Noble gas

Noble gas is translated from the German noun {{lang|de|Edelgas}}, first used in 1900 by Hugo Erdmann{{cite journal|journal=Science|year=1901|volume=13|pages=268–270|last=Renouf|first=Edward|title=Noble gases|doi=10.1126/science.13.320.268|issue=320|bibcode= 1901Sci....13..268R |s2cid=34534533|url=https://archive.org/details/lehrbuchderanor01erdmgoog/page/78/mode/2up}} to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known.{{harvnb|Ozima|2002|p=30}} Rare gases is another term that was used,{{harvnb|Ozima|2002|p=4}} 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 due to decay of radioactive potassium-40.{{cite encyclopedia|encyclopedia=Encyclopædia Britannica|year=2008|title=argon|url=https://www.britannica.com/eb/article-9009382/argon}}

File:Helium spectrum.jpgs.|alt=A line spectrum chart of the visible spectrum showing sharp lines on top.]]

Pierre Janssen and Joseph Norman Lockyer had discovered a new element on 18 August 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, {{lang|grc|ἥλιος}} ({{lang|grc-Latn|hḗlios}}).Oxford English Dictionary (1989), s.v. "helium". Retrieved 16 December 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." No chemical analysis was possible at the time, but helium was later found to be a noble gas. 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.{{harvnb|Ozima|2002|p=1}} A century later, in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with Scottish scientist William Ramsay at University College, London, 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, from the Greek word {{lang|grc|ἀργός}} ({{lang|grc-Latn|argós}}, "idle" or "lazy"). With this discovery, they realized an entire class of gases 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 0 in his arrangement of the elements, which would later become the periodic table.{{harvnb|Mendeleev|1903|p=497}}

Ramsay continued his 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 {{lang|grc|κρυπτός}} ({{lang|grc-Latn|kryptós}}, "hidden"), {{lang|grc|νέος}} ({{lang|grc-Latn|néos}}, "new"), and {{lang|grc|ξένος}} ({{lang|grc-Latn|ksénos}}, "stranger"), respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn,{{cite journal|title=Discovery of Radon|last=Partington|first=J. R.|journal=Nature|volume=179|issue=4566|pages=912|year=1957|doi=10.1038/179912a0|bibcode=1957Natur.179..912P|s2cid=4251991|doi-access=free}} 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.{{cite encyclopedia|encyclopedia=Encyclopædia Britannica|year=2008|title=Noble Gas|url=https://www.britannica.com/eb/article-9110613/noble-gas}} Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry, respectively, for their discovery of the noble gases;{{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}}{{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}} 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".

The discovery of the noble gases aided in the development of a general understanding of atomic structure. In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine, the most 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 electrons in atoms are arranged in shells surrounding the nucleus, and that for all noble gases except helium the outermost shell always contains eight electrons. 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.{{cite journal |author1=Gillespie, R. J. |author2=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 |pmid=17109437 |doi=10.1002/jcc.20545|doi-access=free }}

In 1962, Neil Bartlett discovered the first chemical compound of a noble gas, xenon hexafluoroplatinate.{{cite journal|title=Xenon hexafluoroplatinate Xe⁺[PtF₆]⁻|last=Bartlett|first=N.|journal=Proceedings of the Chemical Society|issue=6|page=218|year=1962|doi=10.1039/PS9620000197}} Compounds of other noble gases were discovered soon after: in 1962 for radon, radon difluoride ({{chem|Rn||F|2}}),{{cite journal|author1=Fields, Paul R. |author2=Stein, Lawrence |author3=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|bibcode=1962JAChS..84.4164F }} which was identified by radiotracer techniques and in 1963 for krypton, krypton difluoride ({{chem|Kr||F|2}}).{{cite journal|author1=Grosse, A. V. |author2=Kirschenbaum, A. D. |author3=Streng, A. G. |author4=Streng, L. V. |title=Krypton Tetrafluoride: Preparation and Some Properties|journal=Science|year=1963|volume=139|pages=1047–1048|doi=10.1126/science.139.3559.1047|pmid=17812982|issue=3559|bibcode=1963Sci...139.1047G}} The first stable compound of argon was reported in 2000 when argon fluorohydride (HArF) was formed at a temperature of {{convert|40|K}}.{{cite journal|title=A stable argon compound|journal=Nature|issue= 6798|pages=874–876|year=2000|doi=10.1038/35022551|author1=Khriachtchev, Leonid |author2=Pettersson, Mika |author3=Runeberg, Nino |author4=Lundell, Jan |author5=Räsänen, Markku |volume=406|pmid=10972285 |bibcode=2000Natur.406..874K|s2cid=4382128}}

In October 2006, scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully created synthetically oganesson, the seventh element in group 18,{{cite journal|journal=Pure Appl. Chem.|volume=83|issue=7|year=2011|title=Discovery of the elements with atomic numbers greater than or equal to 113 (IUPAC Technical Report)*|author1=Barber, Robert C. |author2=Karol, Paul J. |author3=Nakahara, Hiromichi |author4=Vardaci, Emanuele |author5=Vogt, Erich W. |name-list-style=amp |url=http://iupac.org/publications/pac/pdf/2011/pdf/8307x1485.pdf|access-date=30 May 2014|publisher=IUPAC|doi=10.1515/ci.2011.33.5.25b }} by bombarding californium with calcium.{{cite journal |last1=Oganessian |first1=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 |page=44602 |year=2006 |doi=10.1103/PhysRevC.74.044602 |last2=Utyonkov |first2=V. |last3=Lobanov |first3=Yu. |last4=Abdullin |first4=F. |last5=Polyakov |first5=A. |bibcode=2006PhRvC..74d4602O |display-authors=5 |last6=Shirokovsky |first6=I. |last7=Tsyganov |first7=Yu. |last8=Voinov |first8=A. |last9=Gulbekian |first9=G. |last10=Bogomolov |first10=S. |last11=Gikal |first11=B. |last12=Mezentsev |first12=A. |last13=Iliev |first13=S. |last14=Subbotin |first14=V. |last15=Sukhov |first15=A. |last16=Subotic |first16=K. |last17=Zagrebaev |first17=V. |last18=Vostokin |first18=G. |last19=Itkis |first19=M. |last20=Moody |first20=K. |last21=Patin |first21=J. |last22=Shaughnessy |first22=D. |last23=Stoyer |first23=M. |last24=Stoyer |first24=N. |last25=Wilk |first25=P. |last26=Kenneally |first26=J. |last27=Landrum |first27=J. |last28=Wild |first28=J. |last29=Lougheed |first29=R.|doi-access=free }}

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{{Hatnote|For more data, see Noble gas (data page).}}

