Wikipedia

Noble gas

Thenoble gases(historically theinert gases,sometimes referred to asaerogens[1]) are the naturally occurring members ofgroup18 of theperiodic table:helium(He),neon(Ne),argon(Ar),krypton(Kr),xenon(Xe), andradon(Rn). Understandard conditions,theseelementsare odorless, colorless,monatomicgases with very lowchemical reactivityandcryogenicboiling points.

Noble gases
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
halogens←→alkali metals
IUPAC group number 18
Name by element helium groupor
neon group
Trivial name noble gases
CAS group number
(US, pattern A-B-A)
 VIIIA
old IUPAC number
(Europe, pattern A-B)
 0
↓Period
1
Image: Helium discharge tube
Helium(He)
2
2
Image: Neon discharge tube
Neon(Ne)
10
3
Image: Argon discharge tube
Argon(Ar)
18
4
Image: Krypton discharge tube
Krypton(Kr)
36
5
Image: Xenon discharge tube
Xenon(Xe)
54
6 Radon(Rn)
86
7 Oganesson(Og)
118

Legend

primordial element
element by radioactive decay

The noble gases'inertness,or tendency not toreactwith otherchemical substances,results from theirelectron configuration:theirouter shellofvalence electronsis "full", giving them little tendency to participate inchemical reactions.Only a few hundrednoble gas compoundsare known to exist. For the same reason[clarification needed],noble gasatomsare small, and the onlyintermolecular forcebetween them is the very weakLondon dispersion force,so their boiling points are all cryogenic, below 165 K (−108 °C; −163 °F).[2]

The inertness of noble gases makes them useful whenever chemical reactions are unwanted. For example, argon is used as ashielding gasinweldingand as a filler gas inincandescent light bulbs.Helium is used to provide buoyancy inblimpsandballoons.Helium and neon are also used asrefrigerantsdue to their lowboiling points.Industrialquantities of the noble gases, except for radon, are obtained by separating them fromairusing the methods ofliquefaction of gasesandfractional distillation.Helium is also a byproduct of the mining ofnatural gas.Radon is usually isolated from theradioactive decayof dissolvedradium,thorium,oruraniumcompounds.

The seventh member of group 18 isoganesson(Og), anunstablesynthetic elementwhose chemistry is still uncertain because only five very short-lived atoms (t1/2= 0.69 ms) have ever been synthesized (as of 2020[3]).IUPACuses the term "noble gas" interchangeably with "group 18" and thus includes oganesson;[4]however, due torelativistic effects,oganesson is predicted to be asolidunder standard conditions and reactive enough not to qualify functionally as "noble".[3]In the rest of this article, the term "noble gas" should be understoodnotto include oganesson unless it is specifically mentioned.

History

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Noble gasis translated from theGermannounEdelgas,first used in 1900 byHugo Erdmann[5]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 asinert gases,but this label is deprecated as manynoble gas compoundsare now known.[6]Rare gasesis another term that was used,[7]but this is also inaccurate becauseargonforms a fairly considerable part (0.94% by volume, 1.3% by mass) of theEarth's atmospheredue to decay of radioactivepotassium-40.[8]

Helium was first detected in the Sun due to its characteristicspectral lines.

Pierre JanssenandJoseph Norman Lockyerhad discovered a new element on 18 August 1868 while looking at thechromosphereof theSun,and named itheliumafter the Greek word for the Sun,ἥλιος(hḗlios).[9]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 physicistHenry Cavendishhad discovered that air contains a small proportion of a substance less reactive thannitrogen.[10]A century later, in 1895,Lord Rayleighdiscovered that samples of nitrogen from the air were of a differentdensitythan nitrogen resulting fromchemical reactions.Along with Scottish scientistWilliam RamsayatUniversity 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 wordἀργός(argós,"idle" or "lazy" ).[10]With this discovery, they realized an entire class ofgaseswas missing from the periodic table. During his search for argon, Ramsay also managed to isolate helium for the first time while heatingcleveite,a mineral. In 1902, having accepted the evidence for the elements helium and argon,Dmitri Mendeleevincluded these noble gases as group 0 in his arrangement of the elements, which would later become the periodic table.[11]

