Respect all members: no insults, harassment, or hate speech.
Be tolerant of different viewpoints, cultures, and beliefs. If you do not agree with others, just create separate note, article or collection.
Clearly distinguish between personal opinion and fact.
Verify facts before posting, especially when writing about history, science, or statistics.
Promotional content must be published on the “Related Services and Products” page—no more than one paragraph per service. You can also create subpages under the “Related Services and Products” page and publish longer promotional text there.
Do not post materials that infringe on copyright without permission.
Always credit sources when sharing information, quotes, or media.
Be respectful of the work of others when making changes.
Discuss major edits instead of removing others' contributions without reason.
If you notice rule-breaking, notify community about it in talks.
Do not share personal data of others without their consent.
The seventh member of group 18 is oganesson, an unstablesynthetic 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 to relativistic effects, oganesson is predicted to be a solid under standard conditions and reactive enough not to qualify functionally as "noble".[3]
Noble gas is translated from the German noun Edelgas, first used in 1900 by Hugo 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 as inert gases, but this label is deprecated as many noble gas compounds are now known.[6]Rare gases is another term that was used,[7] 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.[8]
Helium was first detected in the Sun due to its characteristic spectral lines.
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, 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 physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen.[10] 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 ἀργός (argós, "idle" or "lazy").[10] 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.[11]
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 κρυπτός (kryptós, "hidden"), νέος (néos, "new"), and ξένος (ksénos, "stranger"), respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn,[12] 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.[13] Rayleigh and Ramsay received the 1904 Nobel 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 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".[15]
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.[13] 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.[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 when argon fluorohydride (HArF) was formed at a temperature of 40 K (−233.2 °C; −387.7 °F).[20]
The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomicgases under standard conditions, including the elements with larger atomic masses than many normally solid elements.[13]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[29] (an effect explained by quantum mechanics as its zero point energy is too high to permit freezing)[30] – a pressure of 25 standard atmospheres (2,500 kPa; 370 psi) must be applied at a temperature of 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 at room temperature.[31] 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.[13] 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.
This is a plot of ionization potential 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.
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.[23] 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.[17] Noble gases cannot accept an electron to form stable anions; that is, they have a negative electron affinity.[32]
The macroscopicphysical 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.[23]
Neon, like all noble gases, has a full valence 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 under standard conditions.[34] 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.[13]
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.[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 1s2 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 of atomic orbitals.[36]
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,[37][38][39] but this change has not generally been adopted.
Structure of xenon tetrafluoride (XeF 4), one of the first noble gas compounds to be discovered
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.[40]
Some of these compounds have found use in chemical synthesis as oxidizing agents; XeF 2, in particular, is commercially available and can be used as a fluorinating 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 gas matrices, 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 of radon isotopes, only a few fluorides and oxides 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 the oxidation 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 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.[40] 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.[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 like silicon or tin 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 stable molecular ions in the gas phase. The simplest is the helium 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 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,[60] 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.[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 form endohedral fullerene compounds, in which the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when C 60, a spherical molecule consisting of 60 carbon atoms, is exposed to noble gases at high pressure, complexes such as He@C 60 can be formed (the @ notation indicates He is contained inside C 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 the nuclear magnetic resonance of the noble gas atom.[65]
Bonding in XeF 2 according to the 3-center-4-electron bond model
Noble gas compounds such as xenon difluoride (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.[66][67] This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding in 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.[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 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).[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 no primordial helium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth's gravitational field.[71] 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.[71] 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.[72]
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.[73][74] 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.[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 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.[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 as cryogenicrefrigerants.[82] In particular, liquid helium, which boils at 4.2 K (−268.95 °C; −452.11 °F), is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear 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 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.[84] 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.[85] Helium's reduced solubility offers further advantages for the condition known as decompression sickness, or the 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 a drysuit 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 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.[91] 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.[79] 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.[78]
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.[78] 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.[78] 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.[78]
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.[92]
Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people with asthma.[78] 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.[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 in radiotherapy.[13]
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.[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 across geological settings.[96][98]
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 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 as subcontinental 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 unmodified primordial signature is reported to have ~ 330 RA.[103]
Neon has three main stable isotopes:20Ne, 21Ne and 22Ne, with 20Ne produced by cosmic nucleogenic reactions, causing high abundance in the atmosphere.[98][104]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.[105] 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.[106]
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.[98] 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.[104][112]40Ar is a product of radiogenic decay of 40K. Different endmembers values for 40Ar/36Ar have been reported; Air = 295.5,[113] MORB = 40,000,[113] and crust = 3000.[98]
Krypton has several isotopes, with 78, 80, 82Kr being primordial, while 83,84, 86Kr results from spontaneous fission of 244Pu and radiogenic decay of 238U.[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 as chondritic material, solar wind and cometary.[116][117]
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.[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 like volcanic vents, springs, and geothermal 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 used magnetic sector mass spectrometry, which is time-consuming and has low sensitivity due to "peak jumping mode".[125][126] Multiple-collector mass spectrometers, like Quadrupole 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 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.
^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."
^Bent Weberg, Libby (18 January 2019). ""The" periodic table". Chemical & Engineering News. 97 (3). Archived from the original on 1 February 2020. Retrieved 27 March 2020.
^Zupan, Marko; Iskra, Jernej; Stavber, Stojan (1998). "Fluorination with XeF2. 44. Effect of Geometry and Heteroatom on the Regioselectivity of Fluorine Introduction into an Aromatic Ring". J. Org. Chem. 63 (3): 878–880. doi:10.1021/jo971496e. PMID11672087.
^Liebman, Joel F. (1975). "Conceptual Problems in Noble Gas and Fluorine Chemistry, II: The Nonexistence of Radon Tetrafluoride". Inorg. Nucl. Chem. Lett. 11 (10): 683–685. doi:10.1016/0020-1650(75)80185-1.
^Kulsha, A. V. "Есть ли граница у таблицы Менделеева?" [Is there a boundary to the Mendeleev table?] (PDF). www.primefan.ru (in Russian). Retrieved 8 September 2018.
^Hogness, T. R.; Lunn, E. G. (1925). "The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis". Physical Review. 26 (1): 44–55. Bibcode:1925PhRv...26...44H. doi:10.1103/PhysRev.26.44.
^Saunders, Martin; Jimenez-Vazquez, Hugo A.; Cross, R. James; Mroczkowski, Stanley; Gross, Michael L.; Giblin, Daryl E.; Poreda, Robert J. (1994). "Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure". J. Am. Chem. Soc.116 (5): 2193–2194. Bibcode:1994JAChS.116.2193S. doi:10.1021/ja00084a089.