The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements. Helium has several unique qualities when compared with other elements: its boiling point at 1 atm is lower than those of any other known substance; it is the only element known to exhibit superfluidity; and, it is the only element that cannot be solidified by cooling at atmospheric pressure (an effect explained by quantum mechanics as its zero point energy is too high to permit freezing){{cite web|year = 2008|url = http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm|url-status = dead|title = John Beamish's Research on Solid Helium|archive-url = https://web.archive.org/web/20080531145546/http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm|archive-date = 31 May 2008|publisher = Department of Physics, University of Alberta}} – a pressure of {{convert|25|atm|kPa psi|lk=on}} must be applied at a temperature of {{convert|0.95|K}} to convert it to a solid{{cite book|title = The Properties of Liquid and Solid Helium|chapter-url = https://archive.org/details/propertiesofliqu0000wilk|chapter-url-access = registration|first = John|last = Wilks|year = 1967|place = Oxford|publisher = Clarendon Press|isbn = 978-0-19-851245-5|chapter = Introduction}} while a pressure of about {{cvt|113500|atm|kPa psi}} is required at room temperature.{{cite journal|last1 = Pinceaux|first1 = J.-P.|last2 = Maury|first2 = J.-P.|last3 = Besson|first3 = J.-M.|title = Solidification of helium, at room temperature under high pressure|journal = Journal de Physique Lettres|year = 1979|volume = 40|issue = 13|pages = 307–308|doi = 10.1051/jphyslet:019790040013030700| s2cid=40164915 |url = https://hal.archives-ouvertes.fr/jpa-00231630/file/ajp-jphyslet_1979_40_13_307_0.pdf}} The noble gases up to xenon have multiple stable isotopes; krypton and xenon also have naturally occurring radioisotopes, namely ⁷⁸Kr, ¹²⁴Xe, and ¹³⁶Xe, all have very long lives (> 10²¹ years) and can undergo double electron capture or double beta decay. Radon has no stable isotopes; its longest-lived isotope, ²²²Rn, has a half-life of 3.8 days and decays to form helium and polonium, which ultimately decays to lead. Oganesson also has no stable isotopes, and its only known isotope ²⁹⁴Og is very short-lived (half-life 0.7 ms). Melting and boiling points increase going down the group.

File:First Ionization Energy blocks.svg versus atomic number. The noble gases have the largest ionization potential for each period, although period 7 is expected to break this trend because the predicted first ionization energy of oganesson (Z = 118) is lower than those of elements 110-112.|alt=A graph of ionization energy vs. atomic number showing sharp peaks for the noble gas atoms.]]

The noble gas atoms, like atoms in most groups, increase steadily in atomic radius from one 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 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. Some of the heavier noble gases, however, have ionization potentials small enough to be comparable to those of other elements and molecules. It was the insight that xenon has an ionization potential similar to that of the oxygen molecule that led Bartlett to attempt oxidizing xenon using platinum hexafluoride, an oxidizing agent known to be strong enough to react with oxygen. Noble gases cannot accept an electron to form stable anions; that is, they have a negative electron affinity.{{cite journal |journal=Journal of Chemical Education|last=Wheeler |first=John C. |year=1997 |volume=74 |issue=1 |pages=123–127 |title=Electron Affinities of the Alkaline Earth Metals and the Sign Convention for Electron Affinity|bibcode= 1997JChEd..74..123W |doi= 10.1021/ed074p123 }}; {{cite journal|journal=Chemical Reviews |year=1994 |volume=94 |pages=2291–2318|author1=Kalcher, Josef |author2=Sax, Alexander F. |title=Gas Phase Stabilities of Small Anions: Theory and Experiment in Cooperation |doi=10.1021/cr00032a004|issue=8}}

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 18, the atomic radius increases, and with it the interatomic forces increase, 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.{{harvnb|Greenwood|1997|p=891}}

The noble gases are nearly ideal gases 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.{{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|doi-access=free}} 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.

File:Electron shell 010 Neon - no label.svg. Noble gases have eight electrons in their outermost shell, except in the case of helium, which has two.|alt=An atomic shell diagram with neon core, 2 electrons in the inner shell and 8 in the outer shell.]]

The noble gases are colorless, odorless, tasteless, and nonflammable under standard conditions.{{cite book |title=Ullmann's Encyclopedia of Industrial Chemistry – Volume 23 |year=2003 |publisher=John Wiley & Sons |author=Wiley-VCH |page=217 }} They were once labeled group 0 in the periodic table because it was believed they had a valence of zero, meaning their atoms cannot combine with those of other elements to form compounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse.

{{Further|Noble gas configuration}}

Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells resulting in trends in chemical behavior:

The noble gases have full valence electron shells. Valence electrons are the outermost electrons of an atom and are normally the only electrons that participate in chemical bonding. 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.{{harvnb|Ozima|2002|p=35}} 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

phosphorus is {{nowrap|1s² 2s² 2p⁶ 3s² 3p³}}, while the noble gas notation is {{nowrap|[Ne] 3s² 3p³}}. This more compact notation makes it easier to identify elements, and is shorter than writing out the full notation of atomic orbitals.{{harvnb|CliffsNotes|2007|p=15}}

The noble gases cross the boundary between blocks—helium is an s-element whereas the rest of members are p-elements—which is unusual among the IUPAC groups. All other IUPAC groups contain elements from one block each. This causes some inconsistencies in trends across the table, and on those grounds some chemists have proposed that helium should be moved to group 2 to be with other s² elements,{{cite journal |last1=Grochala |first1=Wojciech |date=1 November 2017 |title=On the position of helium and neon in the Periodic Table of Elements |journal=Foundations of Chemistry |volume=20 |pages=191–207 |issue=2018 |doi=10.1007/s10698-017-9302-7 |doi-access=free }}{{cite journal |last1=Bent Weberg |first1=Libby |date=18 January 2019 |title="The" periodic table |url=https://cen.acs.org/articles/97/i3/Reactions.html |journal=Chemical & Engineering News |volume=97 |issue=3 |access-date=27 March 2020 |archive-date=1 February 2020 |archive-url=https://web.archive.org/web/20200201200009/https://cen.acs.org/articles/97/i3/Reactions.html |url-status=live }}{{cite journal |last1=Grandinetti |first1=Felice |date=23 April 2013 |title=Neon behind the signs |journal=Nature Chemistry |volume=5 |issue=2013 |page=438 |doi=10.1038/nchem.1631 |pmid=23609097 |bibcode=2013NatCh...5..438G |doi-access=free }} but this change has not generally been adopted.

{{Main|Noble gas compound}}

File:Xenon-tetrafluoride-3D-vdW.png ({{chem|Xe||F|4}}), one of the first noble gas compounds to be discovered|alt=A model of planar chemical molecule with a blue center atom (Xe) symmetrically bonded to four peripheral atoms (fluorine).]]

The noble gases show extremely low chemical reactivity; consequently, only a few hundred noble gas compounds have been formed. Neutral compounds in which helium and neon are involved in chemical bonds have not been formed (although some helium-containing ions exist and there is some theoretical evidence for a few neutral helium-containing ones), while xenon, krypton, and argon have shown only minor reactivity.{{cite journal|last=Grochala|first=Wojciech|title=Atypical compounds of gases, which have been called noble|journal=Chemical Society Reviews|year=2007|issue=10|pages=1632–1655|doi=10.1039/b702109g|volume=36|pmid=17721587|url=http://www.chem.uw.edu.pl/people/WGrochala/Ng_essay.pdf?origin%3Dpublication_detail|access-date=25 October 2017|archive-date=26 October 2017|archive-url=https://web.archive.org/web/20171026111020/http://beta.chem.uw.edu.pl/people/WGrochala/Ng_essay.pdf?origin%3Dpublication_detail|url-status=dead}} The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn ≪ Og.

