helium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth's [[gravitational field]].<ref name=morrison>{{cite journal|first=P.|last=Morrison|coauthors=Pine, J.|year=1955|title=Radiogenic Origin of the Helium Isotopes in Rock|journal=Annals of the New York Academy of Sciences|volume=62|issue=3|pages=71–92|doi=10.1111/j.1749-6632.1955.tb35366.x}}</ref> Helium on Earth comes from the [[alpha decay]] of heavy elements such as [[uranium]] and [[thorium]] found in the Earth's [[Crust (geology)|crust]], and tends to accumulate in [[Natural gas field|natural gas deposit]]s.<ref name=morrison /> The abundance of argon, on the other hand, is increased as a result of the [[beta decay]] of [[potassium-40]], also found in the Earth's crust, to form [[argon-40]], which is the most abundant isotope of argon on Earth despite being relatively rare in the [[Solar System]]. This process is the base for the [[potassium-argon dating]] method.<ref name=iso>{{cite web|url=http://www.geoberg.de/text/geology/07011601.php|title=<sup>40</sup>Ar/<sup>39</sup>Ar dating and errors|accessdate=2008-06-26|publisher=[[Technische Universität Bergakademie Freiberg]]|date=2007-01-16|last=Scherer|first=Alexandra}}</ref> Xenon has an unexpectedly low abundance in the atmosphere, in what has been called the ''missing xenon problem''; one theory is that the missing xenon may be trapped in minerals inside the Earth's crust.<ref>{{cite journal|first=Chrystèle|last=Sanloup|coauthors=''et al''|title=Retention of Xenon in Quartz and Earth's Missing Xenon|journal=Science|year=2005|volume=310|issue=5751|pages=1174–1177|doi= 10.1126/science.1119070|pmid=16293758 }}</ref> Radon is formed in the [[lithosphere]] as from the [[alpha decay]] of [[radium]]. It can seep into buildings through cracks in their foundation and accumulate in areas that are not well ventilated. Due to its high radioactivity, radon presents a significant health hazard; it is implicated in an estimated 21,000 [[lung cancer]] deaths per year in the United States alone.<ref>{{cite web| title = A Citizen's Guide to Radon| publisher = U.S. Environmental Protection Agency| date = 2007-11-26| url = http://www.epa.gov/radon/pubs/citguide.html|accessdate = 2008-06-26}}</ref> <center> {| class="wikitable" style="text-align: center;" ! Abundance || [[Helium]] || [[Neon]] || [[Argon]] || [[Krypton]] || [[Xenon]] || [[Radon]] |- |align="left" | Solar System (for each atom of silicon)<ref>{{cite journal |title=Solar System Abundances and Condensation Temperatures of the Elements |author= Lodders, Katharina|journal=The Astrophysical Journal |volume=591 |pages=1220–1247 |year=2003 |doi=10.1086/375492}}</ref>|| 2343 || 2.148 || 0.1025 || 5.515 × 10<sup>−5</sup> || 5.391 × 10<sup>−6</sup> || – |- |align="left" | Earth's atmosphere (volume fraction in [[parts per million|ppm]])<ref name=nws>{{cite web|accessdate=2008-06-01|url=http://www.srh.noaa.gov/jetstream//atmos/atmos_intro.htm|title=The Atmosphere |publisher=[[National Weather Service]] }}</ref> || 5.20 || 18.20 || 9340.00 || 1.10 || 0.09 || (0.06 – 18) × 10<sup>−19</sup><ref name=ullmann/> |- |align="left" | Igneous rock (mass fraction in ppm)<ref name=greenwood891/> || 3 × 10<sup>−3</sup> || 7 × 10<sup>−5</sup> || 4 × 10<sup>−2</sup> || – || – || 1.7 × 10<sup>−10</sup> |} </center> <div style="float: right; padding-left: 10px;"> {| class="wikitable" style="text-align: center;" ! Gas || 2004 price ([[United States Dollar|USD]]/m<sup>3</sup>)<ref name=kirk>{{cite book |title=Kirk Othmer Encyclopedia of Chemical Technology |author= Hwang, Shuen-Chen; Lein, Robert D.; Morgan, Daniel A.|chapter=Noble Gases |doi=10.1002/0471238961.0701190508230114.a01 |pages=343–383 |year=2005 |publisher=Wiley}}</ref> |- |align=left| Helium (industrial grade) || 4.20–4.90 |- |align=left| Helium (laboratory grade) || 22.30–44.90 |- |align=left| Argon || 2.70–8.50 |- |align=left| Neon || 60–120 |- |align=left| Krypton || 400–500 |- |align=left| Xenon || 4000–5000 |} </div> Neon, argon, krypton, and xenon are obtained from air using the methods of [[liquefaction of gases]], to convert elements to a liquid state, and [[fractional distillation]], to separate mixtures into component parts. Helium is typically produced by separating it from [[natural gas]], and radon is isolated from the radioactive decay of [[radium]] compounds.