Noble gas

The noble gases (historically also the inert gases; sometimes referred to as aerogens[1]) make up a group of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity. The six noble gases that occur naturally are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). Oganesson (Og) is variously predicted to be a noble gas as well or to break the trend due to relativistic effects; its chemistry has not yet been investigated.

For the first six periods of the periodic table, the noble gases are exactly the members of group 18. Noble gases are typically highly unreactive except when under particular extreme conditions. The inertness of noble gases makes them very suitable in applications where reactions are not wanted. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; also, helium is used in breathing gas by deep-sea divers to prevent oxygen, nitrogen and carbon dioxide (hypercapnia) toxicity.

The properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, and it has been possible to prepare only a few hundred noble gas compounds. The melting and boiling points for a given noble gas are close together, differing by less than 10 °C (18 °F); that is, they are liquids over only a small temperature range.

Neon, argon, krypton, and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation. Helium is sourced from natural gas fields that have high concentrations of helium in the natural gas, using cryogenic gas separation techniques, and radon is usually isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. Noble gases have several important applications in industries such as lighting, welding, and space exploration. A helium-oxygen breathing gas is often used by deep-sea divers at depths of seawater over 55 m (180 ft). After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons.

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
Helium discharge tube
Helium (He)
2
2
Neon discharge tube
Neon (Ne)
10
3
Argon discharge tube
Argon (Ar)
18
4
Krypton discharge tube
Krypton (Kr)
36
5
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

History

Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann[2] 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.[3] Rare gases is another term that was used,[4] but this is also inaccurate because argon forms a fairly considerable part (0.94% by volume, 1.3% by mass) of the Earth's atmosphere due to decay of radioactive potassium-40.[5]

Helium spectrum
Helium was first detected in the Sun due to its characteristic spectral lines.

Pierre Janssen and Joseph Norman Lockyer discovered a new element on August 18, 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, ἥλιος (hḗlios).[6] 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.[7] 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").[7] 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.[8]

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,[9] 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.[10] Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry, respectively, for their discovery of the noble gases;[11][12] 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".[12]

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.[10] 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.[13]

In 1962, Neil Bartlett discovered the first chemical compound of a noble gas, xenon hexafluoroplatinate.[14] Compounds of other noble gases were discovered soon after: in 1962 for radon, radon difluoride (RnF
2
),[15] which was identified by radiotracer techniques and in 1963 for krypton, krypton difluoride (KrF
2
).[16] 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).[17]

In December 1998, scientists at the Joint Institute for Nuclear Research working in Dubna, Russia bombarded plutonium (Pu) with calcium (Ca) to produce a single atom of element 114,[18] flerovium (Fl).[19] Preliminary chemistry experiments have indicated this element may be the first superheavy element to show abnormal noble-gas-like properties, even though it is a member of group 14 on the periodic table.[20] In October 2006, scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully created synthetically oganesson (Og), the seventh element in group 18,[21] by bombarding californium (Cf) with calcium (Ca).[22]

Physical and atomic properties

Property[10][23] Helium Neon Argon Krypton Xenon Radon
Density (g/dm3) 0.1786 0.9002 1.7818 3.708 5.851 9.97
Boiling point (K) 4.4 27.3 87.4 121.5 166.6 211.5
Melting point (K) 0.95
(at 25 bar)
24.7 83.6 115.8 161.7 202.2
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
Atomic radius (calculated) (pm) 31 38 71 88 108 120
Ionization energy (kJ/mol) 2372 2080 1520 1351 1170 1037
Allen electronegativity[24] 4.16 4.79 3.24 2.97 2.58 2.60

The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements.[10] Helium has several unique qualities when compared with other elements: its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit superfluidity; it is the only element that cannot be solidified by cooling under standard conditions—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.[25] The noble gases up to xenon have multiple stable isotopes. 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.[10] Melting and boiling points generally increase going down the group.

Ionization energies
This is a plot of ionization potential versus atomic number. The noble gases, which are labeled, have the largest ionization potential for each period.

