Noble gas compounds are
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s that include an
elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
from
GroupIn chemistry, a group is a vertical column in the periodic table of the chemical elements. The name family is derived from the fact that the elements share similar characteristics and traits, just as members of any human family would...
18 of the
periodic tableThe periodiс table of the chemical elements is a tabular display of the chemical elements...
, the
noble gasThe noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity...
es.
History and background
It was initially believed that the noble gases could not form compounds due to their full valence shell of electrons that rendered them very chemically stable and unreactive.
All noble gases have full
s and
p outer
electron shellAn electron shell may be thought of as an orbit followed by electrons around an atom nucleus. Because each shell can contain only a fixed number of electrons, each shell is associated with a particular range of electron energy, and thus...
s (except
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
, which has no
p orbital at all), and so do not form
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s easily. Because of their high ionization energy and almost zero
electron affinityThe electron affinity, Eea, of an atom or molecule is the amount of energy required to detach an electron from a singly charged negative ion, i.e., the energy change for the processAn equivalent definition is the energy released when an electron is attached to a neutral atom or...
, they were not expected to be reactive at all.
In 1933, however,
Linus PaulingLinus Carl Pauling was an American chemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists in any field of the 20th century. Pauling was among the first scientists to work in the fields of quantum...
predicted that the heavier noble gases would be able to form compounds with
fluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F
2 molecule. F
2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the...
and
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
. Specifically, he predicted the existence of krypton hexafluoride and
xenon hexafluorideXenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three binary fluorides of xenon, the other two being XeF2 and XeF4. All are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of...
(
XeXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
FFluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F
2 molecule. F
2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the...
6), speculated that XeF
8 might exist as an unstable compound, and suggested that
xenic acidXenic acid is a noble gas compound formed by the dissolution of xenon trioxide in water. Its chemical structure is H2XeO4. It is a very powerful oxidizing agent, and decomposition of this chemical is dangerous as it liberates a large amount of gaseous products - xenon,...
would form
perxenateThe perxenates are salts of perxenic acid, H4XeO6.-Properties:Perxenic acid and the anion XeO are both strong oxidizing agents, capable of oxidising silver to silver and copper to copper. The acid can be formed by dissolving xenon tetroxide into water...
salts. These predictions proved quite accurate, although subsequent predictions for XeF
8 indicated that it would be not only thermodynamically unstable, but kinematically unstable. As of 2009, XeF
8 has not been made, although the octafluoroxenon(VI) anion (XeF) has been observed.
The heavier noble gases have more electron shells than the lighter ones. Hence, the outermost electrons experience a shielding effect from the inner electrons that makes them more easily ionized, since they are less strongly attracted to the positively-charged
nucleusThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
. This results in an ionization energy low enough to form stable compounds with the most
electronegativeElectronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond. An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus...
elements, fluorine and oxygen, and even with less electronegative elements such as nitrogen and carbon under certain circumstances.
Pre-1962 compounds
Prior to 1962, the only isolated compounds of noble gases were
clathratesA clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule...
(including clathrate
hydrateHydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
s). Other compounds such as
coordination compoundsIn chemistry, a coordination complex or metal complex, is a structure consisting of a central atom or ion , bonded to a surrounding array of molecules or anions ....
were observed only by spectroscopic means.
Clathrates
Clathrates (also known as cage compounds) are compounds of noble gases in which they are 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 β-quinol, but He and Ne cannot fit because they are too small.
Clathrates have been used for separation of He and Ne from Ar, Kr, and Xe, and also for the transportation of Ar, Kr, and Xe. In addition,
85Kr clathrate provides a safe source of
beta particleBeta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
s, while
133Xe clathrate provides a useful source of
gamma rayGamma rays are electromagnetic radiation of high frequency . They are produced by sub-atomic particle interactions, such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes...
s.
Coordination compounds
Coordination compounds such as Ar·BF
3 have been postulated to exist at low temperatures, but have never been confirmed. Also, compounds such as WHe
2 and HgHe
2 were reported to have been formed by electron bombardment, but recent research has shown that these are probably the result of He being adsorbed on the surface of the metal; therefore, these compounds cannot truly be considered chemical compounds.
Hydrates
Hydrates are formed by compressing the noble gases in water. It is believed that the water molecule, a strong dipole, induces a weak dipole in the noble gas atoms, resulting in dipole-dipole interaction. Heavier atoms are more influenced than smaller ones, hence Xe·6H
2O is the most stable hydrate. The existence of these compounds has, however, been disputed in recent years.
True noble gas compounds
The first published report, in June 1962, of a noble gas compound was by
Neil BartlettNeil Bartlett was a chemist best known for his work on noble gas compounds. He taught chemistry at the University of California, Berkeley.-Biography:Neil Bartlett was born September 15, 1932 in Newcastle-upon-Tyne, England...
, who noticed that the highly oxidising compound
platinum hexafluoridePlatinum hexafluoride is the chemical compound with the formula PtF6. It is a dark-red volatile solid that forms a red gas. The compound is a unique example of platinum in the 6+ oxidation state...
ionised
O2Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
to
O2+The dioxygenyl ion, , is a rarely-encountered oxycation in which both oxygen atoms have an oxidation state of +½. It is formally derived from oxygen by the removal of an electron:...
. As the ionisation energy of O
2 to O
2+ (1165 kJ mol
–1) is nearly equal to the ionisation energy of Xe to Xe
+ (1170 kJ mol
–1), he tried the reaction of Xe with PtF
6. This yielded a crystalline product
xenon hexafluoroplatinateXenon hexafluoroplatinate is the name of the product of the reaction of platinum hexafluoride and xenon, in an experiment that proved the chemical reactivity of the noble gases...
, whose formula was proposed to be .
