Noble gas

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Group	18
Period
1	2 He
2	10 Ne
3	18 Ar
4	36 Kr
5	54 Xe
6	86 Rn
7	118 Uuo

The noble gases are the nonmetal, inert elements in group 18 (previously known as group 0) of the periodic table. Chemically, they are very stable because they have the maximum number of valence electrons their outer shell can hold, causing them to rarely react with other elements. Under normal conditions, they are odorless, colorless, monatomic gases. The melting and boiling points for each noble gas are close together; consequently, only a small temperature range exists for each to be in a liquid state.

The five noble gases that have stable isotopes are helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe). They have several important applications in industries such as lighting, welding, and space exploration. There is also one naturally-occuring radioactive noble gas, radon (Rn), which to its radioactivity, has been studied much less than the previous five members of the group. A synthetic member of the group ununoctium (Uuo) has been discovered, but very little is known of its properties due to its extremely limited availability. Another synthetic element, ununquadium (Uuq), may turn out to have similar chemical and physical characteristics as other noble gases, and therefore may be a "noble gas" without being a member of group 18.

Contents 1 History and etymologies 2 Chemical properties 2.1 Compounds 2.2 Bonding 3 Physical properties 4 Applications 5 See also 6 Notes 7 References

[edit] History and etymologies

"Noble gas" is translated from the German word Edelgas first used in 1898 by Hugo Erdmann^[1] to refer to the extremely low level of reactivity that most of the elements in Group 18 exhibit under normal conditions. The noble gases have also been referred to as "inert gases", but this is an inaccurate label because several of them participate in chemical reactions.^[2] Another term that is seldom used is "rare gases",^[3] 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.^[4]

Pierre Janssen and Joseph Norman Lockyer, were the first to discover a noble gas in 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek name of the Sun, ἥλιος (Helios).^[5][6] Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance that was less reactive than nitrogen.^[7] A century later, in 1895, Lord Rayleigh discovered that some samples of nitrogen from the air were a different density than nitrogen that resulted from chemical reactions. Along with scientist William Ramsay, Lord Rayleigh theorized that the nitrogen extracted from air was associated with another gas, leading to an experiment that successfully isolated a new element, argon, for the Greek αργό(ν) (inactive).^[7] With this discovery, they realized that an entire class of gases was missing from the periodic table. In 1895, in his search for argon, Ramsay also managed to isolate helium for the first time, while heating the mineral clevite. In 1902, having accepted the evidence for elements helium and argon, Mendeleev placed these Noble Gases in Group 0 in his arrangement of the elements.^[8]

Ramsay continued to search for these gases using the method of fractional distillation to separate liquid air into several components, and in 1898 discovered the elements krypton, neon, and xenon, and named them after the Greek κρυπτός (kryptos, hidden), νέος (neos, new), and ξένος (xenos, stranger). Radon was first identified in 1898 by Friedrich Ernst Dorn,^[9] and was named as radium emanation, but was not considered a noble gas until 1904, when its characteristics were similar to other noble gases.^[10]

The discovery of the noble gases helped to develop general understanding of atomic structure. In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine and argon, one of the noble gases, but this failed. Scientists were unable to prepare chemical compounds of argon until the early 20th century, although these attempts still 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 for helium, the outermost shell always contains eight electrons. In 1916, research concluded that an octet of electrons in the outer shell was the most stable arrangement for any atom, causing them to be unreactive with any other element because they did not require any more electrons to complete the outer shell.^[10]

It was not until 1962 when Neil Bartlett discovered the first true chemical compound of a noble gas, xenon hexafluoroplatinate (Xe⁺[PtF₆]⁻).^[11] Soon, the first compounds of other noble gases were discovered: in 1962 for radon, radon fluoride,^[12] and in 1963 for krypton, krypton difluoride (KrF₂).^[13] The first stable compound of argon was reported in 2000, when argon fluorohydride (HArF) was characterized at very low temperatures (40 Kelvins).^[14]

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,^[15] temporarily named ununquadium (Uuq).^[16] First chemistry experiments indicate that this element may be the first superheavy element to show abnormal noble-gas-like properties.^[17] Another discovery was made in October 2006 when scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully obtained ununoctium (Uuo) by bombarding californium (Cf) with calcium (Ca),^[18] a synthetically created element in Group 18.^[19] There is currently no practical use for the element because it is very unstable.^[20]

[edit] Chemical properties

The noble gases make up Group 18 of the periodic table. The confirmed members are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).^[21] All these noble gases are colorless, odorless, tasteless, and nonflammable under normal conditions. They were once labeled Group 0 in the periodic table because it was believed that they had a valence of zero, so their atoms could not combine with those of other elements to form chemical compounds. However, it was later discovered that some of them do indeed form compounds, causing this label to fall into disuse.^[10] Very little is known about the properties of the last member, ununoctium (Uuo).