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}}) and speculated that xenon octafluoride ({{chem|XeF|8}}) might exist as an unstable compound, and suggested that xenic acid could form perxenate salts.{{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|year=1933| doi=10.1021/ja01332a016|bibcode=1933JAChS..55.1895P }}{{harvnb|Holloway|1968}} These predictions were shown to be generally accurate, except that {{chem|XeF|8}} is now thought to be both thermodynamically and kinetically unstable.{{cite journal|last=Seppelt|first=Konrad|year=1979|title=Recent developments in the Chemistry of Some Electronegative Elements|journal=Accounts of Chemical Research|volume=12|pages=211–216|doi=10.1021/ar50138a004|issue=6}}

Xenon compounds are the most numerous of the noble gas compounds that have been formed.{{cite journal|last=Moody|first=G. J.|title=A Decade of Xenon Chemistry|journal=Journal of Chemical Education|year=1974|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|access-date=16 October 2007|doi=10.1021/ed051p628|bibcode= 1974JChEd..51..628M }} 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|XeF|2}}), xenon tetrafluoride ({{chem|XeF|4}}), xenon hexafluoride ({{chem|XeF|6}}), xenon tetroxide ({{chem|XeO|4}}), and sodium perxenate ({{chem|Na|4|XeO|6}}). Xenon reacts with fluorine to form numerous xenon fluorides according to the following equations:

::Xe + F₂ → XeF₂

::Xe + 2F₂ → XeF₄

::Xe + 3F₂ → XeF₆

Some of these compounds have found use in chemical synthesis as oxidizing agents; {{chem|XeF|2}}, in particular, is commercially available and can be used as a fluorinating agent.{{cite journal |title=Fluorination with XeF₂. 44. Effect of Geometry and Heteroatom on the Regioselectivity of Fluorine Introduction into an Aromatic Ring |author1=Zupan, Marko |author2=Iskra, Jernej |author3=Stavber, Stojan |journal=J. Org. Chem. |year=1998 |volume=63 |issue=3 |pages=878–880 |doi=10.1021/jo971496e |pmid=11672087}} As of 2007, about five hundred compounds of xenon bonded to other elements have been identified, including organoxenon compounds (containing xenon bonded to carbon), and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon itself.{{harvnb|Harding|2002|pp=90–99}} 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 matrices, or in supersonic noble gas jets.

Radon is more reactive than xenon, and forms chemical bonds more easily than xenon does. However, due to the high radioactivity and short half-life of radon isotopes, only a few fluorides and oxides of radon have been formed in practice..{{cite journal|title=The Chemistry of Radon|volume=51|journal=Russian Chemical Reviews|year=1982|issue=1|pages=12–20|author1=Avrorin, V. V. |author2=Krasikova, R. N. |author3=Nefedov, V. D. |author4=Toropova, M. A. |doi=10.1070/RC1982v051n01ABEH002787|bibcode= 1982RuCRv..51...12A |s2cid=250906059 }} Radon goes further towards metallic behavior than xenon; the difluoride RnF₂ is highly ionic, and cationic Rn²⁺ is formed in halogen fluoride solutions. For this reason, kinetic hindrance makes it difficult to oxidize radon beyond the +2 state. Only tracer experiments appear to have succeeded in doing so, probably forming RnF₄, RnF₆, and RnO₃.{{cite journal |last1=Stein |first1=Lawrence |date=1983 |title=The Chemistry of Radon |journal=Radiochimica Acta |volume=32 |issue=1–3 |pages=163–171 |doi=10.1524/ract.1983.32.13.163|s2cid=100225806 }}{{cite journal |last1=Liebman |first1=Joel F. |date=1975 |title=Conceptual Problems in Noble Gas and Fluorine Chemistry, II: The Nonexistence of Radon Tetrafluoride |journal=Inorg. Nucl. Chem. Lett. |volume=11 |issue=10 |pages=683–685 |doi=10.1016/0020-1650(75)80185-1}}{{cite journal |last1=Seppelt |first1=Konrad |date=2015 |title=Molecular Hexafluorides |journal=Chemical Reviews |volume=115 |issue=2 |pages=1296–1306 |doi=10.1021/cr5001783|pmid=25418862 }}

Krypton is less reactive than xenon, but several compounds have been reported with krypton in the oxidation state of +2. Krypton difluoride is the most notable and easily characterized. Under extreme conditions, krypton reacts with fluorine to form KrF₂ according to the following equation:

::Kr + F₂ → KrF₂

Compounds in which krypton forms a single bond to nitrogen and oxygen have also been characterized,{{cite journal|doi=10.1016/S0010-8545(02)00202-3|title=The chemistry of krypton|year=2002|author=Lehmann, J|journal=Coordination Chemistry Reviews|volume=233–234|pages=1–39}} but are only stable below {{convert|-60|C}} and {{convert|-90|C}} respectively.

Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine, carbon) as well as some late transition metals (copper, silver, gold) have also been observed, but only either at low temperatures in noble gas matrices, or in supersonic noble gas jets. 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. As of 2007, no stable neutral molecules involving covalently bound helium or neon are known.

Extrapolation from periodic trends predict that oganesson should be the most reactive of the noble gases; more sophisticated theoretical treatments indicate greater reactivity than such extrapolations suggest, to the point where the applicability of the descriptor "noble gas" has been questioned.{{cite journal |last=Roth |first=Klaus |year=2017 |title=Ist das Element 118 ein Edelgas? |trans-title=Is Element 118 a Noble Gas? |journal=Chemie in unserer Zeit |language=German |volume=51 |issue=6 |pages=418–426 |doi=10.1002/ciuz.201700838}}
Translated into English by W. E. Russey and published in three parts in ChemViews Magazine:
{{cite magazine |last=Roth |first=Klaus |date=3 April 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 1 |url=https://www.chemistryviews.org/details/ezine/10907570/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_1.html |magazine=ChemViews Magazine |doi=10.1002/chemv.201800029}}
{{cite magazine |last=Roth |first=Klaus |date=1 May 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 2 |url=https://www.chemistryviews.org/details/ezine/11002036/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_2.html |magazine=ChemViews Magazine |doi=10.1002/chemv.201800033}}
{{cite magazine |last=Roth |first=Klaus |date=5 June 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 3 |url=https://www.chemistryviews.org/details/ezine/11046703/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_3.html |magazine=ChemViews Magazine |doi=10.1002/chemv.201800046}} Oganesson is expected to be rather like silicon or tin in group 14:{{cite web |url=http://www.primefan.ru/stuff/chem/ptable/ptable.pdf |title=Есть ли граница у таблицы Менделеева? |trans-title=Is there a boundary to the Mendeleev table? |last=Kulsha |first=A. V. |website=www.primefan.ru |access-date=8 September 2018 |language=ru}} a reactive element with a common +4 and a less common +2 state,{{Cite journal |last1=Fricke |first1=Burkhard |year=1975 |title=Superheavy elements: a prediction of their chemical and physical properties |journal=Recent Impact of Physics on Inorganic Chemistry |volume=21 |pages=[https://archive.org/details/recentimpactofph0000unse/page/89 89–144] |doi=10.1007/BFb0116498 |url=https://archive.org/details/recentimpactofph0000unse/page/89 |access-date=4 October 2013 |series=Structure and Bonding |isbn=978-3-540-07109-9}}[http://www.primefan.ru/stuff/chem/front2019.png Russian periodic table poster] by A. V. Kulsha and T. A. Kolevich which at room temperature and pressure is not a gas but rather a solid semiconductor. Empirical / experimental testing will be required to validate these predictions.{{cite journal |last1=Mewes |first1=Jan-Michael |last2=Smits |first2=Odile Rosette |first3=Paul |last3=Jerabek |first4=Peter |last4=Schwerdtfeger |date=25 July 2019 |title=Oganesson is a Semiconductor: On the Relativistic Band-Gap Narrowing in the Heaviest Noble-Gas Solids |journal=Angewandte Chemie |volume=58 |issue=40 |pages=14260–14264 |doi=10.1002/anie.201908327 |pmid=31343819 |pmc=6790653 }} (On the other hand, flerovium, despite being in group 14, is predicted to be unusually volatile, which suggests noble gas-like properties.){{Cite journal |last=Kratz |first=J. V. |date=2012-08-01 |title=The impact of the properties of the heaviest elements on the chemical and physical sciences |journal=Radiochimica Acta |language=en |volume=100 |issue=8–9 |pages=569–578 |doi=10.1524/ract.2012.1963 |s2cid=97915854 |issn=2193-3405|doi-access=free }}{{Cite web |title=Indication for a volatile element 114 |url=http://doc.rero.ch/record/290779/files/ract.2010.1705.pdf |website=doc.rero.ch}}

