Noble gas

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

Legend

primordial element
element by radioactive decay
Atomic number color: red=gas

The noble gases (historically the inert gases, sometimes referred to as aerogens

chemical reactivity and cryogenic
boiling points.

The noble gases' inertness, or tendency not to react with other chemical substances, results from their electron configuration: their outer shell of valence electrons is "full", giving them little tendency to participate in chemical reactions. Only a few hundred noble gas compounds are known to exist. For the same reason[clarification needed], noble gas atoms are small, and the only intermolecular force between them is the very weak London dispersion force, so their boiling points are all cryogenic, below 165 K (−108 °C; −163 °F).[2]

The inertness of noble gases makes them useful whenever chemical reactions are unwanted. For example, argon is used as a

air using the methods of liquefaction of gases and fractional distillation. Helium is also a byproduct of the mining of natural gas. Radon is usually isolated from the radioactive decay of dissolved radium, thorium, or uranium
compounds.

The seventh member of group 18 is

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]
In the rest of this article, the term "noble gas" should be understood not to include oganesson unless it is specifically mentioned.

History

Noble gas is translated from the

Earth's atmosphere due to decay of radioactive potassium-40.[8]

A line spectrum chart of the visible spectrum showing sharp lines on top.
Helium was first detected in the Sun due to its characteristic spectral lines.

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]

In October 2006, scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully created synthetically oganesson, the seventh element in group 18,[21] by bombarding californium with calcium.[22]

Physical and atomic properties

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

The noble gases have weak

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.

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

anions; that is, they have a negative electron affinity.[32]

The

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]

The noble gases are nearly

isotropic
.

Chemical properties

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]

Electron configuration

Further information:
Noble gas configuration

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

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

The noble gases have full valence

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.

Compounds

Main article: Noble gas compound
A model of planar chemical molecule with a blue center atom (Xe) symmetrically bonded to four peripheral atoms (fluorine).
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] The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn ≪ Og.

In 1933,

thermodynamically and kinetically unstable.[43]

electronegative atoms such as fluorine or oxygen, as in xenon difluoride (XeF
2), xenon tetrafluoride (XeF
4), xenon hexafluoride (XeF
6), xenon tetroxide (XeO
4), and sodium perxenate
(Na
4XeO
6). Xenon reacts with fluorine to form numerous xenon fluorides according to the following equations:

Xe + F2 → XeF2
Xe + 2F2 → XeF4
Xe + 3F2 → XeF6

Some of these compounds have found use in

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, sulphur, 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

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]

A skeletal structure of buckminsterfullerene with an extra atom in its center.
An endohedral fullerene compound containing a noble gas atom

Noble gases can form

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]

Schematic illustration of bonding and antibonding orbitals (see text)
Bonding in XeF
2 according to the 3-center-4-electron bond model

Noble gas compounds such as

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.

Occurrence and production

The abundances of the noble gases in the universe decrease as their

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] After the discovery of xenon dioxide, research showed that Xe can substitute for Si in quartz.[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.

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

For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain up to 7% helium.[80]

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.

Applications

A large solid cylinder with a hole in its center and a rail attached to its side.
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

nuclear magnetic resonance imaging and nuclear magnetic resonance.[82] 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

drysuit inflation gas for scuba diving.[86] Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.[87]

Cigar-shaped blimp with "Good Year" written on its side.
Goodyear Blimp

Since the Hindenburg disaster in 1937,[88] helium has replaced hydrogen as a lifting gas in blimps and balloons: despite an 8.6%[89] decrease in buoyancy compared to hydrogen, helium is not combustible.[13]

In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis of

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]

xenon short-arc lamp used in IMAX
projectors

Noble gases are commonly used in

continuous spectrum that resembles daylight, find application in film projectors and as automobile headlamps.[78]

The noble gases are used in

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.[91]

Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people with

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.

Discharge color

Colors and spectra (bottom row) of electric discharge in noble gases; only the second row represents pure gases.
Glass tube shining violet light with a wire wound over it Glass tube shining orange light with a wire wound over it Glass tube shining purple light with a wire wound over it Glass tube shining white light with a wire wound over it Glass tube shining blue light with a wire wound over it
Glass tube shining light red Glass tube shining reddish-orange Glass tube shining purple Glass tube shining bluish-white Glass tube shining bluish-violet
Illuminated light red gas discharge tubes shaped as letters H and e Illuminated orange gas discharge tubes shaped as letters N and e Illuminated light blue gas discharge tubes shaped as letters A and r Illuminated white gas discharge tubes shaped as letters K and r Illuminated violet gas discharge tubes shaped as letters X and e
Helium line spectrum Neon line spectrum Argon line spectrum Krypton line spectrum Xenon line spectrum
Helium Neon Argon Krypton Xenon

The color of gas discharge emission depends on several factors, including the following:[94]

  • 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.

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References

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Noble gas
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