The seventh member of group 18 is oganesson, anunstablesynthetic element whose chemistry is still uncertain because only five very short-lived atoms (t1/2 = 0.69 ms) have ever been synthesized (as of 2020[update][3]).IUPAC uses the term "noble gas" interchangeably with "group 18" and thus includes oganesson;[4] however, due torelativistic effects, oganesson is predicted to be asolid under standard conditions and reactive enough not to qualify functionally as "noble".[3]
Noble gas is translated from theGerman nounEdelgas, 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 compounds are now known.[6]Rare gases is another term that was used,[7] but this is also inaccurate becauseargon forms a fairly considerable part (0.94% by volume, 1.3% by mass) of theEarth's atmosphere due to decay of radioactivepotassium-40.[8]
Helium was first detected in the Sun due to its characteristicspectral lines.
Pierre Janssen andJoseph Norman Lockyer had discovered a new element on 18 August 1868 while looking at thechromosphere of theSun, and named ithelium after 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 Cavendish had discovered that air contains a small proportion of a substance less reactive thannitrogen.[10] A century later, in 1895,Lord Rayleigh discovered that samples of nitrogen from the air were of a differentdensity than nitrogen resulting fromchemical reactions. Along with Scottish scientistWilliam Ramsay atUniversity 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 ofgases was 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 Mendeleev included 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 distillation to separateliquid air into 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.Radon was 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 Prizes in 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 Moissan attempted to form a reaction betweenfluorine, the mostelectronegative 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 physicistNiels Bohr proposed in 1913 that theelectrons in atoms are arranged inshells surrounding thenucleus, and that for all noble gases except helium the outermost shell always contains eight electrons.[13] In 1916,Gilbert N. Lewis formulated 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 Bartlett discovered 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]
This is a plot ofionization potential versusatomic 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 energy of oganesson (Z = 118) is lower than those of elements 110-112.
The noble gasatoms, like atoms in most groups, increase steadily inatomic radius from oneperiod to the next due to the increasing number ofelectrons. Thesize of the atom is related to several properties. For example, theionization potential decreases with an increasing radius because thevalence electrons in the larger noble gases are farther away from thenucleus 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 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 theoxygen molecule that ledBartlett to attempt oxidizing xenon usingplatinum hexafluoride, anoxidizing agent known 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]
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 labeledgroup 0 in theperiodic table because it was believed they had avalence of zero, meaning theiratoms cannot combine with those of otherelements to formcompounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse.[13]
Like other groups, the members of thisfamily show patterns in itselectron configuration, especially the outermost shells resulting in trends in chemical behavior:
The noble gases have full valenceelectron shells.Valence electrons are the outermostelectrons of an atom and are normally the only electrons that participate inchemical bonding. Atoms with full valence electron shells are extremelystable and therefore do not tend to formchemical bonds and have little tendency togain or lose electrons.[35] However, heavier noble gases such as radon are held less firmly together byelectromagnetic 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 theelectron configuration notation 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 ofphosphorus is1s2 2s2 2p6 3s2 3p3, while the noble gas notation is[Ne] 3s2 3p3. 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-element whereas the rest of members arep-elements—which is unusual among theIUPAC groups. All other IUPAC groups contain elements fromone block each. This causes some inconsistencies in trends across the table, and on those grounds somechemists have proposed that helium should be moved togroup 2 to be with other s2 elements,[37][38][39] but this change has not generally been adopted.
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 compounds have been formed. Neutralcompounds in which helium and neon are involved inchemical bonds have not been formed (although some helium-containingions exist and there is some theoretical evidence for a few neutral helium-containing ones), while xenon, krypton, and argon have shown only minor reactivity.[40]
Some of these compounds have found use inchemical synthesis asoxidizing agents;XeF 2, in particular, is commercially available and can be used as afluorinating agent.[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, sulfur, 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 fewfluorides andoxides of radon have been formed in practice.[47] 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.[48][49][50]
Krypton is less reactive than xenon, but several compounds have been reported with krypton in theoxidation state of +2.[40]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,[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 likesilicon ortin in 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 ions in 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 neutralexcimers of 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-covalent compounds. Theclathrates, first described in 1949,[60] consist of a noble gas atom trapped within cavities ofcrystal 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 withhydroquinone, but helium and neon do not because they are too small or insufficientlypolarizable to 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 fullerene compounds, in which the noble gas atom is trapped inside afullerene molecule. In 1993, it was discovered that whenC 60, a spherical molecule consisting of 60 carbon atoms, is exposed to noble gases at high pressure,complexes such asHe@C 60 can be formed (the@ notation indicates He is contained insideC 60 but 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 resonance of the noble gas atom.[65]
Bonding inXeF 2 according to the 3-center-4-electron bond model
Noble gas compounds such asxenon difluoride (XeF 2) are considered to behypervalent because they violate theoctet rule. Bonding in such compounds can be explained using athree-center four-electron bond model.[66][67] This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding inXeF 2 is described by a set of threemolecular orbitals (MOs) derived fromp-orbitals on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from eachF atom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an emptyantibonding orbital. Thehighest 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.[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.
The abundances of the noble gases in the universe decrease as theiratomic numbers increase. Helium is the most common element in theuniverse after 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 decay of 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 noprimordial helium 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 decay of heavy elements such asuranium andthorium found 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 decay ofpotassium-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 dating method.[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][74] Radon is formed in thelithosphere by thealpha 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 21,000lung cancer deaths per year in the United States alone.[75] Oganesson does not occur in nature and is instead created manually by scientists.
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.