<ref name="brit" /> The prices of the noble gases are influenced by their natural abundance, with argon being the cheapest and xenon the most expensive. As an example, the table to the right lists the 2004 prices in the United States for laboratory quantities of each gas. {{clear}} ==Applications== [[Image:Modern 3T MRI.JPG|thumb|left|Liquid helium is used to cool the superconducting magnets in modern MRI scanners.]] <!-- cryogenics --> Noble gases have very low boiling and melting points, which makes them useful as [[cryogenic]] [[refrigerant]]s.<ref>{{cite encyclopedia|title=Neon|encyclopedia=[[Encarta]]|date=2008}}</ref> In particular, [[liquid helium]], which boils at {{convert|4.2|K}}, is used for [[superconducting magnet]]s, such as those needed in [[nuclear magnetic resonance imaging]] and [[nuclear magnetic resonance]].<ref>{{cite journal|title=Demountable coaxial gas-cooled current leads for MRI superconducting magnets|author=Zhang, C. J.; Zhou, X. T.; Yang, L.|journal=Magnetics, IEEE Transactions on|publisher=[[IEEE]]|volume=28|issue=1|date=January 1992|pages=957–959|doi=10.1109/20.120038}}</ref> Liquid neon, although it does not reach temperatures as low as liquid helium, also finds use in cryogenics because it has over 40&nbsp;times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen.<ref name=ullmann/> <!-- diving --> Helium is used as a component of [[breathing gases]] to replace nitrogen, due its low [[solubility]] in fluids, especially in [[lipids]]. Gases are absorbed by the [[blood]] and [[body tissue]]s when under pressure like in [[scuba diving]], which causes an [[anesthetic]] effect known as [[nitrogen narcosis]].<ref name=Fowler>{{cite journal |last=Fowler |first=B |coauthors=Ackles, K. N.; Porlier, G. |title=Effects of inert gas narcosis on behavior—a critical review |journal=Undersea Biomed. Res. |volume=12 |issue=4 |pages=369–402 |date=1985 |issn=0093-5387 |oclc=2068005 |pmid=4082343 |url=http://archive.rubicon-foundation.org/3019 |accessdate=2008-04-08 }}</ref> Due to its reduced solubility, little helium is taken into [[cell membranes]], and when helium is used to replace part of the breathing mixtures, such as in [[Trimix (breathing gas)|trimix]] or [[heliox]], a decrease in the narcotic effect of the gas at depth is obtained.<ref>{{harvnb|Bennett|1998|p=176}}</ref> Helium's reduced solubility offers further advantages for the condition known as [[Decompression_sickness#Helium|decompression sickness]], or ''the bends''.<ref name="brit"/><ref name=38uhms>{{cite journal |last=Vann|first=R. D. (ed)|title=The Physiological Basis of Decompression|journal=38th Undersea and Hyperbaric Medical Society Workshop |volume=75(Phys)6-1-89 |date=1989 |pages=437 |url=http://archive.rubicon-foundation.org/6853 |accessdate=2008-05-31 }}</ref> The reduced amount of dissolved gas in the body means that fewer gas bubbles form during the decrease in pressure of the ascent. Another noble gas, argon, is considered the best option for use as a [[drysuit]] inflation gas for scuba diving.<ref>{{cite web |last=Maiken |first=Eric |title=Why Argon? |url=http://www.decompression.org/maiken/Why_Argon.htm |accessdate=2008-06-26|publisher=Decompression|date=2004-08-01}}</ref> <!-- lifting --> [[Image:Goodyear-blimp.jpg|thumb|right|''The Spirit of Goodyear'', one of the iconic [[Goodyear Blimp]]s]] Since the [[Hindenburg disaster|''Hindenburg'' disaster]] in 1937,<ref>{{cite news|title=Disaster Ascribed to Gas by Experts|work=[[The New York Times]]|date=[[1937-05-07]]|page=1}}</ref> helium has replaced hydrogen as a lifting gas in [[blimp]]s and [[balloon]]s due to its lightness and incombustibility, despite an 8.6%<ref>{{cite web | last=Freudenrich | first=Craig | year=2008 | url=http://science.howstuffworks.com/blimp2.htm | title=How Blimps Work | publisher=HowStuffWorks | accessdate=2008-07-03 }}</ref> decrease in buoyancy.