Ramsay continued his search for these gases using the method offractional distillationto separateliquid airinto several components. In 1898, he discovered the elementskrypton,neon,andxenon,and named them after the Greek wordsκρυπτός(kryptós,"hidden" ),νέος(néos,"new" ), andξένος(ksénos,"stranger" ), respectively.Radonwas first identified in 1898 byFriedrich Ernst Dorn,[12]and was namedradium emanation,but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases.[13]Rayleigh and Ramsay received the 1904Nobel Prizesin Physics and in Chemistry, respectively, for their discovery of the noble gases;[14][15]in the words of J. E. Cederblom, then president of theRoyal 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".[15]

The discovery of the noble gases aided in the development of a general understanding ofatomic structure.In 1895, French chemistHenri Moissanattempted to form a reaction betweenfluorine,the mostelectronegativeelement, 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 physicistNiels Bohrproposed in 1913 that theelectronsin atoms are arranged inshellssurrounding thenucleus,and that for all noble gases except helium the outermost shell always contains eight electrons.[13]In 1916,Gilbert N. Lewisformulated theoctet 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.[16]

In 1962,Neil Bartlettdiscovered the first chemical compound of a noble gas,xenon hexafluoroplatinate.[17]Compounds of other noble gases were discovered soon after: in 1962 for radon,radon difluoride(RnF
2),[18]which was identified by radiotracer techniques and in 1963 for krypton,krypton difluoride(KrF
2).[19]The first stable compound of argon was reported in 2000 whenargon fluorohydride(HArF) was formed at a temperature of 40 K (−233.2 °C; −387.7 °F).[20]

In October 2006, scientists from the Joint Institute for Nuclear Research andLawrence Livermore National Laboratorysuccessfully created syntheticallyoganesson,the seventh element in group 18,[21]by bombardingcaliforniumwith calcium.[22]

Physical and atomic properties

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Property[13][23] Helium Neon Argon Krypton Xenon Radon Oganesson
Density(g/dm3) 0.1786 0.9002 1.7818 3.708 5.851 9.97 7200 (predicted)[24]
Boiling point(K) 4.4 27.3 87.4 121.5 166.6 211.5 450±10 (predicted)[24]
Melting point(K) –[25] 24.7 83.6 115.8 161.7 202.2 325±15 (predicted)[24]
Enthalpy of vaporization(kJ/mol) 0.08 1.74 6.52 9.05 12.65 18.1 –
Solubilityin water at 20 °C (cm3/kg) 8.61 10.5 33.6 59.4 108.1 230 –
Atomic number 2 10 18 36 54 86 118
Atomic radius(calculated) (pm) 31 38 71 88 108 120 –
Ionization energy(kJ/mol) 2372 2080 1520 1351 1170 1037 839 (predicted)[26]
Electronegativity[27] 4.16 4.79 3.24 2.97 2.58 2.60 2.59[28]
For more data, seeNoble gas (data page).

The noble gases have weakinteratomic force,and consequently have very lowmeltingandboiling points.They are allmonatomicgasesunderstandard conditions,including theelementswith largeratomic massesthan many normallysolidelements.[13]Heliumhas several unique qualities when compared with other elements: its boiling point at 1 atm is lower than those of any other knownsubstance;it is the only element known to exhibitsuperfluidity;and, it is the only element that cannot be solidified by cooling atatmospheric pressure[29](an effect explained byquantum mechanicsas itszero point energyis too high to permitfreezing)[30]– apressureof 25standard atmospheres(2,500kPa;370psi) must be applied at atemperatureof 0.95 K (−272.200 °C; −457.960 °F) to convert it to a solid[29]while a pressure of about 113,500 atm (11,500,000 kPa; 1,668,000 psi) is required atroom temperature.[31]The noble gases up to xenon have multiple stableisotopes;krypton and xenon also have naturally occurringradioisotopes,namely78Kr,124Xe,and136Xe,all have very long lives (> 1021years) and can undergodouble electron captureordouble beta decay.Radon has nostable isotopes;its longest-lived isotope,222Rn,has ahalf-lifeof 3.8 days and decays to form helium andpolonium,which ultimately decays tolead.[13]Oganesson also has no stable isotopes, and its only known isotope294Ogis very short-lived (half-life 0.7 ms). Melting and boiling points increase going down the group.