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 imaging andnuclear 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 gases to replace nitrogen, due its lowsolubility in fluids, especially inlipids. Gases are absorbed by theblood andbody tissues when under pressure like inscuba diving, which causes ananesthetic effect 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 intrimix orheliox, 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 adrysuit inflation gas for scuba diving.[87] Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.[88]
In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis ofair-sensitive compounds that are sensitive to nitrogen. Solid argon is also used for the study of very unstable compounds, such asreactive intermediates, by trapping them in an inertmatrix at 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 counter and thebubble chamber.[79] Helium and argon are both commonly used to shieldwelding arcs and the surroundingbase 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.[78]
Noble gases are commonly used inlighting because 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 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 ofiodine orbromine.[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 spectrum that resembles daylight, find application in film projectors.[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, 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 produceultraviolet light, which, due to its shortwavelength (193nm for ArF and 248 nm for KrF), allows for high-precision imaging. Excimer lasers have many industrial, medical, and scientific applications. They are used formicrolithography andmicrofabrication, which are essential forintegrated circuit manufacture, and forlaser surgery, including laserangioplasty andeye 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 ananesthetic because 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 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 importantgeochemical tracing tool inearth science.[95][96] They can unravel the Earth's degassing history and its effects to the surrounding environment (i.e.,atmosphere composition[97]). 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 acrossgeological settings.[96][98]
Helium has two abundant isotopes:helium-3, which isprimordial with high abundance inearth's core andmantle, andhelium-4, which originates from decay ofradionuclides (232Th,235,238U) abundant in theearth's crust. Isotopic ratios of helium are represented by RA value, a value relative to air measurement (3He/4He = 1.39*10−6).[99]Volatiles that originate from the earth's crust have a 0.02-0.05 RA, which indicate an enrichment of helium-4.[100] Volatiles that originate from deeper sources such assubcontinental lithospheric mantle (SCLM), have a 6.1± 0.9 RA[101] and mid-oceanic ridge basalts (MORB) display higher values (8 ± 1 RA).Mantle plume samples have even higher values than > 8 RA.[101][102]Solar wind, which represent an unmodifiedprimordial signature is reported to have ~ 330 RA.[103]
Neon has three main stable isotopes:20Ne,21Ne and22Ne, with20Ne produced by cosmicnucleogenic reactions, causing high abundance in the atmosphere.[98][104]21Ne and22Ne are produced in the earth's crust as a result of interactions between alpha and neutron particles with light elements;18O,19F and24,25Mg.[105] The neon ratios (20Ne/22Ne and21Ne/22Ne) are systematically used to discern the heterogeneity in theEarth's mantle and volatile sources. Complimenting He isotope data, neon isotope data additionally provide insight to thermal evolution of Earth's systems.[106]
Argon has three stable isotopes:36Ar,38Ar and40Ar.36Ar and38Ar areprimordial, with their inventory on the earth's crust dependent on the equilibration ofmeteoric water with the crustal fluids.[98] This explains huge inventory of36Ar in the atmosphere. Production of these two isotopes (36Ar and38Ar) is negligible within the earth's crust, only limited concentrations of38Ar can be produced by interaction between alpha particles from decay of235,238U and232Th and light elements (37Cl and41K). While36Ar is continuously being produced by Beta-decay of36Cl.[104][112]40Ar is a product of radiogenic decay of40K. Different endmembers values for40Ar/36Ar have been reported; Air = 295.5,[113] MORB = 40,000,[113] and crust = 3000.[98]
Krypton has severalisotopes, with78, 80, 82Kr beingprimordial, while83,84, 86Kr results from spontaneous fission of244Pu and radiogenic decay of238U.[95][98] Krypton's isotopes geochemical signature in mantle reservoirs resembling the modern atmosphere. preserves the solar-like primordial signature.[114] 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.[115] This is largely due to a clear distinction of krypton isotope signature from various sources such aschondritic material,solar wind andcometary.[116][117]
Xenon hasnine isotopes, most of which are produced by theradiogenic decay. Krypton and xenon noble gases requires pristine, robust geochemical sampling protocol to avoid atmospheric contamination.[118] 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 likevolcanic vents,springs, andgeothermal wells following specific sampling protocols.[122] 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.
Sampling of noble gases using a Giggenbach bottle, a funnel is placed on top of the hot spring to focus the stream of sample towards the bottle via the Tygon tube. A geochemist is controlling the flow of the sample inlet using a Teflon valve. Note the condensation process inside the evacuated Giggenbach bottle.Giggenbach 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.[123] These bottles were first invented and distributed by a Werner F. Giggenbach, a German chemist.[124]
Noble gases have numerous isotopes and subtle mass variation that requires high-precision detection systems. Originally, scientists usedmagnetic sector mass spectrometry, which is time-consuming and has low sensitivity due to "peak jumping mode".[125][126] Multiple-collector mass spectrometers, likeQuadrupole mass spectrometers (QMS), enable simultaneous detection of isotopes, improving sensitivity and throughput.[126] Before analysis, sample preparation is essential due to the low abundance of noble gases, involving extraction, purification system.[96] 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.[127] Cryogenic traps are used for sequential analysis without peak interference by stepwise temperature raise.[128]
Research labs have successfully developed miniaturized field-based mass spectrometers, such as the portable mass spectrometer (miniRuedi), which can analyze noble gases with an analytical uncertainty of 1-3% using low-cost vacuum systems and quadrupole mass analyzers.[129]
Extraction and purification (clean up) mass spectrometer line.
The color of gas discharge emission depends on several factors, including the following:[130]
discharge parameters (local value ofcurrent density andelectric 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.
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