<ref name="brit"/> <!-- scientific and some miscellaneous uses --> In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis of [[air sensitive|air-sensitive compounds]] that are sensitive to nitrogen. Solid argon is also used for the study of very unstable compounds, such as [[reactive intermediate]]s, by trapping them in an inert [[matrix isolation|matrix]] at very low temperatures.<ref>{{cite journal |journal=Chem. Soc. Rev. |year=1980 |volume=9 |pages=1–23 |doi=10.1039/CS9800900001 |title=The matrix isolation technique and its application to organic chemistry |author=Dunkin, I. R. }}</ref> Helium is used as the carrier medium in [[gas chromatography]], as a filler gas for thermometers, and in devices for measuring radiation, such as the [[Geiger counter]] and the [[bubble chamber]].<ref name=kirk/> Helium and argon are both commonly used to shield [[welding arc]]s and the surrounding [[base metal]] from the atmosphere during welding and cutting, as well as in other metallurgical processes and in the production of silicon for the semiconductor industry.<ref name="ullmann" /> <!-- lighting --> [[Image:Xenon short arc 1.jpg|thumb|left|15,000-watt [[xenon short-arc lamp]] used in [[IMAX]] projectors]] Noble gases are commonly used in [[lighting]] because of their lack of chemical reactivity. Argon, mixed with nitrogen, is used as a filler gas for [[incandescent light bulb]]s.<ref name=ullmann>{{cite book |author= Häussinger, Peter; Glatthaar, Reinhard; Rhode, Wilhelm; Kick, Helmut; Benkmann, Christian; Weber, Josef; Wunschel, Hans-Jörg; Stenke, Viktor; Leicht, Edith; Stenger, Hermann|chapter=Noble gases |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |year=2002 |doi=10.1002/14356007.a17_485}}</ref> Krypton is used in high-performance light bulbs, which have higher [[color temperature]]s and greater efficiency, because it reduces the rate of evaporation of the filament more than argon; [[halogen lamps]], in particular, use krypton mixed with small amounts of compounds of [[iodine]] or [[bromine]].<ref name=ullmann/> The noble gases glow in distinctive colors when used inside [[gas-discharge lamp]]s, such as [[neon light]]s, which produce an orange-red color. Xenon is commonly used in [[xenon arc lamp]]s which, due to their nearly [[continuous spectrum]] that resembles daylight, find application in film projectors and as automobile headlamps.<ref name=ullmann/> <!-- lasers --> The noble gases are used in [[excimer laser]]s, which are based on short-lived electronically excited molecules known as [[excimer]]s. The excimers used for lasers may be noble gas dimers such as Ar<sub>2</sub>, Kr<sub>2</sub> or Xe<sub>2</sub>, or more commonly, the noble gas is combined with a halogen in excimers such as ArF, KrF, XeF, or XeCl. These lasers produce [[ultraviolet]] light which, due to its short [[wavelength]] (193 [[nanometer|nm]] for ArF and 248 nm for KrF), allows for high-precision imaging. Excimer lasers have many industrial, medical, and scientific applications. They are used for [[microlithography]] and [[microfabrication]], which are essential for [[integrated circuit]] manufacture, and for [[laser surgery]], including laser [[angioplasty]] and [[eye surgery]].<ref>{{cite book |title=Excimer Laser Technology |author= Basting, Dirk; Marowsky, Gerd|publisher=Springer |year=2005 |isbn=3540200568}}</ref> <!-- medical uses --> Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of [[asthma]] sufferers.<ref name=ullmann/> Xenon is used as an [[anesthetic]] because of it is highly solubility in lipids, which makes it more potent than the usual [[nitrous oxide]], and because it is readily eliminated from the body, resulting in faster recovery.<ref name=Sanders>{{cite journal | author=Sanders, Robert D.; Ma, Daqing; Maze, Mervyn | title=Xenon: elemental anaesthesia in clinical practice | journal=British Medical Bulletin | year=2005 | volume=71 | issue=1 | pages=115–135 | doi= 10.1093/bmb/ldh034 | accessdate=2007-10-02 | pmid=15728132 }}</ref> Xenon finds application in medical imaging of the lungs through hyperpolarized MRI.