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.[14] Noble gases cannot accept an electron to form stable anions; that is, they have a negative electron affinity.[26]

The macroscopic physical properties of the noble gases are dominated by the weak van der Waals forces between the atoms. The attractive force increases with the size of the atom as a result of the increase in polarizability and the decrease in ionization potential. This results in systematic group trends: as one goes down group 18, the atomic radius, and with it the interatomic forces, increases, 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 ideal gases under standard conditions, but their deviations from the ideal gas law provided important clues for the study of intermolecular interactions. The Lennard-Jones potential, often used to model intermolecular interactions, was deduced in 1924 by John Lennard-Jones from experimental data on argon before the development of quantum mechanics provided the tools for understanding intermolecular forces from first principles.[27] The theoretical analysis of these interactions became tractable because the noble gases are monatomic and the atoms spherical, which means that the interaction between the atoms is independent of direction, or isotropic.

Chemical properties

Electron shell 010 Neon - no label
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.[28] 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.[10]

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

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.[29] 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.[30]

Compounds

Xenon-tetrafluoride-3D-vdW
Structure of 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 there is some theoretical evidence for a few helium compounds), while xenon, krypton, and argon have shown only minor reactivity.[31] The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn.

In 1933, Linus Pauling predicted that the heavier noble gases could form compounds with fluorine and oxygen. He predicted the existence of krypton hexafluoride (KrF
6
) and xenon hexafluoride (XeF
6
), speculated that XeF
8
might exist as an unstable compound, and suggested that xenic acid could form perxenate salts.[32][33] These predictions were shown to be generally accurate, except that XeF
8
is now thought to be both thermodynamically and kinetically unstable.[34]

Xenon compounds are the most numerous of the noble gas compounds that have been formed.[35] Most of them have the xenon atom in the oxidation state of +2, +4, +6, or +8 bonded to highly electronegative atoms such as fluorine or oxygen, as in xenon difluoride (XeF
2
), xenon tetrafluoride (XeF
4
), xenon hexafluoride (XeF
6
), xenon tetroxide (XeO
4
), and sodium perxenate (Na
4
XeO
6
). Xenon reacts with fluorine to form numerous xenon fluorides according to the following equations:

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

Some of these compounds have found use in chemical synthesis as oxidizing agents; XeF
2
, in particular, is commercially available and can be used as a fluorinating agent.[36] 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.[31][37] 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.[31]

In theory, radon is more reactive than xenon, and therefore should form 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.[38]

Krypton is less reactive than xenon, but several compounds have been reported with krypton in the oxidation state of +2.[31] 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,[39] but are only stable below −60 °C (−76 °F) and −90 °C (−130 °F) respectively.[31]

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.[31] 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.[31] As of 2007, no stable neutral molecules involving covalently bound helium or neon are known.[31]

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.[40] Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it is believed to occur naturally in the interstellar medium, although it has not been detected yet.[41] 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,[42] 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.[43] Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice.[44]

Endohedral fullerene
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).[45] As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been obtained.[46] 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.[47]

XeF2
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.[48][49] 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.[50]

The chemistries 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 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).[51][52] 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.[53] 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.[53] 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.[54] 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.[55] After the discovery of xenon dioxide, research showed that Xe can substitute for Si in quartz.[56] 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.[57]

Abundance Helium Neon Argon Krypton Xenon Radon
Solar System (for each atom of silicon)[58] 2343 2.148 0.1025 5.515 × 10−5 5.391 × 10−6
Earth's atmosphere (volume fraction in ppm)[59] 5.20 18.20 9340.00 1.10 0.09 (0.06–18) × 10−19[60]
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)[61]
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.[62]

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.[10] 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

Modern 3T MRI
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 cryogenic refrigerants.[63] 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.[64] 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.[60]

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.[65] 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.[66] Helium's reduced solubility offers further advantages for the condition known as decompression sickness, or the bends.[10][67] 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.[68] Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.[69]