^Pimentel, G. C. (1951). "The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method". The Journal of Chemical Physics. 19 (4): 446–448. Bibcode:1951JChPh..19..446P. doi:10.1063/1.1748245.
^ abHwang, Shuen-Chen; Lein, Robert D.; Morgan, Daniel A. (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. doi:10.1002/0471238961.0701190508230114.a01.
^Maiken, Eric (1 August 2004). "Why Argon?". Decompression. Retrieved 26 June 2008.
^Horhoianu, G.; Ionescu, D. V.; Olteanu, G. (1999). "Thermal behaviour of CANDU type fuel rods during steady state and transient operating conditions". Annals of Nuclear Energy. 26 (16): 1437–1445. Bibcode:1999AnNuE..26.1437H. doi:10.1016/S0306-4549(99)00022-5.
^"Disaster Ascribed to Gas by Experts". The New York Times. 7 May 1937. p. 1.
The noble gases are the chemical elements comprising group 18 (also known as group VIII or 0) of the periodic table, consisting of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og).[1][2] These elements are distinguished by their extreme chemical inertness, arising from completely filled valence electron shells that confer high stability and minimal tendency to form chemical bonds.[1] Under standard conditions, they exist as colorless, odorless, tasteless, and nonflammable monatomic gases with very low melting and boiling points due to weak interatomic forces, increasing gradually down the group from helium's boiling point of 4.216 K to radon's 211.5 K.[1]The discovery of the noble gases revolutionized the periodic table in the late 19th century, filling a previously unrecognized group of inert elements.[3] In 1894, Lord Rayleigh and William Ramsayidentifiedargon as a component of air denser than nitrogen, marking the first terrestrial noble gas discovery.[4]Helium, first observed in the sun's spectrum in 1868 by Pierre Janssen and Norman Lockyer, was isolated on Earth in 1895 by Ramsay from the mineral cleveite.[3] Ramsay, collaborating with Morris Travers, then isolated neon, krypton, and xenon from liquid air in 1898, while radon was discovered in 1900 by Friedrich Ernst Dorn as a radioactive emanation from radium.[3][5] These findings, led primarily by Ramsay—who received the 1904 Nobel Prize in Chemistry—confirmed the existence of a new family of elements with no affinity for chemical combination.[4]Although long considered completely unreactive, noble gases exhibit high first ionization energies—ranging from 2372.3 kJ/mol for helium to 1037.1 kJ/mol for radon—and show limited reactivity under extreme conditions.[1] The breakthrough came in 1962 when Neil Bartlett synthesized the first noble gas compound, xenon hexafluoroplatinate (XePtF₆), demonstrating that xenon and krypton can form bonds with highly electronegative elements like fluorine and oxygen.[6]Radon, the heaviest naturally occurring noble gas, is radioactive with ^{222}Rn having a half-life of about 3.8 days, while the others are stable.[1] Noble gases find widespread applications: helium in cryogenics and as a lifting gas due to its low density; neon in advertising signs for its red glow; argon in welding to provide an inert atmosphere; krypton and xenon in lighting and medical imaging; and radon in radiation therapy despite its hazards.[1]
History and Discovery
Early Observations
In the 18th and 19th centuries, scientists and observers documented luminous phenomena in the Earth's atmosphere, such as the colorful glows produced during auroras and lightning strikes, which involved electrical discharges ionizing atmospheric gases and creating visible emissions. These events, including the red and green hues of the aurora borealis noted in European and North American records from the 1700s onward, hinted at the complex composition of air but were not yet linked to specific inert components. Similarly, lightning's bright flashes and associated glows were studied as electrical phenomena, with early experiments replicating similar effects in glass tubes under reduced pressure, producing colored lights from gas ionization as early as the late 18th century.[7][8][9]A significant experimental hint came from Henry Cavendish's work in 1785, when he sparked mixtures of atmospheric air and excess oxygen, absorbing the resulting nitric acid with alkali. Cavendish found that after these processes, a small residue of about 1% of the original volume remained, which resisted further chemical reaction, later recognized as a mixture including noble gases like argon and neon. This residue, isolated but unidentified at the time, demonstrated the presence of inert atmospheric components denser than nitrogen.[10][11]Spectroscopic studies in the mid-19th century further revealed anomalies in light emissions from celestial and terrestrial sources. During the total solar eclipse of August 18, 1868, French astronomer Pierre Janssen and English astronomer Norman Lockyer independently observed a bright yellow line at 587.49 nm in the solar chromosphere's spectrum, which did not match any known terrestrial element and was provisionally named helium from the Greek "helios" for sun. This discovery suggested the existence of unknown gases in the universe, predating helium's identification on Earth.[12][13]By the late 19th century, precise measurements exposed discrepancies in atmospheric gas properties. In 1892, Lord Rayleigh reported that nitrogen extracted from air had a density of 1.2572 g/L, slightly higher than the 1.2506 g/L of chemically prepared nitrogen from ammonia or other compounds, indicating an impurity of heavier, inert gases in the atmosphere. This small but consistent difference, confirmed through meticulous weighing and volume experiments, prompted further investigation into the air's composition. These early observations laid the groundwork for the systematic isolation of noble gases in the 1890s.[14][10]
Isolation and Recognition
The discovery of argon in 1894 by English physicist Lord Rayleigh and Scottish chemist Sir William Ramsay initiated the identification of the noble gases as a unique family of elements. Rayleigh's observation of a density discrepancy between atmospheric nitrogen and nitrogen derived from chemical sources prompted their collaboration; they isolated the new gas chemically from air by absorbing nitrogen with hot magnesium, obtaining about 1% of the atmospheric volume as a residue. Density measurements revealed it to be heavier than nitrogen yet monatomic and inert, with no evidence of chemical combination under standard conditions.[15][3]Argon was named from the Greek argos, meaning "lazy" or "inactive," alluding to its chemical unreliability at the time.[16] In 1895, helium became the second noble gas isolated on Earth, extracted by Ramsay and Swedish chemist Per Teodor Cleve from cleveite, a uranium-bearing mineral treated with acids; this followed its identification via spectroscopy in the solar spectrum decades earlier. The name derives from the Greek helios for "sun."[17][18]By 1898, Ramsay and his student Morris William Travers had advanced air liquefaction techniques to isolate three more noble gases: neon, krypton, and xenon from the residues after evaporating liquid air components. Spectroscopic analysis confirmed their distinct emission lines, with neon appearing as a bright red glow, krypton as a greenish-yellow, and xenon as blue. Their names stem from Greek roots: neos ("new") for neon, kryptos ("hidden") for krypton, and xenos ("stranger") for xenon.[3][19]In 1900, German physicist Friedrich Ernst Dorn identified radon as a radioactive emanation produced during the decay of radium, isolating it from radium chloride. Initially termed "radium emanation," it was later named radon from "radium." By 1902, the growing collection of these inert elements led to their classification as Group 0 in the periodic table, a placement proposed by chemists like Bohuslav Brauner despite initial skepticism from Dmitri Mendeleev, who doubted argon's status as a simple element and its atomic weight-based positioning ahead of potassium.[20][21][22]