The noble gases—including helium—can form stable molecular ions in the gas phase. The simplest is the helium hydride molecular ion, HeH⁺, discovered in 1925.{{cite journal |author1=Hogness, T. R. |author2=Lunn, E. G. |title=The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis |journal=Physical Review |year=1925 |volume=26 |issue=1 |pages=44–55 |doi=10.1103/PhysRev.26.44 |bibcode=1925PhRv...26...44H}} Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it was believed to occur naturally in the interstellar medium, and it was finally detected in April 2019 using the airborne SOFIA telescope. In addition to these ions, there are many known neutral excimers of the noble gases. These are compounds such as ArF and KrF that are stable only when in an excited electronic state; some of them find application in excimer lasers.

In addition to the compounds where a noble gas atom is involved in a covalent bond, noble gases also form non-covalent compounds. The clathrates, first described in 1949,{{cite journal |title=An Inert Gas Compound |author1=Powell, H. M. |author2=Guter, M. |name-list-style=amp |journal=Nature |volume=164 |pages=240–241 |year=1949 |doi=10.1038/164240b0 |pmid=18135950 |issue=4162|bibcode= 1949Natur.164..240P |s2cid=4134617 }} consist of a noble gas atom trapped within cavities of crystal lattices 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 hydroquinone, but helium and neon do not because they are too small or insufficiently polarizable to be retained.{{harvnb|Greenwood|1997|p=893}} Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice.{{cite journal |author=Dyadin, Yuri A.|doi=10.1070/MC1999v009n05ABEH001104 |title=Clathrate hydrates of hydrogen and neon |journal=Mendeleev Communications |volume=9 |issue=5 |year=1999 |pages=209–210|display-authors=etal}}

File:Endohedral fullerene.png

Noble gases can form 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 carbon atoms, is exposed to noble gases at high pressure, complexes 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).{{cite journal|title=Stable compounds of helium and neon. He@C60 and Ne@C60|author1=Saunders, M. |author2=Jiménez-Vázquez, H. A. |author3=Cross, R. J. |author4=Poreda, R. J. |journal=Science|year=1993|volume=259|pages=1428–1430|doi=10.1126/science.259.5100.1428|pmid=17801275|issue=5100|bibcode= 1993Sci...259.1428S |s2cid=41794612 }} As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been created.{{cite journal|title=Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure|author1=Saunders, Martin |author2=Jimenez-Vazquez, Hugo A. |author3=Cross, R. James |author4=Mroczkowski, Stanley |author5=Gross, Michael L. |author6=Giblin, Daryl E. |author7=Poreda, Robert J. |journal=J. Am. Chem. Soc.|year=1994|volume=116|issue=5|pages=2193–2194|doi=10.1021/ja00084a089|bibcode=1994JAChS.116.2193S }} 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.{{cite journal|last1=Frunzi|first1=Michael|last2=Cross|first2=R. Jame|last3=Saunders|first3=Martin|title=Effect of Xenon on Fullerene Reactions|journal=Journal of the American Chemical Society|year=2007|volume=129|doi=10.1021/ja075568n|pages=13343–13346|pmid=17924634|issue=43|bibcode=2007JAChS.12913343F |url=https://figshare.com/articles/journal_contribution/2977702}}

File:XeF2.svg

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 three-center four-electron bond model.{{harvnb|Greenwood|1997|p=897}}{{harvnb|Weinhold|2005|pp=275–306}} 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 orbitals (MOs) derived from p-orbitals on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from each 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 that is facilitated by the high electronegativity of fluorine.{{cite journal|last=Pimentel|first=G. C.|title= The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method|year=1951|issue=4|pages=446–448|doi=10.1063/1.1748245|journal=The Journal of Chemical Physics|volume=19|bibcode= 1951JChPh..19..446P }}

The chemistry of the heavier noble gases, krypton and xenon, are well established. The chemistry of the lighter ones, argon and helium, is still at an early stage, while a neon compound is yet to be identified.

{{clear}}

The abundances of the noble gases in the universe decrease as their atomic numbers 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 (and, to a very slight degree, the alpha decay of heavy elements).{{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|access-date=23 June 2008|archive-url=https://web.archive.org/web/20070208212728/http://www.einstein-online.info/en/spotlights/BBN_obs/index.html|archive-date=8 February 2007|url-status=dead}}{{cite journal|author=Coc, A.|title=Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements|journal=Astrophysical Journal|volume=600|year=2004|pages=544–552|doi=10.1086/380121|bibcode=2004ApJ...600..544C|issue=2|arxiv= astro-ph/0309480 |s2cid=16276658|display-authors=etal}}

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 helium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth's gravitational field.{{cite journal|first1=P.|last1=Morrison|last2=Pine|first2=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|bibcode= 1955NYASA..62...71M |s2cid=85015694}} Helium on Earth comes from the alpha decay of heavy elements such as uranium and thorium found in the Earth's crust, and tends to accumulate in natural gas deposits. 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 basis for the potassium-argon dating method.{{cite web|url=http://www.geoberg.de/text/geology/07011601.php|title=⁴⁰Ar/³⁹Ar dating and errors|access-date=26 June 2008|publisher=Technische Universität Bergakademie Freiberg|date=16 January 2007|last=Scherer|first=Alexandra |archive-url= https://web.archive.org/web/20071014042248/http://geoberg.de/text/geology/07011601.php |archive-date=14 October 2007}}

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.{{cite journal |first1=Chrystèle|last1=Sanloup |first2=Burkhard C. |last2=Schmidt |first3=Eva Maria Chamorro|last3=Perez |first4=Albert |last4=Jambon |first5=Eugene |last5=Gregoryanz |first6=Mohamed |last6=Mezouar |display-authors=2 |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|bibcode= 2005Sci...310.1174S |s2cid=31226092 }}{{Cite journal |last=Zhu |first=Li |last2=Liu |first2=Hanyu |last3=Pickard |first3=Chris J. |last4=Zou |first4=Guangtian |last5=Ma |first5=Yanming |date=July 2014 |title=Reactions of xenon with iron and nickel are predicted in the Earth's inner core |url=https://www.nature.com/articles/nchem.1925 |journal=Nature Chemistry |language=en |volume=6 |issue=7 |pages=644–648 |doi=10.1038/nchem.1925 |issn=1755-4349|arxiv=1309.2169 }} Radon is formed in the lithosphere by 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.{{cite web| title= A Citizen's Guide to Radon| publisher= U.S. Environmental Protection Agency| date= 26 November 2007| url= http://www.epa.gov/radon/pubs/citguide.html| access-date= 26 June 2008}} Oganesson does not occur in nature and is instead created manually by scientists.