All of the noble gases have full valence electron shells.^[22] 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. 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. In 1962, an experiment successfully removed electrons from xenon, a noble gas, by the chemical process of oxidation.^[10]

As a result of this closed 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 carbon is 1s²2s² 2p², and the noble gas notation is [He] 2s²2p². This notation makes it easier to identify elements, and is shorter and easier than writing out the full notation of atomic orbitals.^[23]

[edit] Compounds

Main article: Noble gas compound

The noble gases exhibit extremely low chemical reactivity, and only a few hundred noble gas compounds have been characterized.^[24] No true chemical compounds of helium or neon have yet been formed, while xenon, krypton, and argon have shown some reactivity in the laboratory. Lack of reactivity among the noble gases is caused by a full valence shell, resulting in little tendency to gain or lose electrons.^[25] The noble gases have very weak interatomic force, and consequently they have very low melting and boiling points. All of them are monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements.^[10]

In 1933, Linus Pauling predicted that the heavier noble gases would be able to form compounds with fluorine and oxygen. Specifically, he predicted the existence of krypton hexafluoride and xenon hexafluoride (XeF₆), speculated that XeF₈ might exist as an unstable compound, and suggested that xenic acid would form perxenate salts.^[26][27] These predictions proved quite accurate, except that XeF₈ is now predicted to be not only thermodynamically unstable, but kinematically unstable,^[28] and as of 2006 has not been made.

Xenon compounds are the most numerous of the noble gas compounds that have been characterized.^[24][29][30] Most of them have the xenon atom in the oxidation state of +2, +4, +6, or +8 bonded to an electronegative atom of fluorine or oxygen, such as in xenon difluoride (XeF₂), xenon tetrafluoride (XeF₄), xenon hexafluoride (XeF₆), xenon tetroxide (XeO₄), and sodium perxenate (Na₄XeO₆). Some of these compounds have found practical application as oxidizing agents. As of 2007, close to a hundred compounds of xenon bonded to other elements, such as organoxenon compounds (those bonded to hydrogen or carbon), or those bonded to nitrogen, or chlorine, or even to xenon itself, or even gold or mercury.^[24][31]

In theory, radon is more reactive than xenon, and therefore should form chemical bonds much easier than xenon does. In practice though, due to the high radioactivity of the radon isotopes, only a few fluorides and oxides have been characterized.^[32] Krypton on the other hand is less reactive than xenon, but quite a few compounds of it have been reported with krypton in the oxydation state of +2, inluding some bonded to nitrogen and oxygen (but only stable under −60 °C and −90 °C respectively).^[24] No compounds of helium, or neon, are known and as of 2007 HArF is still the only confirmed compound of argon.^[24]

In addition to the compounds where a noble gas atom is involved in a covalent bond, noble gases also form non-covalent compounds. The first to be discovered were the clathrates, where a noble gas atom is 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 should be of appropriate size to fit in the cavities of the host crystal lattice. For instance, Ar, Kr and Xe can form clathrates with hydroquinone, but He and Ne cannot fit because they are too small.

Noble gases can form endohedral fullerene compounds where the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when C₆₀ is exposed to noble gases at high pressure, complexes such as He@C₆₀ can be formed (the "@" notation means that He is contained inside C₆₀ but not covalently bound to it).^[33] Endohedral complexes with He, Ne, Ar, Kr, and Xe have been obtained.^[34] 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.

[edit] Bonding

Noble gas compounds such as XeF₂ are said to be hypervalent because they violate the octet rule. Bonding in such compounds can be explained using a 3-center-4-electron bond model.^[35][36] This model, first proposed in 1951,^[37] considers bonding of three colinear atoms; for example, bonding in XeF₂ 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 HOMO is localized on the two terminal atoms. This localization of charge is accommodated by the fact that the fluorine atoms are highly electronegative.