Physical Properties
Atomic Structure
Noble gases, also known as group 18 elements, consist of helium (He, atomic number 2), neon (Ne, 10), argon (Ar, 18), krypton (Kr, 36), xenon (Xe, 54), radon (Rn, 86), and the synthetic oganesson (Og, 118).[23] These elements exhibit distinctive atomic structures characterized by fully filled valence electron shells, which confer exceptional stability. The general electron configuration for noble gases (except helium) follows the pattern [noble gas core] ns²np⁶, where n is the principal quantum number of the outermost shell, resulting in eight valence electrons that complete the octet rule. Helium, with its 1s² configuration, achieves a stable duet.[23][24]The specific electron configurations are as follows:
Element
Atomic Number
Electron Configuration
He
2
1s²
Ne
10
[He] 2s² 2p⁶
Ar
18
[Ne] 3s² 3p⁶
Kr
36
[Ar] 3d¹⁰ 4s² 4p⁶
Xe
54
[Kr] 4d¹⁰ 5s² 5p⁶
Rn
86
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶
Og
118
[Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶ (predicted)
This closed-shell arrangement, with completely filled s and p subshells in the valence level, minimizes electron repulsion and maximizes exchange energy, leading to high chemical stability and low reactivity.[24]Noble gas atoms possess relatively small atomic radii compared to elements in adjacent groups, measured as van der Waals radii due to their monatomic nature. These radii increase down the group—from 140 pm for He to 220 pm for Rn—primarily because of the addition of new electron shells, which outweighs the concurrent increase in effective nuclear charge (Zeff) from greater proton count.[25] The rising Zeff, defined as the net positive charge experienced by valence electrons after accounting for shielding by inner electrons, pulls electrons closer but is insufficient to reverse the size increase from larger principal quantum numbers./08%3A_Periodic_Properties_of_the_Elements/8.06%3A_Periodic_Trends_in_the_Size_of_Atoms_and_Effective_Nuclear_Charge)Ionization energies, reflecting the energy required to remove a valence electron, are the highest among all elements for noble gases, underscoring their stability. Helium has the highest first ionization energy at 24.59 eV, decreasing down the group to 10.75 eV for radon, as larger atomic sizes and increased shielding reduce the attraction between the nucleus and valence electrons.[16] For oganesson, theoretical predictions suggest an even lower value around 8.7 eV, though experimental confirmation is lacking due to its short half-life.[26]
States and Phase Behavior
The noble gases are monatomic and exist as colorless, odorless gases at standard temperature and pressure (STP), including radon despite its radioactivity and relatively higher boiling point.[27] Due to their weak interatomic van der Waals forces, these elements exhibit very low melting and boiling points compared to other groups in the periodic table, with both values increasing down the group as atomic mass and size grow, enhancing intermolecular attractions.[27]The following table summarizes key phase transition temperatures and gas densities at STP for the noble gases:
These low transition temperatures reflect the minimal energy required to overcome the weak forces holding the atoms together, resulting in gases that liquefy only at cryogenic conditions.[27] Densities also increase down the group, from helium as the second-lightest gas after hydrogen to radon, which is about 55 times denser than helium at STP.[27]The noble gases possess low critical temperatures and pressures, allowing relatively straightforward liquefaction compared to many other substances; for instance, helium's critical point is at 5.2 K and 0.227 MPa, while argon's is at 150.7 K and 4.86 MPa.[28] This property underpins their widespread use in cryogenics. Helium displays exceptional phase behavior: at atmospheric pressure, it lacks a conventional triple point and remains liquid down to absolute zero (0 K), refusing to solidify without applied pressure due to its high zero-point energy.[30] Below the λ-point of 2.17 K, liquid helium transitions to a superfluid state (helium II), characterized by zero viscosity, infinite thermal conductivity, and the ability to flow without friction through narrow channels.[31] In contrast, the heavier noble gases follow standard phase diagrams with defined triple points where solid, liquid, and gas phases coexist in equilibrium.[27]
Chemical Properties
Electronic Configuration
The electronic configurations of noble gases are characterized by fully filled valence shells, conferring exceptional stability. For neon through radon, the ground-state configuration follows the general pattern [noble gas core] ns² np⁶, where n ranges from 2 to 6, resulting in eight valence electrons that complete the octet in the outermost shell.[32] This filled ns² np⁶ subshell leaves no unpaired electrons available for bonding, as the s and p orbitals are maximally occupied according to the Aufbau principle, Pauli exclusion principle, and Hund's rule.[32]Helium represents an exception, with its configuration 1s², achieving a stable duet rather than an octet due to the limited capacity of the single 1s orbital, which can hold only two electrons of opposite spin; despite this difference, helium exhibits similar inert behavior.[33]The stability of these configurations arises from the high effective nuclear charge (Z_eff) experienced by the valence electrons, which tightly binds them to the nucleus. Qualitatively, the first ionization energy (IE) for noble gases trends with the formula IE ≈ 13.6 (Z_eff)² / n² eV, adapted from the hydrogen-like atom model, where increasing Z_eff across a period enhances orbital binding, while larger n down the group reduces it despite rising nuclear charge. This results in progressively decreasing ionization energies from He (24.59 eV) to Rn (10.75 eV), reflecting weaker binding in larger atoms.In heavier noble gases such as krypton, xenon, and radon, relativistic effects introduce deviations from non-relativistic predictions, notably lowering ionization energies and facilitating limited reactivity. These effects stem from high velocities of inner electrons near the nucleus, causing s and p orbital contraction (relativistic stabilization) while expanding d and f orbitals; for valence np electrons, this leads to a slight destabilization, reducing ionization energies by up to 5-10% for Xe and Rn compared to scalar-relativistic estimates.[34] Such modifications enable electron promotion and hybridization, as seen in xenon where the energy required to promote an electron from the 5s orbital to 5d is approximately 8 eV, allowing sp³d hybridization in compounds like XeF₂.[35]
Reactivity and Inertness