For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain up to 7% helium.{{cite web| author = Winter, Mark| title = Helium: the essentials| publisher = University of Sheffield|year = 2008| url = http://www.webelements.com/helium/| access-date = 14 July 2008}}

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. 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 adjacent table lists the 2004 prices in the United States for laboratory quantities of each gas.

None of the elements in this group has any biological importance.{{cite journal |last1=Remick |first1=Kaleigh |last2=Helmann |first2=John D. |title=The Elements of Life: A Biocentric Tour of the Periodic Table |journal=Advances in Microbial Physiology |publisher=PubMed Central |date=30 January 2023 |volume=82 |pages=1–127 |doi=10.1016/bs.ampbs.2022.11.001 |pmid=36948652 |pmc=10727122 |isbn=978-0-443-19334-7 |quote=The group 18 elements (noble gases) are non-reactive and not biologically important.}}

File:Modern 3T MRI.JPG

Noble gases have very low boiling and melting points, which makes them useful as cryogenic refrigerants.{{cite encyclopedia|title=Neon|encyclopedia=Encarta|year=2008}} In particular, liquid helium, which boils at {{convert|4.2|K}}, is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear magnetic resonance.{{cite journal|title=Demountable coaxial gas-cooled current leads for MRI superconducting magnets|author1=Zhang, C. J. |author2=Zhou, X. T. |author3=Yang, L. |journal=IEEE Transactions on Magnetics|publisher=IEEE|volume=28|issue=1|year=1992|pages=957–959|doi=10.1109/20.120038|bibcode= 1992ITM....28..957Z }} Liquid neon, although it does not reach temperatures as low as liquid helium, also finds use in cryogenics because it has over 40 times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen.

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 tissues when under pressure like in scuba diving, which causes an anesthetic effect known as nitrogen narcosis.{{cite journal |last1=Fowler |first1=B. |last2=Ackles |first2=K. N. |last3=Porlier |first3=G. |title=Effects of inert gas narcosis on behavior—a critical review |journal=Undersea Biomed. Res. |volume=12 |issue=4 |pages=369–402 |year=1985 |issn=0093-5387 |oclc=2068005 |pmid=4082343 |url=http://archive.rubicon-foundation.org/3019 |access-date=8 April 2008 |archive-url=https://web.archive.org/web/20101225052236/http://archive.rubicon-foundation.org/3019 |archive-date=25 December 2010 |url-status=dead }} 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 or heliox, a decrease in the narcotic effect of the gas at depth is obtained.{{harvnb|Bennett|1998|p=176}} Helium's reduced solubility offers further advantages for the condition known as decompression sickness, or the bends.{{cite journal |editor-last=Vann|editor-first=R. D. |title=The Physiological Basis of Decompression|journal=38th Undersea and Hyperbaric Medical Society Workshop |volume=75(Phys)6-1-89 |year=1989 |pages=437 |url=http://archive.rubicon-foundation.org/6853 |archive-url=https://web.archive.org/web/20081007192913/http://archive.rubicon-foundation.org/6853 |url-status=dead |archive-date=7 October 2008 |access-date=31 May 2008}} 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.{{cite web |last=Maiken |first=Eric |title=Why Argon? |url=http://www.decompression.org/maiken/Why_Argon.htm |access-date=26 June 2008|publisher=Decompression|date=1 August 2004}} Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.{{cite journal|last1=Horhoianu|first1=G.|title=Thermal behaviour of CANDU type fuel rods during steady state and transient operating conditions|journal=Annals of Nuclear Energy|volume=26|pages=1437–1445|year=1999|doi=10.1016/S0306-4549(99)00022-5|issue=16|last2=Ionescu|first2=D. V.|last3=Olteanu|first3=G.|bibcode=1999AnNuE..26.1437H }}

File:Goodyear-blimp.jpg

Since the Hindenburg disaster in 1937,{{cite news|title=Disaster Ascribed to Gas by Experts|work=The New York Times|date=7 May 1937|page=1}} helium has replaced hydrogen as a lifting gas in blimps and balloons: despite an 8.6%{{cite web

|last=Freudenrich|first=Craig|year=2008

|url=http://science.howstuffworks.com/blimp2.htm

|title=How Blimps Work|publisher=HowStuffWorks

|access-date=3 July 2008}} decrease in buoyancy compared to hydrogen, helium is not combustible.

In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis of air-sensitive compounds that are sensitive to nitrogen. Solid argon is also used for the study of very unstable compounds, such as reactive intermediates, by trapping them in an inert matrix at very low temperatures.{{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.}} 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. Helium and argon are both commonly used to shield welding arcs 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.

File:Xenon short arc 1.jpg used in IMAX projectors|alt=Elongated glass sphere with two metal rod electrodes inside, facing each other. One electrode is blunt and another is sharpened.]]

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 bulbs.{{cite book |author1=Häussinger, Peter |author2=Glatthaar, Reinhard |author3=Rhode, Wilhelm |author4=Kick, Helmut |author5=Benkmann, Christian |author6=Weber, Josef |author7=Wunschel, Hans-Jörg |author8=Stenke, Viktor |author9=Leicht, Edith |author10=Stenger, Hermann |chapter=Noble gases |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |year=2002 |doi=10.1002/14356007.a17_485|isbn=3-527-30673-0 }} Krypton is used in high-performance light bulbs, which have higher color temperatures 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. The noble gases glow in distinctive colors when used inside gas-discharge lamps, such as "neon lights". These lights are called after neon but often contain other gases and phosphors, which add various hues to the orange-red color of neon. Xenon is commonly used in xenon arc lamps, which, due to their nearly continuous spectrum that resembles daylight, find application in film projectors and as automobile headlamps.

The noble gases are used in excimer lasers, which are based on short-lived electronically excited molecules known as excimers. The excimers used for lasers may be noble gas dimers such as Ar₂, Kr₂ or Xe₂, 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 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.{{cite book |title=Excimer Laser Technology |author1=Basting, Dirk |author2=Marowsky, Gerd |publisher=Springer |year=2005 |isbn=3-540-20056-8}}

Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people with asthma. Xenon is used as an anesthetic because of its high 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.{{cite journal|author1=Sanders, Robert D. |author2=Ma, Daqing |author3=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|pmid=15728132|doi-access=free}} Xenon finds application in medical imaging of the lungs through hyperpolarized MRI.{{cite journal|last1=Albert|first1=M. S.|last2=Balamore|first2=D.|title=Development of hyperpolarized noble gas MRI |journal=Nuclear Instruments and Methods in Physics Research A|year=1998|volume=402|pages=441–453|doi= 10.1016/S0168-9002(97)00888-7|pmid=11543065|issue=2–3|bibcode= 1998NIMPA.402..441A }} Radon, which is highly radioactive and is only available in minute amounts, is used in radiotherapy.