Goodyear-blimp
Goodyear Blimp

Since the Hindenburg disaster in 1937,[70] helium has replaced hydrogen as a lifting gas in blimps and balloons due to its lightness and incombustibility, despite an 8.6%[71] decrease in buoyancy.[10]

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.[72] 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.[61] 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.[60]

Xenon short arc 1
15,000-watt xenon short-arc lamp used in IMAX projectors

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.[60] 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.[60] The noble gases glow in distinctive colors when used inside gas-discharge lamps, such as "neon lights". These lights are called after neon but often contain other gases and phosphors, which add various hues to the orange-red color of neon. Xenon is commonly used in xenon arc lamps, which, due to their nearly continuous spectrum that resembles daylight, find application in film projectors and as automobile headlamps.[60]

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.[73]

Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of asthma sufferers.[60] 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.[74] Xenon finds application in medical imaging of the lungs through hyperpolarized MRI.[75] Radon, which is highly radioactive and is only available in minute amounts, is used in radiotherapy.[10]

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.

Discharge color

Colors and spectra (bottom row) of electric discharge in noble gases; only the second row represents pure gases.
Helium-glow Neon-glow Argon-glow Krypton-glow Xenon-glow
Helium discharge tube Neon discharge tube Argon discharge tube Krypton discharge tube Xenon discharge tube
HeTube NeTube ArTube KrTube XeTube
Helium spectra Neon spectra Argon Spectrum Krypton Spectrum Xenon Spectrum
Helium Neon Argon Krypton Xenon

The color of gas discharge emission depends on several factors, including the following:[76]

  • 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.

See also

Notes

  1. ^ Bauzá, Antonio; Frontera, Antonio (2015). "Aerogen Bonding Interaction: A New Supramolecular Force?". Angewandte Chemie International Edition. 54 (25): 7340–3. doi:10.1002/anie.201502571.
  2. ^ Renouf, Edward (1901). "Noble gases". Science. 13 (320): 268–270. Bibcode:1901Sci....13..268R. doi:10.1126/science.13.320.268.
  3. ^ Ozima 2002, p. 30
  4. ^ Ozima 2002, p. 4
  5. ^ "argon". Encyclopædia Britannica. 2008.
  6. ^ Oxford English Dictionary (1989), s.v. "helium". Retrieved December 16, 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."
  7. ^ a b Ozima 2002, p. 1
  8. ^ Mendeleev 1903, p. 497
  9. ^ Partington, J. R. (1957). "Discovery of Radon". Nature. 179 (4566): 912. Bibcode:1957Natur.179..912P. doi:10.1038/179912a0.
  10. ^ a b c d e f g h i j "Noble Gas". Encyclopædia Britannica. 2008.
  11. ^ Cederblom, J. E. (1904). "The Nobel Prize in Physics 1904 Presentation Speech".
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References

Argon

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive", as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.

Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. Argon is also used in incandescent, fluorescent lighting, and other gas-discharge tubes. Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters.

Argonium

Argonium, an ion combining a proton and an argon atom (and so also called protonated argon), (ArH+) can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space.

Chassigny (meteorite)

Chassigny is a Mars meteorite which fell on October 3, 1815, at approximately 8:00 am, in Chassigny, Haute-Marne, France. Chassigny is the meteorite for which the chassignites are named, and gives rise to the "C" in SNCs. Chassigny is an olivine cumulate rock (dunite). It consists almost entirely of olivine with intercumulous pyroxene, feldspar, and oxides. Chassigny was the only known chassignite until NWA2737 was found in the Moroccan Sahara in northwest Africa.

Chassigny is particularly important because, unlike most SNCs, it contains noble gas compositions different from the current Martian atmosphere. These differences are presumably due to its cumulate (mantle-derived) nature.

Chemically inert

In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. Most Group 8 or 18 elements that appear in the last column of the periodic table (Helium, Neon, Argon, Krypton, Xenon and Radon) are classified as inert (or unreactive). These elements are stable in their naturally occurring form (gaseous form) and they are called inert gases.