Noble gases are renowned for their chemical inertness, a property arising from their stable electron configurations with fully occupied valence shells. This electronic stability leads to exceptionally high ionization energies, which increase the energy required to remove electrons and form positive ions, and effectively zero electronegativity, minimizing their tendency to attract electrons in bonding interactions. As a result, these elements do not readily participate in chemical reactions under standard conditions, distinguishing them from more reactive groups in the periodic table.[36][37]Early efforts to induce reactivity in noble gases, such as the late 19th-century experiments by Henri Moissan exposing argon to fluorine gas, consistently failed to yield stable compounds, reinforcing the perception of these elements as chemically unreactive. This view persisted until 1962, when Neil Bartlett synthesized the first noble gas compound, xenon hexafluoroplatinate (XePtF₆), by reacting xenon with platinum hexafluoride. Shortly after, binary xenon fluorides such as XeF₂ (via photochemical reaction of xenon and fluorine under UV irradiation) and XeF₄ (via thermal reaction) were prepared, demonstrating that noble gases could form bonds under specific, forcing conditions involving highly electronegative reagents.[6][38]Reactivity among the noble gases increases down the group, correlating with decreasing ionization energies that facilitate electron promotion or removal for bonding. Helium remains unreactive under standard conditions, with no stable covalent compounds, though an ionic compound Na₂He is stable at high pressures (>113 GPa). Neon remains entirely unreactive, with no stable compounds observed even under extreme conditions as of 2025. Argon exhibits only rare, unstable reactivity, typically in cryogenic matrix isolations, while krypton, xenon, and radon show progressively greater propensity for compound formation; xenon, in particular, displays oxidation states ranging from +2 to +8 in its fluorides and oxides. These relativistic effects are even more pronounced in the synthetic superheavy element oganesson (Og), which computational studies predict to exhibit metallic or reactive behavior rather than inertness. The bonds formed, such as the Xe–F linkage in xenon fluorides, are notably weak compared to typical covalent bonds, with energies around 147 kJ/mol, underscoring the reluctance of noble gases to engage in stable chemistry.[37][6][39][40][41]
Occurrence in Nature
Abundance in the Universe
Noble gases, particularly helium, were primarily formed during Big Bang nucleosynthesis (BBN), the earliest process of element creation in the universe's first few minutes. In BBN, fusion of protons and neutrons produced helium-4 as the second most abundant element after hydrogen, with a primordial mass fraction Y_p ≈ 0.25, meaning about 25% of the baryonic mass was helium.[42] Heavier noble gases like neon and argon were not significantly produced in BBN due to the rapid expansion and cooling of the universe, which halted fusion at light elements; their primordial abundances are thus negligible or trace.[42]Subsequent stellar nucleosynthesis enriched the universe with additional noble gases through fusion in stars. Helium is generated in stellar cores via hydrogen burning, where four protons fuse into one helium-4 nucleus through processes like the proton-proton chain or CNO cycle at temperatures exceeding millions of kelvin.[43] Neon arises during helium burning at around 10^8 K, primarily through alpha-particle captures on oxygen-16 (e.g., ^{16}O + ^4He → ^{20}Ne + γ) and carbon burning reactions such as ^{12}C + ^{12}C → ^{20}Ne + ^4He.[43] Argon, mainly ^{36}Ar, forms in more advanced stages during oxygen burning at approximately 10^9 K in massive stars, via fusion of lighter nuclei and alpha-process side reactions, with significant production in supernova explosions that disperse these elements into the interstellar medium.[43]In stellar environments like the Sun, noble gas abundances reflect this nucleosynthetic history, serving as benchmarks for cosmic composition. The solar photosphere has a helium number abundance relative to hydrogen of N_He / N_H ≈ 0.085 (or log ε(He) = 10.93), neon at N_Ne / N_H ≈ 1.1 × 10^{-4} (log ε(Ne) = 8.06), and argon at N_Ar / N_H ≈ 2.5 × 10^{-6} (log ε(Ar) = 6.40), determined from helioseismology, solar wind, and coronal data since direct spectroscopy is challenging for these inert elements.[44] These values indicate helium dominates among noble gases, with neon and argon present at much lower levels.In the interstellar medium (ISM), noble gas abundances are measured via ultraviolet (UV) spectroscopy of absorption lines, revealing depletions relative to solar values due to adsorption onto dust grains, though less severe than for reactive metals. Neon, being highly inert, shows minimal depletion and serves as a reference for total gas column density, with observed abundances close to solar in diffuse clouds.[45]Argon exhibits moderate depletion by factors of 2–3 in some sightlines, attributed to physisorption on dust surfaces in colder, denser regions, as inferred from UV observations of Ar I lines. Helium, the most abundant, is largely undepleted but traces ISM dynamics through its high mobility.Meteorites and comets preserve primordial noble gas ratios from the early solar system, offering insights into cosmic origins unaltered by later processing. Carbonaceous chondrites and asteroid samples like those from Ryugu contain a planetary noble gas component (P1) in a carrier phase called Q, with isotopic ratios such as ^{20}Ne/^{22}Ne ≈ 12.5 and mass-fractionated Xe and Kr relative to solar wind, indicating trapping of presolar material.[46] Cometary ices and primitive meteorites show similar primordial He, Ne, and Ar ratios to BBN and early stellar outputs, with excesses like ^{129}Xe from short-lived isotopes confirming ancient nucleosynthetic signatures.[46] These compositions contrast sharply with Earth's depleted atmospheric noble gases, highlighting cosmic versus planetary fractionation.
Terrestrial Distribution
Noble gases are present in Earth's atmosphere primarily as trace components, with argon being the most abundant at 0.934% by volume in dry air, followed by neon at 18.18 ppm, helium at 5.24 ppm, krypton at 1.14 ppm, and xenon at 0.09 ppm; radon exists only in trace amounts due to its radioactivity and short half-life.[47] These fractions reflect the overall inert nature of noble gases, which do not participate in chemical reactions under standard conditions, allowing their abundances to be maintained through long-term atmospheric mixing. The isotopic composition of atmospheric helium, for example, shows a ³He/⁴He ratio of approximately 1.38 × 10⁻⁶, serving as the standard reference (Ra) for comparing other reservoirs.[48]In the Earth's crust, noble gases such as helium and radon are primarily produced through alpha decay of uranium and thorium in minerals, leading to radiogenic signatures with low ³He/⁴He ratios typically ranging from 0.01 to 0.1 Ra, in contrast to the higher primordial atmospheric value.[48] This process generates ⁴He and short-lived radon isotopes, contributing to crustal degassing, though overall crustal noble gas abundances are low compared to the atmosphere due to limited mobility and trapping in rocks. Mantle outgassing, observed in volcanic gases, exhibits elevated ³He/⁴He ratios up to 40 Ra (equivalent to about 5.5 × 10⁻⁵), indicating a primordial component preserved from Earth's early history, with mid-ocean ridge basalts averaging around 8 Ra.[48]Noble gases dissolve in ocean waters in equilibrium with the atmosphere, where argon reaches saturation concentrations of approximately 0.45 ppm under typical surface conditions, varying with temperature and salinity; these dissolved levels are utilized in paleoclimate studies to reconstruct past oceantemperatures through noble gas recharge proxies.[49] Relative to cosmic abundances derived from solar and meteoritic compositions, Earth shows significant depletion in heavier noble gases (Kr, Xe) by factors of 10 to 100, attributed to their high volatility during planetary accretion, which preferentially lost these elements to space or the nebula.[50] This depletion pattern underscores the role of high-temperature processes in differentiating terrestrial noble gas inventories from primordial solar nebula values.