Noble gases, particularly xenon, are predominantly used in ion engines due to their inertness. Since ion engines are not driven by chemical reactions, chemically inert fuels are desired to prevent unwanted reaction between the fuel and anything else on the engine.

Oganesson is too unstable to work with and has no known application other than research.

The relative isotopic abundances of noble gases serve as an important geochemical tracing tool in earth science.{{Cite book |url=http://link.springer.com/10.1007/978-3-642-28836-4 |title=The Noble Gases as Geochemical Tracers |date=2013 |publisher=Springer Berlin Heidelberg |isbn=978-3-642-28835-7 |editor-last=Burnard |editor-first=Pete |series=Advances in Isotope Geochemistry |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-642-28836-4|bibcode=2013nggt.book.....B }} They can unravel the Earth's degassing history and its effects to the surrounding environment (i.e., atmosphere composition{{Citation |last1=Ballentine |first1=Chris J. |title=13. Tracing Fluid Origin, Transport and Interaction in the Crust |date=2018-12-17 |pages=539–614 |url=https://www.degruyter.com/document/doi/10.1515/9781501509056-015/html |access-date=2024-09-23 |publisher=De Gruyter |language=en |doi=10.1515/9781501509056-015 |isbn=978-1-5015-0905-6 |last2=Burgess |first2=Ray |last3=Marty |first3=Bernard}}). Due to their inert nature and low abundances, change in the noble gas concentration and their isotopic ratios can be used to resolve and quantify the processes influencing their current signatures across geological settings.{{Cite journal |last1=Ballentine |first1=C. J. |last2=Burnard |first2=P. G. |date=2002-01-01 |title=Production, Release and Transport of Noble Gases in the Continental Crust |url=https://pubs.geoscienceworld.org/rimg/article/47/1/481-538/235410 |journal=Reviews in Mineralogy and Geochemistry |language=en |volume=47 |issue=1 |pages=481–538 |doi=10.2138/rmg.2002.47.12 |bibcode=2002RvMG...47..481B |issn=1529-6466}}   

Helium has two abundant isotopes: helium-3, which is primordial with high abundance in earth's core and mantle, and helium-4, which originates from decay of radionuclides (²³²Th, ^235,238U) abundant in the earth's crust. Isotopic ratios of helium are represented by R_A value, a value relative to air measurement (³He/⁴He = 1.39*10⁻⁶).{{Cite book |last1=Ozima |first1=Minoru |url=https://books.google.com/books?id=TMIAfSIe428C&pg=PP1 |title=Noble Gas Geochemistry |last2=Podosek |first2=Frank A. |date=2002 |publisher=Cambridge University Press |isbn=978-0-521-80366-3 |language=en}} Volatiles that originate from the earth's crust have a 0.02-0.05 R_A, which indicate an enrichment of helium-4.{{Cite journal |last1=Ballentine |first1=Chris J. |last2=Sherwood Lollar |first2=Barbara |date=July 2002 |title=Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703702008505 |journal=Geochimica et Cosmochimica Acta |language=en |volume=66 |issue=14 |pages=2483–2497 |doi=10.1016/S0016-7037(02)00850-5|bibcode=2002GeCoA..66.2483B }} Volatiles that originate from deeper sources such as subcontinental lithospheric mantle (SCLM), have a 6.1± 0.9 R_A{{Cite journal |last1=Gautheron |first1=Cécile |last2=Moreira |first2=Manuel |date=2002-05-30 |title=Helium signature of the subcontinental lithospheric mantle |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X02005630 |journal=Earth and Planetary Science Letters |volume=199 |issue=1 |pages=39–47 |doi=10.1016/S0012-821X(02)00563-0 |bibcode=2002E&PSL.199...39G |issn=0012-821X}} and mid-oceanic ridge basalts (MORB) display higher values (8 ± 1 R_A). Mantle plume samples have even higher values than > 8 R_A.{{Cite journal |last=Graham |first=D. W. |date=2002-01-01 |title=Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs |url=https://pubs.geoscienceworld.org/rimg/article/47/1/247-317/235393 |journal=Reviews in Mineralogy and Geochemistry |language=en |volume=47 |issue=1 |pages=247–317 |doi=10.2138/rmg.2002.47.8 |bibcode=2002RvMG...47..247G |issn=1529-6466}} Solar wind, which represent an unmodified primordial signature is reported to have ~ 330 R_A.{{Cite journal |last1=Benkert |first1=Jean-Paul |last2=Baur |first2=Heinrich |last3=Signer |first3=Peter |last4=Wieler |first4=Rainer |date=1993-07-25 |title=He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/93JE01460 |journal=Journal of Geophysical Research: Planets |language=en |volume=98 |issue=E7 |pages=13147–13162 |doi=10.1029/93JE01460 |bibcode=1993JGR....9813147B |issn=0148-0227}}   

Neon has three main stable isotopes:²⁰Ne, ²¹Ne and ²²Ne, with ²⁰Ne produced by cosmic nucleogenic reactions, causing high abundance in the atmosphere.{{Cite journal |last=Wetherill |first=George W. |date=1954-11-01 |title=Variations in the Isotopic Abundances of Neon and Argon Extracted from Radioactive Minerals |url=https://journals.aps.org/pr/abstract/10.1103/PhysRev.96.679 |journal=Physical Review |volume=96 |issue=3 |pages=679–683 |doi=10.1103/PhysRev.96.679|bibcode=1954PhRv...96..679W }} ²¹Ne and ²²Ne are produced in the earth's crust as a result of interactions between alpha and neutron particles with light elements; ¹⁸O, ¹⁹F and ^24,25Mg.{{Cite journal |last1=Yatsevich |first1=Igor |last2=Honda |first2=Masahiko |date=1997-05-10 |title=Production of nucleogenic neon in the Earth from natural radioactive decay |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/97JB00395 |journal=Journal of Geophysical Research: Solid Earth |language=en |volume=102 |issue=B5 |pages=10291–10298 |doi=10.1029/97JB00395 |bibcode=1997JGR...10210291Y |issn=0148-0227}} The neon ratios (²⁰Ne/²²Ne and ²¹Ne/²²Ne) are systematically used to discern the heterogeneity in the Earth's mantle and volatile sources. Complimenting He isotope data, neon isotope data additionally provide insight to thermal evolution of Earth's systems.{{Cite journal |last1=Tremblay |first1=Marissa M. |last2=Shuster |first2=David L. |last3=Balco |first3=Greg |last4=Cassata |first4=William S. |date=2017-05-15 |title=Neon diffusion kinetics and implications for cosmogenic neon paleothermometry in feldspars |url=https://linkinghub.elsevier.com/retrieve/pii/S001670371730090X |journal=Geochimica et Cosmochimica Acta |volume=205 |pages=14–30 |doi=10.1016/j.gca.2017.02.013 |bibcode=2017GeCoA.205...14T |issn=0016-7037}}       