<ref>{{cite journal | last=Albert | first=M. S. | coauthors=Balamore, D. | title=Development of hyperpolarized noble gas MRI | journal=Nuclear Instruments and Methods in Physics Research A | year=1998 | volume=402 | pages=441–453 | url=http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=11543065&dopt=AbstractPlus | accessdate=2007-10-01 | doi = 10.1016/S0168-9002(97)00888-7}}</ref> Radon, which is highly radioactive and is only available in minute amounts, is used in [[radiotherapy]].<ref name=brit /> <center><gallery caption="Colors produced by different noble gases when used in [[Neon sign|neon tubes]]" perrow="5"> Image:HeTube.jpg|Helium Image:NeTube.jpg|Neon Image:ArTube.jpg|Argon (with some mercury) Image:KrTube.jpg|Krypton Image:XeTube.jpg|Xenon </gallery></center> ==See also== *[[Noble gas (data page)]], for extended tables of physical properties. *[[Noble metal]], for metals that are resistant to corrosion or oxidation. *[[Inert gas]], for any gas that is not reactive under normal circumstances. ==Notes== {{reflist|colwidth=30em}} ==References== * {{cite book|title=The Physiology and Medicine of Diving|last= Bennett|first= Peter B.|coauthors= Elliott, David H.|publisher=SPCK Publishing|year=1998|isbn=0702024104|ref=CITEREFBennett1998}} * {{cite book|author=Bobrow Test Preparation Services|title=CliffsAP Chemistry|publisher=[[CliffsNotes]]|date=[[2007-12-05]]|year=2007|isbn=047013500X|ref=CITEREFCliffsNotes2007}} * {{cite book | last=Greenwood |first = N. N. |coauthors = Earnshaw, A. | title=Chemistry of the Elements |edition = 2nd edition |publisher=Oxford:Butterworth-Heinemann | year=1997 | isbn=0-7506-3365-4|ref=CITEREFGreenwood1997}} * {{cite book | last=Harding|first=Charlie J.|coauthors=Janes, Rob | year=2002 | title=Elements of the P Block | publisher=[[Royal Society of Chemistry]] | isbn= 0854046909 |ref=CITEREFHarding2002}} * {{cite book | last = Holloway | first = John H. | year = 1968 | title = Noble-Gas Chemistry | publisher = [[Methuen Publishing]] | location = [[London]]|isbn=0412211009|ref=CITEREFHolloway1968}} * {{cite book|last=Mendeleev|first=D.|authorlink=Dmitri Mendeleev|title= Osnovy Khimii (The Principles of Chemistry)|edition=7th edition| year=1903|date=1902–1903|language=[[Russian language|Russian]]|url=http://www.archive.org/details/principlesofchem00menduoft|ref=CITEREFMendeleev1903}} * {{cite book|first=Minoru|last=Ozima|coauthors=Podosek, Frank A.|title=Noble Gas Geochemistry|year=2002|publisher=[[Cambridge University Press]]|isbn=0521803667|url=http://books.google.com/books?id=CBM2LJDvRtgC|ref=CITEREFOzima2002}} * {{cite book|last=Weinhold|first=F.|coauthors=Landis, C.|title=Valency and bonding|publisher=[[Cambridge University Press]]|year=2005|isbn=0521831288|ref=CITEREFWeinhold2005}} {{PeriodicTablesFooter}} {{compact periodic table}} {{featured article}} [[Category:Periodic table]] [[Category:Noble gases| ]] <!-- interwiki --> [[ar:غاز نبيل]] [[ast:Gas noble]] [[bn:নিষ্ক্রিয় গ্যাস]] [[bs:Plemeniti plinovi]] [[bg:Благороден газ]] [[ca:Gas noble]] [[cv:Сӳрĕк газсем]] [[cs:Inertní plyn]] [[cy:Nwyon nobl]] [[da:Ædelgas]] [[de:Edelgase]] [[et:Väärisgaasid]] [[el:Ευγενή αέρια]] [[es:Gas noble]] [[eo:Nobla gaso]] [[eu:Gas noble]] [[fa:گازهای نجیب]] [[fo:Tey Óvirknu Loftevnini]] [[fr:Gaz noble]] [[gl:Gas nobre]] [[ko:비활성 기체]] [[hsb:Drohopłun]] [[hr:Plemeniti plinovi]] [[id:Gas mulia]] [[is:Eðallofttegund]] [[it:Gas nobili]] [[he:גז אציל]] [[jv:Gas mulia]] [[sw:Gesi adimu]] [[lv:Cēlgāzes]] [[lt:Inertinės dujos]] [[lmo:Gas nòbil]] [[hu:Nemesgáz]] [[ml:ഉല്‍കൃഷ്ട വാതകങ്ങള്‍]] [[ms:Gas adi]] [[nl:Edelgas]] [[ja:第18族元素]] [[no:Edelgasser]] [[nn:Edelgass]] [[nds:Eddelgas]] [[pl:Gazy szlachetne]] [[pt:Gás nobre]] [[ro:Gaz nobil]] [[qu:Umiña wapsi]] [[ru:Инертные газы]] [[simple:Noble gas]] [[sk:Vzácne plyny]] [[sl:Žlahtni plin]] [[sr:Племенити гасови]] [[sh:Plemeniti gas]] [[su:Gas mulya]] [[fi:Jalokaasut]] [[sv:Ädelgas]] [[te:ఉత్కృష్ట వాయువు]] [[th:แก๊สมีตระกูล]] [[vi:Khí hiếm]] [[tr:Soygaz]] [[uk:Інертні гази]] [[ur:غیر تعامل گیس]] [[zh:稀有气体]]