Production and Extraction
Atmospheric Separation
The primary method for isolating noble gases from the atmosphere is cryogenic distillation, which exploits differences in boiling points to separate components of liquefied air. Air is first compressed to high pressures, typically around 5-10 atmospheres, and then cooled progressively through heat exchangers and expansion valves utilizing the Joule-Thomson effect, achieving liquefaction at approximately -196°C (77 K), the boiling point of nitrogen.[51] During fractional evaporation, nitrogen and oxygen—comprising over 99% of air—are distilled off first, leaving a residue enriched in the less volatile noble gases argon, neon, krypton, and xenon.[52]This residue undergoes further separation in rectification columns, where countercurrent flow of vapor and liquid phases allows precise fractionation based on boiling points: neon at -246°C, argon at -186°C, krypton at -153°C, and xenon at -108°C. Argon, the most abundant noble gas in air at about 0.93% by volume, is recovered at yields approaching 1% of input air volume, with commercial purities exceeding 99.999% after additional purification steps like catalytic deoxygenation.[51] Neon, krypton, and xenon, present at trace levels (0.0018%, 0.00011%, and 0.000009% respectively), require more complex multi-column setups for economic recovery, often as by-products of large-scale argon production.[53]Energy consumption for cryogenic air separation units is approximately 0.3-0.4 kWh per cubic meter of product gas, dominated by the refrigeration cycle needed for liquefaction and distillation.[54] Global argon production, derived almost entirely from atmospheric sources, totals around 700,000 metric tons annually, equivalent to roughly 400 million cubic meters at standard conditions.[55]Unlike the other noble gases, helium is not economically separated from the atmosphere due to its extremely low concentration (about 5 ppm); instead, it is extracted from natural gas deposits containing up to 2% helium. Common methods include pressure swing adsorption, where adsorbents selectively capture impurities like methane and nitrogen under pressure cycles, or low-temperature cryogenic distillation to liquefy heavier components while helium remains gaseous.[56] Global helium production is approximately 180 million cubic meters per year (as of 2024), primarily as a by-product of natural gas processing.[57]
Industrial Sources
Noble gases other than those derived primarily from atmospheric separation have significant industrial sources rooted in geological and radioactive processes. Helium, in particular, is extracted from natural gas reservoirs where it accumulates due to its primordial origin and low solubility. Concentrations in these fields typically range from 0.3% to 2% by volume, making extraction economically viable above the 0.3% threshold.[58] The Hugoton field in the United States, spanning Kansas, Oklahoma, and Texas, exemplifies such a source, with helium contents around 0.6% in prospective areas.[59] Extraction begins with pretreatment to remove methane and other hydrocarbons, followed by methods such as membrane separation or cryogenic distillation to isolate and purify helium.[60]Global helium resources are estimated at approximately 40 billion cubic meters, predominantly in natural gas fields in the United States, Qatar, and Russia.[57] Although shortages persisted into early 2025 due to depleting fields and rising demand from sectors like semiconductors and cryogenics, new production from projects such as Tanzania's Rukwa field has contributed to a supply glut by late 2025.[61][62] These non-atmospheric sources supplement air-derived helium but dominate global production, accounting for over 90% of supply.Radon, the heaviest noble gas, originates from the alpha decay of radium-226, itself a product of uranium-238 decay chains in ores and minerals. Unlike other noble gases, radon is collected directly from these radioactive sources for specialized applications, particularly in radiation therapy. Extraction involves degassing radon-laden air from uranium ore processing or using emanation chambers in former mine sites to capture the gas as it diffuses from radium-bearing materials.[63] With a half-life of 3.8 days, radon decays rapidly, limiting storage and necessitating on-site or short-term production for therapeutic use in Europe, where it is applied in balneotherapy for conditions like arthritis.[64] Annual production for such medical purposes is on the order of tens to hundreds of gigabecquerels, primarily from natural emanation in radon spas rather than synthetic generation.Argon, krypton, and xenon, while mainly sourced from atmospheric separation, can appear as minor byproducts in oxygen production plants where air is cryogenically fractionated; however, their industrial extraction remains overwhelmingly tied to air processing.[51] These geological and decay-based sources thus provide critical, albeit specialized, alternatives to atmospheric methods for helium and radon.