This is a plot ofionization potentialversusatomic number.The noble gases have the largest ionization potential for each period, although period 7 is expected to break this trend because the predictedfirst ionization energyof oganesson (Z = 118) is lower than those of elements 110-112.

The noble gasatoms,like atoms in most groups, increase steadily inatomic radiusfrom oneperiodto the next due to the increasing number ofelectrons.Thesize of the atomis related to several properties. For example, theionization potentialdecreases with an increasing radius because thevalence electronsin the larger noble gases are farther away from thenucleusand 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 ofchemical reactivity.[23]Some of the heavier noble gases, however, have ionization potentials small enough to be comparable to those of other elements andmolecules.It was the insight that xenon has an ionization potential similar to that of theoxygenmolecule that ledBartlettto attempt oxidizing xenon usingplatinum hexafluoride,anoxidizing agentknown to be strong enough to react with oxygen.[17]Noble gases cannot accept an electron to form stableanions;that is, they have a negativeelectron affinity.[32]

Themacroscopicphysical propertiesof the noble gases are dominated by the weakvan der Waals forcesbetween the atoms. The attractiveforceincreases with the size of the atom as a result of the increase inpolarizabilityand the decrease in ionization potential. This results in systematic group trends: as one goes down group 18, the atomic radius increases, and with it theinteratomic forcesincrease, resulting in an increasing melting point, boiling point,enthalpy of vaporization,andsolubility.The increase indensityis due to the increase inatomic mass.[23]

The noble gases are nearlyideal gasesunder standard conditions, but their deviations from theideal gas lawprovided important clues for the study ofintermolecular interactions.TheLennard-Jones potential,often used to modelintermolecular interactions,was deduced in 1924 byJohn Lennard-Jonesfromexperimental dataon argon before the development ofquantum mechanicsprovided the tools for understanding intermolecular forces fromfirst principles.[33]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, orisotropic.

Chemical properties

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Neon, like all noble gases, has a fullvalence shell.Noble gases have eight electrons in their outermost shell, except in the case of helium, which has two.

The noble gases are colorless, odorless, tasteless, and nonflammable understandard conditions.[34]They were once labeledgroup0in theperiodic tablebecause it was believed they had avalenceof zero, meaning theiratomscannot combine with those of otherelementsto formcompounds.However, it was later discovered some do indeed form compounds, causing this label to fall into disuse.[13]

Electron configuration

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Further information:Noble gas configuration

Like other groups, the members of thisfamilyshow patterns in itselectron configuration,especially the outermost shells resulting in trends in chemical behavior:

Z Element No. of electrons/shell
2 helium 2
10 neon 2, 8
18 argon 2, 8, 8
36 krypton 2, 8, 18, 8
54 xenon 2, 8, 18, 18, 8
86 radon 2, 8, 18, 32, 18, 8
118 oganesson 2, 8, 18, 32, 32, 18, 8 (predicted)

The noble gases have full valenceelectron shells.Valence electronsare the outermostelectronsof an atom and are normally the only electrons that participate inchemical bonding.Atoms with full valence electron shells are extremelystableand therefore do not tend to formchemical bondsand have little tendency togain or lose electrons.[35]However, heavier noble gases such as radon are held less firmly together byelectromagnetic forcethan 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 theelectron configurationnotation to form thenoble 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 phosphorusis1s22s22p63s23p3,while the noble gas notation is[Ne] 3s23p3.This more compact notation makes it easier to identify elements, and is shorter than writing out the full notation ofatomic orbitals.[36]

The noble gases cross the boundary betweenblocks—helium is ans-elementwhereas the rest of members arep-elements—which is unusual among theIUPACgroups. All other IUPAC groups contain elements fromoneblock each. This causes some inconsistencies in trends across the table, and on those grounds somechemistshave proposed that helium should be moved togroup 2to be with other s2elements,[37][38][39]but this change has not generally been adopted.