Bonding in XeF₂ can also be represented using the resonant Lewis structures shown below:

In this representation, the octet rule is not broken, the bond orders are 1/2, and there is increased electron density in the fluorine atoms. These is consistent with the molecular orbital picture discussed above.

[edit] Physical properties

Noble gases have several unique qualities when compared with other elements. For example, the boiling and freezing points of helium are lower than those of any other known substance. Helium is also the only element that cannot be solidified by cooling in normal conditions. A pressure of 25 standard atmospheres (2,500 kPa) must be applied at a temperature of 1 K (−272.15 °C/−457.87 °F) to convert it to a solid. Argon is the most plentiful noble gas on Earth, while krypton is the lightest noble gas to be converted into chemical compounds. Xenon is the least volatile of the noble gases obtainable from the air, and although it is an unusually safe anesthetic, its compounds are toxic. Radon is formed from radioactive gas along with helium as radium decays. It takes several days for radon to decay, forming helium and heavy metals, typically lead.^[10]

Property	Noble gases[10]
Element number	2	10	18	36	54	86
Element name	Helium	Neon	Argon	Krypton	Xenon	Radon
Density (g/dm³)	0.1786	0.9002	1.7818	3.708	5.851	9.97
Atomic radius (nm)	0.050	0.070	0.094	0.109	0.130	—
Boiling point (°C)	−268.83	−245.92	−185.81	−151.7	−106.6	−62
Melting point (°C)	−272	−248.52	−189.6	−157	−111.5	−71
Dynamic Viscosity (Pa-s)	1.863E-05	2.974E-05	2.099E-05	2.329E-05	2.110E-05	—
Mean Free Path at STP (nm)	192.665	135.355	68.332	52.337	37.878	—
Atmospheric abundance (ppm)[38]	5.20	18.20	9340.00	1.10	0.09	trace
Abundance in the Universe (ppm)[39]	230,000	1,300	200	40	.01	-

[edit] Applications

The abundance of the noble gases in the universe decrease as their atomic number increases; helium is the most common element in the universe after hydrogen. All of the noble gases are present in the Earth's atmosphere, so they are obtained primarily from air using the methods of liquefaction of gases and fractional distillation, with the exception of helium and radon. Helium is typically produced from oil wells, and radon is usually isolated from the radioactive decomposition of dissolved radium compounds.^[10]

Noble gases have very low boiling and melting points, making them useful as cryogenic refrigerants. In particular, liquid helium, which boils at 4 K, is widely used in superconducting magnets such as those used in MRI and NMR.

Helium has a low solubility in fluids, leading to its use, along with oxygen, for breathing by deep-sea divers because helium does not dissolve in blood and therefore does not form bubbles upon decompression. Nitrogen was used before helium; since it formed bubbles upon decompression, it lead to a condition known as decompression sickness, or "the bends".^[10]

In the early 20th century, hydrogen was used as a lifting gas for lighter-than-air aircraft. However, as the LZ 129 Hindenburg disaster demonstrated when the hydrogen in the envelope engulfed the airship in flames,^[40] its combustibility was a dangerous characteristic and resulted in hydrogen being replaced with helium in blimps and balloons because of its lightness and incombustibility, despite its decrease in buoyancy.^[10]

Noble gases are commonly used in lighting because of their lack of chemical reactivity. Argon is often used inside incandescent light bulbs because it is inert, and it is also used in the synthesis of air- and moisture-sensitive compounds as an alternative to nitrogen. Some of the noble gases glow distinctive colors when used inside lighting tubes, such as neon lights, which produce an orange-red color. Helium and argon are commonly used to shield welding arcs and the surrounding base metal from the atmosphere during welding.^[10] Krypton is used in lasers by doctors performing eye surgery.^[41] Xenon is commonly used in xenon arc lamps, and it is also used as an anaesthetic because it is nonflammable and readily eliminated from the body. Because radon is highly radioactive, its only uses have been those that exploit this property, such as radiotherapy.^[10]

Colors produced by the different noble gases when used in neon tubes

Helium

Neon

Argon

Krypton

Xenon

[edit] See also

• Noble metal

[edit] Notes

[edit] References

• Bobrow (2007-12-05), CliffsAP Chemistry, CliffsNotes, ISBN 047013500X, <http://www.amazon.com/dp/047013500X> .
• Ozima, Minoru (2002), Noble Gas Geochemistry, Cambridge University Press, ISBN 0521803667, <http://books.google.com/books?id=CBM2LJDvRtgC> .

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