Compounds and Chemistry
Binary Compounds
Binary compounds of noble gases are limited to fluorides and a few oxide-fluorides of xenon and krypton, as helium and neon do not form stable binaries due to their high ionization energies and lack of low-lying d-orbitals for bonding.[65] These compounds arise from the ability of heavier noble gases to expand their octet through involvement of 5d orbitals, enabling reaction with the highly electronegative fluorine. No binary compounds of argon, radon, or oganesson have been isolated under standard conditions, though theoretical predictions suggest possible high-pressure formations.[66]The xenon fluorides represent the most studied binary noble gas compounds. Xenon difluoride (XeF₂) is prepared by direct fluorination of xenon with fluorine in a 1:1 molar ratio at 400°C in a nickel vessel or via photochemical reaction at room temperature using UV light.[67] The reaction is represented as:Xe+F2400∘C or UVXeF2XeF₂ adopts a linear molecular geometry with D∞h symmetry, consistent with VSEPR theory predicting three lone pairs in the equatorial plane of a trigonal bipyramidal electron arrangement. The Xe–F bond length is 1.98 Å in the gas phase, determined by rotational spectroscopy. Infrared and Raman spectroscopy reveal vibrational frequencies for the symmetric Xe–F stretch at approximately 515 cm⁻¹ and the antisymmetric Xe–F stretch at approximately 555 cm⁻¹, confirming the linear structure.[68]XeF₂ is a white, crystalline solid stable at room temperature but decomposes above 120°C to its elements.[67]Xenon tetrafluoride (XeF₄) is synthesized by heating xenon and fluorine in a 1:5 molar ratio at 400–600°C in a sealed nickel container.[69] It features a square planar geometry with D₄h symmetry, arising from a octahedral electron arrangement with two lone pairs trans to each other.[69] XeF₄ is a colorless, volatile solid stable up to 117°C, where it sublimes without decomposition.[67]Xenon hexafluoride (XeF₆) is obtained by reacting xenon with excess fluorine (1:20 molar ratio) at 250–300°C under pressure.[70] In the gas phase, it exhibits a distorted octahedral structure with C₃v symmetry due to a stereochemically active lone pair, though solid-state structures show fluxional behavior with bridging fluorines.[65] XeF₆ is a white solid that is highly reactive and hydrolyzes readily, but it is stable under anhydrous conditions up to its melting point of 48°C.[67]Krypton difluoride (KrF₂), the only stable binary compound of krypton, is prepared by electric discharge through a mixture of krypton and fluorine at low temperatures (−196°C to −78°C) or via UV photolysis.[71] It possesses a linear structure similar to XeF₂, with D∞h symmetry and Kr–F bond lengths of about 1.89 Å.[71] KrF₂ is a volatile, colorless solid that is explosive upon shock or heating above −60°C, decomposing to its elements.[65]Among oxide-fluoride hybrids, xenon dioxydifluoride (XeO₂F₂), the first such compound identified, is formed by the reaction of xenon trioxide with xenon oxytetrafluoride: XeO₃ + XeOF₄ → 2 XeO₂F₂.[72] It exhibits a trigonal bipyramidal geometry with axial fluorines and equatorial oxygens, C₂v symmetry, as determined by neutrondiffraction.[73] XeO₂F₂ is a pale yellow solid that decomposes above 100°C and is highly moisture-sensitive.[73]
Complex Molecules
Xenon trioxide (XeO₃) is a polyatomic oxide adopting a pyramidal molecular geometry, with the central xenon atom bonded to three oxygen atoms and featuring a lone pair, consistent with VSEPR theory for AX₃E species. This compound is highly unstable and explosive, decomposing above 25°C into xenon and oxygen gas.[74] Upon hydrolysis with water, XeO₃ forms xenic acid (H₂XeO₄), a moderately stable oxyacid in aqueous solution that serves as a strong oxidizing agent.[75]Xenon tetroxide (XeO₄), another polyatomic oxide, exhibits a tetrahedral structure with four equivalent Xe–O bonds and Td symmetry, determined through gas-phase electron diffraction.[76] Like XeO₃, it is explosive, decomposing violently above −35.9°C into xenon and oxygen.Krypton forms complex ionic compounds beyond simple binaries, such as the fluorokrypton cation salts [KrF]⁺[Sb₂F₁₁]⁻, prepared by reacting krypton difluoride with antimony pentafluoride and characterized via ¹⁹F NMR and Raman spectroscopy.[77] These salts highlight krypton's ability to stabilize higher oxidation states in polyfluoride environments, with the [KrF]⁺ cation acting as a potent fluorinating agent. Noble gas hydrides represent another class of complex species, exemplified by HArF (argon fluoride hydride), a neutral compound observed through matrix isolation in solid argon at approximately 8 K.[78] This molecule features a weak Ar–H bond and is metastable, transitioning to more stable forms upon warming to 25–27 K, as confirmed by infrared spectroscopy.[79]Clathrate hydrates provide inclusion compounds where krypton and xenon atoms are encapsulated within water cages under elevated pressure, forming structure I or II hydrates depending on the noble gas size. For instance, xenon hydrates stabilize at pressures above 0.1 MPa and temperatures below 280 K, with Xe occupying larger 5¹²6² cages, enabling applications in gas storage and separation from mixtures like Kr/Xe.[80]Krypton hydrates similarly form under moderate pressure (around 1.5 MPa at 273 K), offering a cage-based host-guest structure for noble gas containment without covalent bonding.[81]The bonding in many noble gas compounds, such as XeF₂, involves three-center four-electron (3c–4e) bonds, where a p-orbital from xenon overlaps with two fluorine atoms to delocalize four electrons across the F–Xe–F unit, explaining the linear geometry and stability despite formal octet expansion.[82] In 1978, the Xe₂⁺ dimer was isolated in fluoroantimonate superacid media, where oxidation of xenon yields the green [Xe₂]⁺ cation, stable as salts like [Xe₂]⁺[Sb₂F₁₁]⁻ and demonstrating weak Xe–Xe bonding.[83] For oganesson (Og), the heaviest noble gas, all compounds remain theoretical due to its extreme radioactivity and fleeting existence; predictions suggest potential reactivity, such as in OgTs₄ (tetratennesside) with tetrahedral geometry stabilized by relativistic effects, contrasting the inertness of lighter group 18 elements.[84]
Biological and Medical Roles
Interactions with Organisms
Noble gases generally exhibit minimal direct interactions with biological systems due to their chemical inertness, with lighter elements like neon and krypton showing no known biochemical roles or incorporation into living organisms.[19][85]Xenon, however, demonstrates notable physiological effects, primarily through non-covalent binding to proteins via van der Waals forces, which modulates receptor activity without forming chemical bonds.[86]Xenon acts as an anesthetic by antagonizing N-methyl-D-aspartate (NMDA) receptors in the central nervous system, inhibiting excitatory neurotransmission and inducing unconsciousness at concentrations around 70% mixed with oxygen for surgical procedures.[87] This mechanism contributes to its rapid onset and offset, with minimal impact on cardiovascular function, preserving heart rate and blood pressure stability compared to traditional volatile anesthetics.[88] Recent clinical investigations as of 2025 have explored xenon's neuroprotective potential in ischemic conditions, such as post-cardiac arrest brain injury, where inhalation trials show promise in reducing gray matter loss through NMDA antagonism and anti-apoptotic effects.[89]Helium, while inert under normal conditions, is incorporated into heliox breathing mixtures for deep-sea diving to prevent nitrogen narcosis by substituting for nitrogen, thereby avoiding impaired cognition at high pressures.[90] However, at extreme depths exceeding 150 meters, heliox can induce high-pressure nervous syndrome, characterized by tremors, dizziness, and cognitive disturbances due to direct neuronal compression effects.[91]Argon primarily poses risks as a simple asphyxiant, displacing oxygen in confined spaces and causing hypoxia at concentrations above 10%, leading to symptoms like dizziness and unconsciousness if oxygen levels fall below 19.5%.[92] In controlled biological applications, liquid argon supports cell cryopreservation by providing an inert, ultra-low-temperature environment for storing tissues and samples without oxidative damage.[93]Radon, a radioactive noble gas, interacts harmfully through its alpha-emitting decay products, which deposit high energy in lung tissues upon inhalation, significantly elevating lung cancer risk, particularly among smokers.[94] The U.S. Environmental Protection Agency sets an action level of 4 picocuries per liter (pCi/L) for indoor air to mitigate this carcinogenicity.[94]