Compounds

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Main article:Noble gas compound
Structure ofxenon tetrafluoride(XeF
4), one of the first noble gas compounds to be discovered

The noble gases show extremely lowchemical reactivity;consequently, only a few hundrednoble gas compoundshave been formed. Neutralcompoundsin which helium and neon are involved inchemical bondshave not been formed (although some helium-containingionsexist and there is some theoretical evidence for a few neutral helium-containing ones), while xenon, krypton, and argon have shown only minor reactivity.[40]The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn ≪ Og.

In 1933,Linus Paulingpredicted that the heavier noble gases could form compounds with fluorine and oxygen. He predicted the existence ofkrypton hexafluoride(KrF
6) andxenon hexafluoride(XeF
6) and speculated thatxenon octafluoride(XeF
8) might exist as an unstable compound, and suggested thatxenic acidcould formperxenatesalts.[41][42]These predictions were shown to be generally accurate, except thatXeF
8is now thought to be boththermodynamicallyandkineticallyunstable.[43]

Xenon compoundsare the most numerous of the noble gas compounds that have been formed.[44]Most of them have the xenon atom in theoxidation stateof +2, +4, +6, or +8 bonded to highlyelectronegativeatoms such as fluorine or oxygen, as inxenon difluoride(XeF
2),xenon tetrafluoride(XeF
4),xenon hexafluoride(XeF
6),xenon tetroxide(XeO
4), andsodium perxenate(Na
4XeO
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 inchemical synthesisasoxidizing agents;XeF
2,in particular, is commercially available and can be used as afluorinatingagent.[45]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.[40][46]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 gasmatrices,or in supersonic noble gas jets.[40]

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 ofradon isotopes,only a fewfluoridesandoxidesof radon have been formed in practice.[47]Radon goes further towards metallic behavior than xenon; the difluoride RnF2is 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.[48][49][50]

Krypton is less reactive than xenon, but several compounds have been reported with krypton in theoxidation stateof +2.[40]Krypton difluorideis the most notable and easily characterized. Under extreme conditions, krypton reacts with fluorine to form KrF2according to the following equation:

Kr + F2→ KrF2

Compounds in which krypton forms a single bond to nitrogen and oxygen have also been characterized,[51]but are only stable below −60 °C (−76 °F) and −90 °C (−130 °F) respectively.[40]

Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine, carbon) as well as some latetransition metals(copper, silver, gold) have also been observed, but only either at low temperatures in noble gas matrices, or in supersonic noble gas jets.[40]Similar conditions were used to obtain the first few compounds of argon in 2000, such asargon fluorohydride(HArF), and some bound to the late transition metals copper, silver, and gold.[40]As of 2007, no stable neutral molecules involving covalently bound helium or neon are known.[40]

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.[52]Oganesson is expected to be rather likesiliconortinin group 14:[53]a reactive element with a common +4 and a less common +2 state,[54][55]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.[24][56](On the other hand,flerovium,despite being in group 14, is predicted to be unusually volatile, which suggests noble gas-like properties.)[57][58]

The noble gases—including helium—can form stablemolecular ionsin the gas phase. The simplest is thehelium hydride molecular ion,HeH+,discovered in 1925.[59]Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it was believed to occur naturally in theinterstellar medium,and it was finally detected in April 2019 using the airborneSOFIA telescope.In addition to these ions, there are many known neutralexcimersof the noble gases. These are compounds such as ArF and KrF that are stable only when in anexcited electronic state;some of them find application inexcimer lasers.