Argon has three stable isotopes: ³⁶Ar, ³⁸Ar and ⁴⁰Ar. ³⁶Ar and ³⁸Ar are primordial, with their inventory on the earth's crust dependent on the equilibration of meteoric water with the crustal fluids. This explains huge inventory of ³⁶Ar in the atmosphere. Production of these two isotopes (³⁶Ar and ³⁸Ar) is negligible within the earth's crust, only limited concentrations of ³⁸Ar can be produced by interaction between alpha particles from decay of ^235,238U and ²³²Th and light elements (³⁷Cl and ⁴¹K). While ³⁶Ar is continuously being produced by Beta-decay of ³⁶Cl.{{Cite journal |last1=Fleming |first1=W. H. |last2=Thode |first2=H. G. |date=1953-10-15 |title=Neutron and Spontaneous Fission in Uranium Ores |url=https://link.aps.org/doi/10.1103/PhysRev.92.378 |journal=Physical Review |language=en |volume=92 |issue=2 |pages=378–382 |doi=10.1103/PhysRev.92.378 |bibcode=1953PhRv...92..378F |issn=0031-899X}} ⁴⁰Ar is a product of radiogenic decay of ⁴⁰K. Different endmembers values for ⁴⁰Ar/³⁶Ar have been reported; Air = 295.5,{{Cite journal |last1=Burnard |first1=Pete |last2=Graham |first2=David |last3=Turner |first3=Grenville |date=1997-04-25 |title=Vesicle-Specific Noble Gas Analyses of "Popping Rock": Implications for Primordial Noble Gases in Earth |url=http://dx.doi.org/10.1126/science.276.5312.568 |journal=Science |volume=276 |issue=5312 |pages=568–571 |doi=10.1126/science.276.5312.568 |pmid=9110971 |issn=0036-8075}} MORB = 40,000, and crust = 3000.   

Krypton has several isotopes, with ^{78, 80, 82}Kr being primordial, while ^{83,84, 86}Kr results from spontaneous fission of ²⁴⁴Pu and radiogenic decay of ²³⁸U.{{Cite journal |last1=Mukhopadhyay |first1=Sujoy |last2=Parai |first2=Rita |date=2019-05-30 |title=Noble Gases: A Record of Earth's Evolution and Mantle Dynamics |url=https://www.annualreviews.org/doi/10.1146/annurev-earth-053018-060238 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=47 |issue=1 |pages=389–419 |doi=10.1146/annurev-earth-053018-060238 |bibcode=2019AREPS..47..389M |issn=0084-6597}} Krypton's isotopes geochemical signature in mantle reservoirs resembling the modern atmosphere. preserves the solar-like primordial signature.{{Cite journal |last1=Holland |first1=Greg |last2=Ballentine |first2=Chris J. |date=May 2006 |title=Seawater subduction controls the heavy noble gas composition of the mantle |url=https://www.nature.com/articles/nature04761 |journal=Nature |language=en |volume=441 |issue=7090 |pages=186–191 |doi=10.1038/nature04761 |pmid=16688169 |bibcode=2006Natur.441..186H |issn=0028-0836}} Krypton isotopes have been used to decipher the mechanism of volatiles delivery to earth's system, which had great implication to evolution of earth (nitrogen, oxygen, and oxygen) and emergence of life.{{Cite journal |last1=Péron |first1=Sandrine |last2=Mukhopadhyay |first2=Sujoy |last3=Kurz |first3=Mark D. |last4=Graham |first4=David W. |date=December 2021 |title=Deep-mantle krypton reveals Earth's early accretion of carbonaceous matter |url=https://www.nature.com/articles/s41586-021-04092-z |journal=Nature |language=en |volume=600 |issue=7889 |pages=462–467 |doi=10.1038/s41586-021-04092-z |pmid=34912082 |bibcode=2021Natur.600..462P |issn=1476-4687}} This is largely due to a clear distinction of krypton isotope signature from various sources such as chondritic material, solar wind and cometary.{{Cite journal |last=Marty |first=Bernard |date=2012-01-01 |title=The origins and concentrations of water, carbon, nitrogen and noble gases on Earth |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X11006388 |journal=Earth and Planetary Science Letters |volume=313-314 |pages=56–66 |doi=10.1016/j.epsl.2011.10.040 |arxiv=1405.6336 |bibcode=2012E&PSL.313...56M |issn=0012-821X}}{{Cite journal |last1=Marty |first1=B. |last2=Altwegg |first2=K. |last3=Balsiger |first3=H. |last4=Bar-Nun |first4=A. |last5=Bekaert |first5=D. V. |last6=Berthelier |first6=J.-J. |last7=Bieler |first7=A. |last8=Briois |first8=C. |last9=Calmonte |first9=U. |last10=Combi |first10=M. |last11=De Keyser |first11=J. |last12=Fiethe |first12=B. |last13=Fuselier |first13=S. A. |last14=Gasc |first14=S. |last15=Gombosi |first15=T. I. |date=2017-06-09 |title=Xenon isotopes in 67P/Churyumov-Gerasimenko show that comets contributed to Earth's atmosphere |url=https://www.science.org/doi/10.1126/science.aal3496 |journal=Science |language=en |volume=356 |issue=6342 |pages=1069–1072 |doi=10.1126/science.aal3496 |pmid=28596364 |bibcode=2017Sci...356.1069M |issn=0036-8075}}  

{{main|Xenon isotope geochemistry}}

Xenon has nine isotopes, most of which are produced by the radiogenic decay. Krypton and xenon noble gases requires pristine, robust geochemical sampling protocol to avoid atmospheric contamination.{{Cite journal |last1=Farley |first1=K. A. |last2=Neroda |first2=E. |date=May 1998 |title=Noble Gases in the Earth's Mantle |url=https://www.annualreviews.org/doi/10.1146/annurev.earth.26.1.189 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=26 |issue=1 |pages=189–218 |doi=10.1146/annurev.earth.26.1.189 |bibcode=1998AREPS..26..189F |issn=0084-6597}} Furthermore, sophisticated instrumentation is required to resolve mass peaks among many isotopes with narrow mass difference during analysis.   

Noble gas measurements can be obtained from sources like volcanic vents, springs, and geothermal wells following specific sampling protocols.{{Cite journal |last1=Hilton |first1=D. R. |last2=Fischer |first2=T. P. |last3=Marty |first3=B. |date=2002-01-01 |title=Noble Gases and Volatile Recycling at Subduction Zones |url=https://pubs.geoscienceworld.org/rimg/article/47/1/319-370/235395 |journal=Reviews in Mineralogy and Geochemistry |language=en |volume=47 |issue=1 |pages=319–370 |doi=10.2138/rmg.2002.47.9 |bibcode=2002RvMG...47..319H |issn=1529-6466}} The classic specific sampling protocol include the following.