Therapeutic Applications
Noble gases have found several specialized applications in therapeutic medicine, leveraging their inert properties and unique physical characteristics to enhance diagnostic imaging, alleviate respiratory symptoms, and protect organs during surgical procedures.Hyperpolarized noble gases, particularly helium-3 (³He) and xenon-129 (¹²⁹Xe), are inhaled to produce high-contrast magnetic resonance imaging (MRI) of lung ventilation, enabling the detection of regional defects not visible with conventional methods.[95] Hyperpolarization increases the nuclear spin alignment of these isotopes, dramatically boosting MRI signal intensity in air-filled lung spaces and allowing visualization of ventilated versus non-ventilated areas in conditions like asthma, chronic obstructive pulmonary disease, and cystic fibrosis.[95] Clinical studies have demonstrated ³He MRI's utility in quantifying ventilation heterogeneity, with applications in research settings since the early 2000s, though its use has been limited by helium scarcity.[96] In contrast, ¹²⁹Xe hyperpolarized MRI, marketed as Xenoview, received initial FDA approval in December 2022 for evaluating lung ventilation in adults and individuals aged 12 years and older, with the indication expanded in June 2025 to include children aged 6 and older, providing a more accessible alternative for detecting ventilation defects in pediatric and adult populations.[97][98] These techniques require medical-grade helium with purity exceeding 99.999% to ensure hyperpolarization efficiency and minimize impurities that could affect imaging quality.[99]Helium-oxygen mixtures, known as heliox, are employed in the management of acute respiratory distress and severe asthma exacerbations by reducing airway resistance and the work of breathing.[100] The lower density of helium compared to nitrogen decreases turbulent flow in narrowed airways, improving gas distribution and oxygenation in patients with obstructive lung diseases.[101] Clinical trials have shown heliox to be particularly effective in emergency settings for patients with respiratory acidosis, often serving as a bridge therapy to avoid intubation.[102] Typical mixtures contain 70-80% helium and 20-30% oxygen, administered via non-rebreather masks, with benefits observed in reducing hypercapnia and fatigue in both adults and children.[100]Radon inhalation therapy, practiced in European spas, involves low-dose exposure to radon-rich waters or air for treating rheumatism and other chronic pain conditions, predicated on the hormesis principle where low radiation levels may stimulate anti-inflammatory responses.[103] Patients typically undergo serial sessions in radon baths or galleries, with studies reporting sustained pain relief in rheumatic diseases up to nine months post-treatment.[104] A multicenter randomized trial confirmed additive analgesic effects, though the therapy remains controversial due to radon's carcinogenic risks at higher exposures and the need for further mechanistic validation.[105][106]Krypton-ion lasers emitting at 647 nm are utilized in ophthalmic procedures for retinal photocoagulation, targeting conditions such as branch retinal vein occlusion and diabetic macular edema with minimal scattering in ocular media.[107] The red wavelength is preferentially absorbed by melanin in the retinal pigment epithelium, allowing precise energy delivery to seal leaks or ablate neovascularization while reducing damage to overlying hemoglobin compared to shorter wavelengths.[108] Clinical outcomes include resolution of macular edema and improved visual acuity in treated eyes, positioning krypton lasers as an alternative to argon in select retinal therapies.[109]Xenon gas is incorporated into cardioplegia solutions for myocardial protection during heart transplants and cardiac surgeries, exploiting its neuroprotective and cardioprotective effects to mitigate ischemia-reperfusion injury.[110] Experimental models have shown that oxygen-xenon mixtures in high-pressure perfusion preserve rat heart function post-storage, outperforming standard crystalloid solutions by reducing oxidative stress and maintaining tissue viability for up to six hours.[111] In clinical contexts, xenon-enriched cardioplegia enhances graft preservation during transplantation, with preclinical data indicating better hemodynamic recovery upon reperfusion.[112]
Practical Applications
Lighting and Displays
Noble gases play a crucial role in various lighting and display technologies, leveraging their unique emission properties during electrical discharges to produce visible light efficiently.Neon signs utilize low-pressure neon gas excited by a high-voltage discharge of 12-15 kV, resulting in a distinctive red-orange glow from the gas's characteristic emission lines. Mixtures of noble gases, such as helium and neon, enable a range of colors; for example, helium-neon combinations produce yellow hues by blending their respective spectral emissions.[113]Fluorescent lamps employ a low-pressure mixture of argon as the primary noble gas buffer and mercury vapor, where the electrical discharge excites mercury atoms to emit ultraviolet radiation at 254 nm, which in turn stimulates a phosphor coating on the tube interior to convert the UV into visible white light.[114]In plasma display panels (PDPs) used for large-screen televisions, cells filled with a neon-xenon gas mixture—typically containing 5-10% xenon—undergo micro-discharges that generate vacuum ultraviolet emission from xenon, exciting red, green, and blue phosphors to form colored pixels.[115]Helium-neon lasers, operating with a gas mixture predominantly helium (about 10:1 ratio to neon) at low pressure, produce a coherent red beam at 632.8 nm through stimulated emission, achieving continuous-wave outputs up to 50 mW for applications in alignment and scanning.[116][117]The global market for neon-based lighting technologies is valued at approximately $500 million annually as of 2025, though it faces decline due to the superior energy efficiency and longevity of LED alternatives.[118]Neon discharge lamps exhibit luminous efficiencies in the range of 20-40 lm/W, providing a benchmark for gas-based illumination though lower than modern solid-state options.[119]
Industrial and Scientific Uses