High-intensity discharge lamp

High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with noble gas and often also contains suitable metal or metal salts. The noble gas enables the arc's initial strike. Once the arc is started, it heats and evaporates the metallic admixture. Its presence in the arc plasma greatly increases the intensity of visible light produced by the arc for a given power input, as the metals have many emission spectral lines in the visible part of the spectrum. High-intensity discharge lamps are a type of arc lamp.

Brand new high-intensity discharge lamps make more visible light per unit of electric power consumed than fluorescent and incandescent lamps, since a greater proportion of their radiation is visible light in contrast to infrared. However, the lumen output of HID lighting can deteriorate by up to 70% over 10,000 burning hours.

Many modern vehicles use HID bulbs for the main lighting systems, some applications are now moving from HID bulbs to LED and laser technology. However, this HID technology is not new and was first demonstrated by Francis Hauksbee in 1705.

Inert

Inert may refer to:

Chemically inert, not chemically reactive

Inert gas

Noble gas, historically called inert gas

Inert knowledge, information which one can express but not use

Inert waste, waste which is neither chemically nor biologically reactive and will not decompose

Inert, a mathematical term in splitting of prime ideals in Galois extensions

Krypton

Krypton (from Ancient Greek: κρυπτός, translit. kryptos "the hidden one") is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. With rare exceptions, krypton is chemically inert.

Krypton, like the other noble gases, is used in lighting and photography. Krypton light has many spectral lines, and krypton plasma is useful in bright, high-powered gas lasers (krypton ion and excimer lasers), each of which resonates and amplifies a single spectral line. Krypton fluoride also makes a useful laser medium. From 1960 to 1983, the official length of a meter was defined by the 605 nm wavelength of the orange spectral line of krypton-86, because of the high power and relative ease of operation of krypton discharge tubes.

Matrix isolation

Matrix isolation is an experimental technique used in chemistry and physics which generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles (atoms, molecules, ions, etc.) are embedded. The guest is said to be isolated within the host matrix. Initially the term matrix-isolation was used to describe the placing of a chemical species in any unreactive material, often polymers or resins, but more recently has referred specifically to gases in low-temperature solids. A typical matrix isolation experiment involves a guest sample being diluted in the gas phase with the host material, usually a noble gas or nitrogen. This mixture is then deposited on a window that is cooled to below the melting point of the host gas. The sample may then be studied using various spectroscopic procedures.

Neon

Neon is a chemical element with symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton and xenon) in 1898 as one of the three residual rare inert elements remaining in dry air, after nitrogen, oxygen, argon and carbon dioxide were removed. Neon was the second of these three rare gases to be discovered and was immediately recognized as a new element from its bright red emission spectrum. The name neon is derived from the Greek word, νέον, neuter singular form of νέος (neos), meaning new. Neon is chemically inert, and no uncharged neon compounds are known. The compounds of neon currently known include ionic molecules, molecules held together by van der Waals forces and clathrates.

During cosmic nucleogenesis of the elements, large amounts of neon are built up from the alpha-capture fusion process in stars. Although neon is a very common element in the universe and solar system (it is fifth in cosmic abundance after hydrogen, helium, oxygen and carbon), it is rare on Earth. It composes about 18.2 ppm of air by volume (this is about the same as the molecular or mole fraction) and a smaller fraction in Earth's crust. The reason for neon's relative scarcity on Earth and the inner (terrestrial) planets is that neon is highly volatile and forms no compounds to fix it to solids. As a result, it escaped from the planetesimals under the warmth of the newly ignited Sun in the early Solar System. Even the outer atmosphere of Jupiter is somewhat depleted of neon, although for a different reason. It is also lighter than air, causing it to escape even from Earth's atmosphere.