• Copper tubes - These are standard refrigeration copper tubes, cut to ~10 cm³ with a 3/8” outer diameter, and are used for sampling volatile discharges by connecting an inverted funnel to the tube via TygonⓇ tubing, ensuring one-way inflow and preventing air contamination. Their malleability allows for cold welding or pinching off to seal the ends after sufficient flushing with the sample.
• File:Giggenbach_Bottle.jpgGiggenbach bottles - Giggenbach bottles are evacuated glass flasks with a Teflon stopcock, used for sampling and storing gases. They require pre-evacuation before sampling, as noble gases accumulate in the headspace.{{Cite web |title=Methods for the collection and analysis of geothermal and volcanic water and gas samples |url=https://search.worldcat.org/title/316795976 |access-date=2024-10-19 |website=search.worldcat.org |language=en}} These bottles were first invented and distributed by a Werner F. Giggenbach, a German chemist.{{Cite journal |last=Giggenbach |first=W. F. |date=1975-03-01 |title=A simple method for the collection and analysis of volcanic gas samples |url=https://link.springer.com/article/10.1007/BF02596953 |journal=Bulletin Volcanologique |language=en |volume=39 |issue=1 |pages=132–145 |doi=10.1007/BF02596953 |bibcode=1975BVol...39..132G |issn=1432-0819}}

Noble gases have numerous isotopes and subtle mass variation that requires high-precision detection systems. Originally, scientists used magnetic sector mass spectrometry, which is time-consuming and has low sensitivity due to "peak jumping mode".{{Cite journal |url=https://pubs.aip.org/rsi/article/27/11/928/299004/High-Sensitivity-Mass-Spectrometer-for-Noble-Gas |access-date=2024-10-16 |journal=Review of Scientific Instruments |doi=10.1063/1.1715415 |title=High Sensitivity Mass Spectrometer for Noble Gas Analysis |date=1956 |last1=Reynolds |first1=John H. |volume=27 |issue=11 |pages=928–934 |bibcode=1956RScI...27..928R }}{{Cite journal |last1=Mark |first1=D. F. |last2=Barfod |first2=D. |last3=Stuart |first3=F. M. |last4=Imlach |first4=J. |date=October 2009 |title=The ARGUS multicollector noble gas mass spectrometer: Performance for 40 Ar/ 39 Ar geochronology |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2009GC002643 |journal=Geochemistry, Geophysics, Geosystems |language=en |volume=10 |issue=10 |doi=10.1029/2009GC002643 |issn=1525-2027}} Multiple-collector mass spectrometers, like Quadrupole mass spectrometers (QMS), enable simultaneous detection of isotopes, improving sensitivity and throughput. Before analysis, sample preparation is essential due to the low abundance of noble gases, involving extraction, purification system. Extraction allows liberation of noble gases from their carrier (major phase; fluid or solid) while purification remove impurities and improve concentration per unit sample volume.{{Cite journal |last1=Mtili |first1=K. M. |last2=Byrne |first2=D. J. |last3=Tyne |first3=R. L. |last4=Kazimoto |first4=E. O. |last5=Kimani |first5=C. N. |last6=Kasanzu |first6=C. H. |last7=Hillegonds |first7=D. J. |last8=Ballentine |first8=C. J. |last9=Barry |first9=P. H. |date=2021-12-20 |title=The origin of high helium concentrations in the gas fields of southwestern Tanzania |url=https://linkinghub.elsevier.com/retrieve/pii/S000925412100485X |journal=Chemical Geology |volume=585 |pages=120542 |doi=10.1016/j.chemgeo.2021.120542 |bibcode=2021ChGeo.58520542M |issn=0009-2541}} Cryogenic traps are used for sequential analysis without peak interference by stepwise temperature raise.{{Cite journal |last1=Li |first1=Yan |last2=Tootell |first2=Damian |last3=Holland |first3=Greg |last4=Zhou |first4=Zheng |date=November 2021 |title=Performance of the NGX High-Resolution Multiple Collector Noble Gas Mass Spectrometer |journal=Geochemistry, Geophysics, Geosystems |language=en |volume=22 |issue=11 |doi=10.1029/2021GC009997 |bibcode=2021GGG....2209997L |issn=1525-2027|doi-access=free }}

Research labs have successfully developed miniaturized field-based mass spectrometers, such as the portable mass spectrometer ([https://gasometrix.com/products/ miniRuedi]), which can analyze noble gases with an analytical uncertainty of 1-3% using low-cost vacuum systems and quadrupole mass analyzers.{{Cite journal |last1=Brennwald |first1=Matthias S. |last2=Schmidt |first2=Mark |last3=Oser |first3=Julian |last4=Kipfer |first4=Rolf |date=2016-12-20 |title=A Portable and Autonomous Mass Spectrometric System for On-Site Environmental Gas Analysis |url=https://pubs.acs.org/doi/10.1021/acs.est.6b03669 |journal=Environmental Science & Technology |language=en |volume=50 |issue=24 |pages=13455–13463 |doi=10.1021/acs.est.6b03669 |pmid=27993051 |bibcode=2016EnST...5013455B |issn=0013-936X}} File:Gchron.copernicus.org.jpg

The color of gas discharge emission depends on several factors, including the following:{{cite book|pages=383–384|url=https://books.google.com/books?id=AEFPNfghI3QC&pg=PA383|title=Scientific photography and applied imaging|author=Ray, Sidney F.|publisher=Focal Press|year=1999|isbn=0-240-51323-1}}

• discharge parameters (local value of current density and electric field, temperature, etc. – note the color variation along the discharge in the top row);
• gas purity (even small fraction of certain gases can affect color);
• material of the discharge tube envelope – note suppression of the UV and blue components in the bottom-row tubes made of thick household glass.

• 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.
• Industrial gas
• Octet rule

{{Reflist|30em}}

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• {{cite book|title=The Physiology and Medicine of Diving|last1= Bennett|first1= Peter B.|last2= Elliott|first2=David H.|publisher=SPCK Publishing |year=1998|isbn=0-7020-2410-4|ref=CITEREFBennett1998}}
• {{cite book|author=Bobrow Test Preparation Services|title=CliffsAP Chemistry|publisher=CliffsNotes|date=5 December 2007|isbn=978-0-470-13500-6|ref=CITEREFCliffsNotes2007}}
• {{cite book|last1=Greenwood |first1= N. N. |last2=Earnshaw|first2=A.|title=Chemistry of the Elements |edition= 2nd |publisher=Oxford:Butterworth-Heinemann|year=1997|isbn=0-7506-3365-4|ref=CITEREFGreenwood1997}}
• {{cite book|last1=Harding|first1=Charlie J.|last2=Janes|first2=Rob|year=2002|title=Elements of the P Block|publisher=Royal Society of Chemistry|isbn= 0-85404-690-9 |ref=CITEREFHarding2002}}
• {{cite book|last= Holloway|first= John H.|year= 1968|title= Noble-Gas Chemistry|publisher= Methuen Publishing|location= London|isbn= 0-412-21100-9|url-access= registration|url= https://archive.org/details/trent_0116400178434}}
• {{cite book|last=Mendeleev|first=D.|author-link=Dmitri Mendeleev|title= Osnovy Khimii (The Principles of Chemistry)|edition=7th|date=1902–1903|publisher=Collier |location=New York |language=ru|url=https://archive.org/details/principlesofchem00menduoft|ref=CITEREFMendeleev1903}}
• {{cite book|first1=Minoru|last1=Ozima|last2=Podosek|first2=Frank A.|title=Noble Gas Geochemistry|year=2002|publisher=Cambridge University Press|isbn=0-521-80366-7|ref=CITEREFOzima2002}}
• {{cite book|last1=Weinhold|first1=F.|last2=Landis|first2=C.|title=Valency and bonding|publisher=Cambridge University Press|year=2005|isbn=0-521-83128-8|ref=CITEREFWeinhold2005}}

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