Noble gases are integral to numerous industrial manufacturing processes owing to their chemical inertness and unique physical properties. Argon serves as a primary shielding gas in arc welding techniques, including Tungsten Inert Gas (TIG) and Metal Inert Gas (MIG) processes, where it envelops the weld pool to exclude oxygen and other reactive atmospheric gases, thereby preventing oxidation and ensuring high-quality joints, especially in alloys like stainless steel and aluminum.[120] For optimal performance in these applications, argon of at least 99.99% purity is employed to minimize impurities that could compromise weld integrity.[121]Helium's low boiling point makes it indispensable for cryogenic cooling in industrial and medical equipment, particularly superconducting magnets in Magnetic Resonance Imaging (MRI) systems, which are maintained at approximately 4 K to enable stable magnetic fields for diagnostic imaging.[122] Global helium demand for cryogenic applications, including MRI and particle accelerators, reaches around 15,000 tons annually, underscoring supply chain challenges and the gas's strategic importance.In advanced manufacturing, krypton and xenon form the active media in excimer lasers, such as the krypton fluoride (KrF) variant emitting at 248 nm, which provides ultraviolet light for photolithography in semiconductor production, enabling the precise patterning of integrated circuits with features below 100 nm.[123]Noble gases also underpin key scientific applications in geosciences. The ⁴⁰Ar/³⁹Ar dating technique, a variant of potassium-argon geochronology, measures the decay of ⁴⁰K to ⁴⁰Ar in potassium-bearing minerals to establish absolute ages for volcanic rocks and meteorites, with applicability spanning from about 10⁴ years to several billion years (up to 10⁹ years).[124] Complementary studies of helium diffusion in mantle minerals, such as aluminum oxide under high-pressure conditions, reveal how noble gases migrate through Earth's deep interior, informing models of convective processes and volatile cycling.[125]In geochemical sampling, mass spectrometry quantifies noble gas isotope ratios in geothermal fluids to trace mantle sources; mid-ocean ridge basalts (MORB) exhibit depleted helium and neon ratios indicative of processed upper mantle, whereas ocean island basalts (OIB) display higher ³He/⁴He ratios (up to 30-40 times atmospheric values) signaling contributions from primitive, undegassed deep mantle reservoirs.[126]As of 2025, xenon continues to advance space propulsion technologies through ion thrusters, where ionized xenon atoms are accelerated electrostatically for efficient, low-thrust trajectories; NASA's Advanced NEXT system achieves thrust levels up to 236 mN at 4190 s specific impulse, supporting extended deep-space missions, while ESA's Henon CubeSat demonstrates compact xenon-based engines for agile microsatellite maneuvers.[127][128]
Spectroscopic and Optical Properties
Emission Spectra
Noble gases exhibit distinct atomic emission spectra characterized by sharp, discrete lines resulting from electron transitions between energy levels in their atoms. These spectra are produced when the gases are excited, such as in electrical discharges or astrophysical environments, leading to the emission of photons at specific wavelengths. The lines are particularly prominent in the ultraviolet, visible, and near-infrared regions, with heavier noble gases showing more lines due to increased complexity in their electronic configurations.[129]For helium (He), the emission spectrum features prominent lines in the visible range from neutral helium (He I) transitions. Key lines include the yellow line at 587.6 nm (intensity 500 relative units) and the green line at 501.6 nm (intensity 100 relative units), arising from transitions such as 3d → 2p and 3p → 2s, respectively. These lines are crucial in astrophysics for stellar classification, particularly identifying hot B-type stars where neutral helium absorption or emission indicates temperatures around 10,000–30,000 K.[130][131][132][133]Neon (Ne) displays a rich spectrum with high-intensity lines in the visible region, making it suitable for detailed spectroscopic analysis. Notable emissions from neutral neon (Ne I) include the red line at 640.2 nm (intensity 200) and the yellow line at 585.2 nm (intensity 200), corresponding to transitions like 3p → 3s configurations. These lines contribute to neon's overall high visibility in the 500–700 nm range, with relative intensities reflecting the population of excited states.[134][135]Argon (Ar) emission lines are observed across the visible and near-infrared, with significant contributions from metastable states. Prominent neutral argon (Ar I) lines include the red emission at 696.5 nm (intensity 300) and the near-infrared line at 763.5 nm (intensity 700), the latter originating from the metastable level at approximately 11.5 eV (specifically the ³P₂ state at 11.55 eV). These metastable states, with long lifetimes, play a key role in sustaining discharges and influencing line ratios.[136][137]Heavier noble gases like krypton (Kr) and xenon (Xe) have spectra extending into the vacuum ultraviolet (VUV), where shorter wavelengths dominate due to higher energy transitions. For neutral krypton (Kr I), resonance lines appear at 116.5 nm and 123.6 nm in the VUV, corresponding to 5s → 4p excitations. Xenon (Xe I) similarly features VUV lines, such as at 129.6 nm and 147.0 nm, with additional emissions around 179 nm from higher-lying states. The following table summarizes selected prominent VUV lines for Kr and Xe:
These VUV lines are critical for applications requiring short-wavelength sources.[138][139]In electrical discharges, the relative intensities of emission lines from noble gases follow the Boltzmann distribution under local thermodynamic equilibrium, where the intensity I of a line is proportional to the statistical weight g, transition probability A, ground state density n, and exponential factor of the upper level energy E: I∝gAnexp(−kTE)
Here, k is Boltzmann's constant and T is the excitation temperature, allowing diagnostics of plasma conditions from observed line ratios. Deviations from this distribution indicate non-equilibrium effects.[140]Noble gas emission spectra serve as standards in spectroscopy due to their well-characterized, stable lines. For instance, helium's 587.6 nm line is used for wavelength calibration in astronomical instruments, while neon's visible lines provide references for visible and near-IR spectrometers. Argon and krypton lines in the VUV are employed in atomic physics experiments for precision measurements.[129][141]
Discharge Characteristics
Noble gases exhibit characteristic colors when ionized in low-pressure discharge tubes due to the excitation and recombination of electrons with gas atoms, producing visible emission spectra. Helium produces a pale yellow glow, neon a red-orange hue, argon a lavender light, krypton a greenish-white discharge, and xenon a blue-violet emission. Radon gas is colorless, but exhibits yellow phosphorescence when cooled to near its freezing point due to alpha-induced radioluminescence; its electric discharge spectrum has not been practically observed due to its radioactivity.[142][2][143][144][29]The glow discharge in noble gases occurs through a structured plasma mechanism involving distinct regions within the tube. Near the cathode, the cathode fall region features a high electric field that accelerates electrons, leading to ionization of gas atoms and the release of secondary electrons from the cathode surface. This sustains the discharge, transitioning into the positive column where the plasma is more uniform, with lower fields and light emission primarily from electron-ion recombination and excited atom de-excitation. These mechanisms ensure stable, visible glows at moderate currents and pressures.[145]Mixtures of noble gases enhance color options in applications like signage. For instance, a 90% helium and 10% neon blend produces a vibrant ruby red glow by combining helium's energy transfer to excite neon's red-emitting lines more efficiently than pure neon.[146]In radiation detection, noble gas mixtures are key in Geiger-Müller counters. Argon mixed with carbon dioxide (typically 90% argon and 10% CO₂) is used for beta particle detection, where incoming betas ionize the gas, triggering an avalanche discharge that produces a detectable pulse. The CO₂ acts as a quenching agent, absorbing energy to prevent continuous discharge after the initial event, allowing the counter to reset for subsequent detections.[147][148]Optimal glow discharge performance in noble gases occurs at pressures of 1-10 Torr, where the mean free path of electrons allows sufficient collisions for ionization without excessive collisions that would quench the plasma. At lower pressures, the discharge may not sustain, while higher pressures lead to arc formation or dimming.While noble gas discharges require high voltages (typically 100-1000 V) to initiate and maintain the plasma, posing risks of electrical shock or burns, the inert nature of these gases eliminates explosion hazards from chemical reactions, unlike discharges in reactive gases.[145]