Neon gives a distinct reddish-orange glow when used in low-voltage neon glow lamps, high-voltage discharge tubes and neon advertising signs. The red emission line from neon also causes the well known red light of helium–neon lasers. Neon is used in some plasma tube and refrigerant applications but has few other commercial uses. It is commercially extracted by the fractional distillation of liquid air. Since air is the only source, it is considerably more expensive than helium.

Noble gas compound

Noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon. From the standpoint of chemistry, the noble gases may be divided into two groups: the relatively reactive krypton (ionisation energy 14.0 eV), xenon (12.1 eV), and radon (10.7 eV) on one side, and the very unreactive argon (15.8 eV), neon (21.6 eV), and helium (24.6 eV) on the other. Consistent with this classification, Kr, Xe, and Rn form compounds that can be isolated in bulk at or near standard temperature and pressure (at least in principle for the highly radioactive radon), whereas He, Ne, Ar have been observed to form true chemical bonds using spectroscopic techniques, but only when frozen into a noble gas matrix at temperatures of 40 K or lower, in supersonic jets of noble gas, or under extremely high pressures with metals.

The heavier noble gases have more electron shells than the lighter ones. Hence, the outermost electrons are subject to a shielding effect from the inner electrons that makes them more easily ionized, since they are less strongly attracted to the positively charged nucleus. This results in an ionization energy low enough to form stable compounds with the most electronegative elements, fluorine and oxygen, and even with less electronegative elements such as nitrogen and carbon under certain circumstances.

Noble gas configuration

Noble gas configuration is the tendency for chemical elements to gain or lose electrons to reach the electronic configuration of a noble gas. This may refer to:

Octet rule, the electronic rule for main group elements.

18-electron rule, the electronic rule for transition elements.

Nonmetal

In chemistry, a nonmetal (or non-metal) is a chemical element that mostly lacks the characteristics of a metal. Physically, a nonmetal tends to have a relatively low melting point, boiling point, and density. A nonmetal is typically brittle when solid and usually has poor thermal conductivity and electrical conductivity. Chemically, nonmetals tend to have relatively high ionization energy, electron affinity, and electronegativity. They gain or share electrons when they react with other elements and chemical compounds. Seventeen elements are generally classified as nonmetals: most are gases (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon); one is a liquid (bromine); and a few are solids (carbon, phosphorus, sulfur, selenium, and iodine). Metalloids such as boron, silicon, and germanium are sometimes counted as nonmetals.

The nonmetals are divided into two categories reflecting their relative propensity to form chemical compounds: reactive nonmetals and noble gases. The reactive nonmetals vary in their nonmetallic character. The less electronegative of them, such as carbon and sulfur, mostly have weak to moderately strong nonmetallic properties and tend to form covalent compounds with metals. The more electronegative of the reactive nonmetals, such as oxygen and fluorine, are characterised by stronger nonmetallic properties and a tendency to form predominantly ionic compounds with metals. The noble gases are distinguished by their great reluctance to form compounds with other elements.

The distinction between categories is not absolute. Boundary overlaps, including with the metalloids, occur as outlying elements in each category show or begin to show less-distinct, hybrid-like, or atypical properties.

Although five times more elements are metals than nonmetals, two of the nonmetals—hydrogen and helium—make up over 99 percent of the observable universe. Another nonmetal, oxygen, makes up almost half of the Earth's crust, oceans, and atmosphere. Living organisms are composed almost entirely of nonmetals: hydrogen, oxygen, carbon, and nitrogen. Nonmetals form many more compounds than metals.

Period (periodic table)

A period in the periodic table is a row of chemical elements. All elements in a row have the same number of electron shells. Each next element in a period has one more proton and is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law. For example, the alkali metals lie in the first column (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration. As of 2016, a total of 118 elements have been discovered and confirmed.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown at right. The filling of each shell corresponds to a row in the table.

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods.

Periodic table (electron configurations)

Configurations of elements 109 and above are not available. Predictions from reliable sources have been used for these elements.

Grayed out electron numbers indicate subshells that are filled to their maximum.