In addition to the compounds where a noble gas atom is involved in acovalent bond,noble gases also formnon-covalentcompounds. Theclathrates,first described in 1949,[60]consist of a noble gas atom trapped within cavities ofcrystal latticesof 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 withhydroquinone,but helium and neon do not because they are too small or insufficientlypolarizableto be retained.[61]Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice.[62]

An endohedral fullerene compound containing a noble gas atom

Noble gases can formendohedral fullerenecompounds, in which the noble gas atom is trapped inside afullerenemolecule. In 1993, it was discovered that whenC
60,a spherical molecule consisting of 60carbonatoms, is exposed to noble gases at high pressure,complexessuch asHe@C
60can be formed (the@notation indicates He is contained insideC
60but not covalently bound to it).[63]As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been created.[64]These compounds have found use in the study of the structure and reactivity of fullerenes by means of thenuclear magnetic resonanceof the noble gas atom.[65]

Bonding inXeF
2according to the 3-center-4-electron bond model

Noble gas compounds such asxenon difluoride(XeF
2) are considered to behypervalentbecause they violate theoctet rule.Bonding in such compounds can be explained using athree-center four-electron bondmodel.[66][67]This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding inXeF
2is described by a set of threemolecular orbitals(MOs) derived fromp-orbitalson each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from eachFatom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an emptyantibondingorbital. Thehighest occupied molecular orbitalis localized on the two terminal atoms. This represents a localization of charge that is facilitated by the high electronegativity of fluorine.[68]

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.

Occurrence and production

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The abundances of the noble gases in the universe decrease as theiratomic numbersincrease. Helium is the most common element in theuniverseafter hydrogen, with a mass fraction of about 24%. Most of the helium in the universe was formed duringBig Bang nucleosynthesis,but the amount of helium is steadily increasing due to the fusion of hydrogen instellar nucleosynthesis(and, to a very slight degree, thealpha decayof heavy elements).[69][70]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 noprimordialhelium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth'sgravitational field.[71]Helium on Earth comes from thealpha decayof heavy elements such asuraniumandthoriumfound in the Earth'scrust,and tends to accumulate innatural gas deposits.[71]The abundance of argon, on the other hand, is increased as a result of thebeta decayofpotassium-40,also found in the Earth's crust, to formargon-40,which is the most abundant isotope of argon on Earth despite being relatively rare in theSolar System.This process is the basis for thepotassium-argon datingmethod.[72]Xenon has an unexpectedly low abundance in the atmosphere, in what has been called themissing xenon problem;one theory is that the missing xenon may be trapped in minerals inside the Earth's crust.[73]After the discovery ofxenon dioxide,research showed that Xe can substitute for Si inquartz.[74]Radon is formed in thelithosphereby thealpha decayof 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 21,000lung cancerdeaths per year in the United States alone.[75]Oganesson does not occur in nature and is instead created manually by scientists.

Abundance Helium Neon Argon Krypton Xenon Radon
Solar System (for each atom of silicon)[76] 2343 2.148 0.1025 5.515 × 10−5 5.391 × 10−6 –
Earth's atmosphere (volume fraction inppm)[77] 5.20 18.20 9340.00 1.10 0.09 (0.06–18) × 10−19[78]
Igneous rock (mass fraction in ppm)[23] 3 × 10−3 7 × 10−5 4 × 10−2 – – 1.7 × 10−10
Gas 2004 price (USD/m3)[79]
Helium (industrial grade) 4.20–4.90
Helium (laboratory grade) 22.30–44.90
Argon 2.70–8.50
Neon 60–120
Krypton 400–500
Xenon 4000–5000

For large-scale use, helium is extracted byfractional distillationfrom natural gas, which can contain up to 7% helium.[80]

Neon, argon, krypton, and xenon are obtained from air using the methods ofliquefaction of gases,to convert elements to a liquid state, andfractional distillation,to separate mixtures into component parts. Helium is typically produced by separating it fromnatural gas,and radon is isolated from the radioactive decay of radium compounds.[13]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.

Biological chemistry

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None of the elements in this group has any biological importance.[81]

Applications

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Liquid helium is used to cool superconducting magnets in modern MRI scanners.