The bracketed noble gas symbols on the left represent the inner configurations that are the same in each period. Written out these are:He, 2, helium : 1s2

Ne, 10, neon : 1s2 2s2 2p6

Ar, 18, argon : 1s2 2s2 2p6 3s2 3p6

Kr, 36, krypton : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

Xe, 54, xenon : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6

Rn, 86, radon : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6

Og, 118, oganesson : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6Note the non-linear shell ordering, which comes about due to the different energies of smaller and larger shells.

Radon difluoride

Radon difluoride (RnF2) is a compound of radon, a noble gas. Radon reacts readily with fluorine to form a solid compound, but this decomposes on attempted vaporization and its exact composition is uncertain. Calculations suggest that it may be ionic, unlike all other known binary noble gas compounds. The usefulness of radon compounds is limited because of the radioactivity of radon. The longest-lived isotope, radon-222, has a half-life of only 3.82 days, which decays by α-emission to yield polonium-218.

Xenon

Xenon is a chemical element with symbol Xe and atomic number 54. It is a colorless, dense, odorless noble gas found in the Earth's atmosphere in trace amounts. Although generally unreactive, xenon can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.Xenon is used in flash lamps and arc lamps, and as a general anesthetic. The first excimer laser design used a xenon dimer molecule (Xe2) as the lasing medium, and the earliest laser designs used xenon flash lamps as pumps. Xenon is used to search for hypothetical weakly interacting massive particles and as the propellant for ion thrusters in spacecraft.Naturally occurring xenon consists of eight stable isotopes. More than 40 unstable xenon isotopes undergo radioactive decay, and the isotope ratios of xenon are an important tool for studying the early history of the Solar System. Radioactive xenon-135 is produced by beta decay from iodine-135 (a product of nuclear fission), and is the most significant (and unwanted) neutron absorber in nuclear reactors.

Xenon hexafluoride

Xenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three known binary fluorides of xenon, the other two being XeF2 and XeF4. All known are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series. At room temperature, it is a colorless solid that readily sublimes into intensely yellow vapors.

Xenon tetrafluoride

Xenon tetrafluoride is a chemical compound with chemical formula XeF4. It was the first discovered binary compound of a noble gas. It is produced by the chemical reaction of xenon with fluorine, F2, according to the chemical equation:

Xe + 2 F2 → XeF4This reaction is exothermic, releasing an energy of 251 kJ/mol of xenon.Xenon tetrafluoride is a colorless crystalline substance under ordinary conditions. Its crystalline structure was determined by both NMR spectroscopy and X-ray crystallography in 1963. The structure is square planar, as has been confirmed by neutron diffraction studies, and is justified by VSEPR theory because xenon has two lone pairs of electrons: one above and one below the plane of the molecule.

Xenon tetrafluoride sublimes at a temperature of 115.7 °C (240.26 °F).

The formation of xenon tetrafluoride, like the other xenon fluorides, is exergonic. They are stable at normal temperatures and pressures. All of them readily react with water, releasing pure xenon gas, hydrogen fluoride, and molecular oxygen. This reaction occurs in slightly moist air; hence, all xenon fluorides must be kept in anhydrous atmospheres.

Xenon tetroxide

Xenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO4, remarkable for being a relatively stable compound of a noble gas. It is a yellow crystalline solid that is stable below −35.9 °C; above that temperature it is very prone to exploding and decomposing into elemental xenon and oxygen (O2).All eight valence electrons of xenon are involved in the bonds with the oxygen, and the oxidation state of the xenon atom is +8. Oxygen is the only element that can bring xenon up to its highest oxidation state; even fluorine can only give XeF6 (+6).

Two other short-lived xenon compounds with an oxidation state of +8, XeO3F2 and XeO2F4, are accessible by the reaction of xenon tetroxide with xenon hexafluoride. XeO3F2 and XeO2F4 can be detected with mass spectrometry. The perxenates are also compounds where xenon has the +8 oxidation state.

Periodic table forms
Sets of elements
Elements
History
See also
Noble gases

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