Noble gases have very low boiling and melting points, which makes them useful ascryogenicrefrigerants.[82]In particular,liquid helium,which boils at 4.2 K (−268.95 °C; −452.11 °F), is used forsuperconducting magnets,such as those needed innuclear magnetic resonance imagingandnuclear magnetic resonance.[83]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.[78]

Helium is used as a component ofbreathing gasesto replace nitrogen, due its lowsolubilityin fluids, especially inlipids.Gases are absorbed by thebloodandbody tissueswhen under pressure like inscuba diving,which causes ananestheticeffect known asnitrogen narcosis.[84]Due to its reduced solubility, little helium is taken intocell membranes,and when helium is used to replace part of the breathing mixtures, such as intrimixorheliox,a decrease in the narcotic effect of the gas at depth is obtained.[85]Helium's reduced solubility offers further advantages for the condition known asdecompression sickness,orthe bends.[13][86]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 adrysuitinflation gas for scuba diving.[87]Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.[88]

Goodyear Blimp

Since theHindenburgdisasterin 1937,[89]helium has replaced hydrogen as alifting gasinblimpsandballoons:despite an 8.6%[90]decrease in buoyancy compared to hydrogen, helium is not combustible.[13]

In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis ofair-sensitive compoundsthat are sensitive to nitrogen. Solid argon is also used for the study of very unstable compounds, such asreactive intermediates,by trapping them in an inertmatrixat very low temperatures.[91]Helium is used as the carrier medium ingas chromatography,as a filler gas forthermometers,and in devices for measuring radiation, such as theGeiger counterand thebubble chamber.[79]Helium and argon are both commonly used to shieldwelding arcsand the surroundingbase metalfrom the atmosphere during welding and cutting, as well as in other metallurgical processes and in the production of silicon for the semiconductor industry.[78]

15,000-wattxenon short-arc lampused inIMAXprojectors

Noble gases are commonly used inlightingbecause of their lack of chemical reactivity. Argon, mixed with nitrogen, is used as a filler gas forincandescent light bulbs.[78]Krypton is used in high-performance light bulbs, which have highercolor temperaturesand 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 ofiodineorbromine.[78]The noble gases glow in distinctive colors when used insidegas-discharge lamps,such as "neon lights".These lights are called after neon but often contain other gases andphosphors,which add various hues to the orange-red color of neon. Xenon is commonly used inxenon arc lamps,which, due to their nearlycontinuous spectrumthat resembles daylight, find application in film projectors and as automobile headlamps.[78]

The noble gases are used inexcimer lasers,which are based on short-lived electronically excited molecules known asexcimers.The excimers used for lasers may be noble gas dimers such as Ar2,Kr2or Xe2,or more commonly, the noble gas is combined with a halogen in excimers such as ArF, KrF, XeF, or XeCl. These lasers produceultravioletlight, which, due to its shortwavelength(193nmfor ArF and 248 nm for KrF), allows for high-precision imaging. Excimer lasers have many industrial, medical, and scientific applications. They are used formicrolithographyandmicrofabrication,which are essential forintegrated circuitmanufacture, and forlaser surgery,including laserangioplastyandeye surgery.[92]

Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people withasthma.[78]Xenon is used as ananestheticbecause of its high solubility in lipids, which makes it more potent than the usualnitrous oxide,and because it is readily eliminated from the body, resulting in faster recovery.[93]Xenon finds application in medical imaging of the lungs through hyperpolarized MRI.[94]Radon, which is highly radioactive and is only available in minute amounts, is used inradiotherapy.[13]

Noble gases, particularly xenon, are predominantly used inion enginesdue 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.

Discharge color

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Colors and spectra (bottom row) of electric discharge in noble gases; only the second row represents pure gases.
Helium Neon Argon Krypton Xenon

The color of gas discharge emission depends on several factors, including the following:[95]

  • discharge parameters (local value ofcurrent densityandelectric 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.

See also

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Notes

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