It was later shown that the compound is actually more complex, containing both XeFPtF
6 and XeFPt
2F
11. This was the first real compound of any noble gas.
In September 1962, Howard Claasen reported the synthesis of a simple (two-element) noble gas compound,
xenon tetrafluorideXenon tetrafluoride, XeF4, is one of the chemical compounds derived from the noble gas xenon. It was the first discovered noble gas compound containing a noble gas and exactly one other element. It forms by the reaction of Xe and F2 in the ratio of 1 mol Xe to 2 mol...
, by subjecting xenon and fluorine to a high temperature. In November 1962,
Rudolf HoppeRudolf Hoppe , a German chemist, discovered the first covalent noble gas compounds.-Academic career:...
of Universitat Munster reported that xenon and fluorine can react to form
xenon difluorideXenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white crystalline solid. It...
.
In recent years, several compounds of noble gases, particularly xenon, have been prepared. Among these are the xenon fluorides (
XeF2Xenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white crystalline solid. It...
,
XeF4Xenon tetrafluoride, XeF4, is one of the chemical compounds derived from the noble gas xenon. It was the first discovered noble gas compound containing a noble gas and exactly one other element. It forms by the reaction of Xe and F2 in the ratio of 1 mol Xe to 2 mol...
,
XeF6Xenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three binary fluorides of xenon, the other two being XeF2 and XeF4. All are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of...
), oxyfluorides (XeOF
2, XeOF
4, XeO
2F
2, XeO
3F
2, XeO
2F
4) and oxides (
XeO3Xenon trioxide is an unstable compound of xenon in its +6 oxidation state. It is a very powerful oxidizing agent, and liberates oxygen from water slowly, accelerated by exposure to sunlight. It is dangerously explosive upon contact with organic materials.-Chemistry:Xenon trioxide is a strong...
and
XeO4Xenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO4, remarkable for being a relatively stable compound of a noble gas...
). Xenon difluoride can be produced by the simple exposure of Xe and F
2 gases to sunlight; while the mixing of the two gases had been tried over 50 years before in an attempt to produce a reaction, nobody had thought to simply expose the mixture to sunlight.
Radon has reacted with fluorine to form
RnF2Radon difluoride 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. The usefulness of radon compounds is limited because...
, which glows with a yellow light in the solid state. Krypton is able to react with fluorine to form KrF
2, and short-lived
excimerAn excimer is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which is in an electronic excited state. Excimers are often diatomic and are formed between two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer...
s of Xe
2 and noble gas halides such as XeCl
2 are used in
excimer laserAn excimer laser is a form of ultraviolet laser which is commonly used in eye surgery and semiconductor manufacturing. The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'...
s. The discovery of
HArFArgon fluorohydride is the first known compound of the chemical element argon.-Discovery:The discovery of this first argon compound is credited to a group of Finnish scientists, led by Markku Räsänen...
was announced in 2000. Although there are some theoretical evidence for a few metastable helium compounds which may be stable at very low temperatures and extreme pressures, none are confirmed by experiments. A neon compound is still yet to be theoretically identified.
Recently xenon has been shown to produce a wide variety of compounds of the type XeO
nX
2 where
n is 1,2 or 3 and X is any electronegative group, such as CF
3, C(SO
2CF
3)
3, N(SO
2F)
2, N(SO
2CF
3)
2, OTeF
5, O(IO
2F
2), etc. The range of compounds is impressive, running into the thousands and involving bonds between xenon and oxygen, nitrogen, carbon, boron and even gold, as well as perxenic acid, several halides, and complex ions; a range of compounds similar to that seen with the neighbouring element
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
. The compound Xe
2Sb
2F
11 contains a Xe–Xe bond, the longest element-element bond known (308.71 pm = 3.0871
ÅThe ångström or angstrom is an internationally recognized unit of length equal to 0.1 nanometre or 1 metres. It is named after Anders Jonas Ångström...
).
Fullerene compounds
Noble gases can also form endohedral fullerene compounds where the noble gas atom is trapped inside a
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
molecule. In 1993, it was discovered that when C
60 is exposed to a pressure of around 3
barThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
of He or Ne, the complexes He@C
60 and Ne@C
60 are formed. Under these conditions, only about one out of every 650,000 C
60 cages was doped with a
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
atom; with higher pressures (3000 bar), it is possible to achieve a yield of up to 0.1%. Endohedral complexes with
argonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
,
kryptonKrypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
and
xenonXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
have also been obtained, as well as numerous
adductAn adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants. The resultant is considered...
s of He@C
60.
Applications
Most applications of noble gas compounds are either as oxidising agents or as a means to store noble gases in a dense form.
Xenic acidXenic acid is a noble gas compound formed by the dissolution of xenon trioxide in water. Its chemical structure is H2XeO4. It is a very powerful oxidizing agent, and decomposition of this chemical is dangerous as it liberates a large amount of gaseous products - xenon,...
is a valuable oxidising agent because it has no potential for introducing impurities: the xenon is simply liberated as a gas. It is rivalled only by
ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
in this respect. The
perxenateThe perxenates are salts of perxenic acid, H4XeO6.-Properties:Perxenic acid and the anion XeO are both strong oxidizing agents, capable of oxidising silver to silver and copper to copper. The acid can be formed by dissolving xenon tetroxide into water...
s are even more powerful oxidising agents, and the xenon fluorides are good fluorinating agents. Stable salts of xenon containing very high proportions of fluorine by weight such as perfluoroammonium heptafluoroxenon, NF
4XeF
7 and the related (NF
4)
2XeF
8 have been developed as highly energetic oxidisers for use as propellants in rocketry.
Radioactive isotopes of krypton and xenon are difficult to store and dispose, and compounds of these elements may be more easily handled than the gaseous forms.
Resources
- http://www.chemsoc.org/exemplarchem/entries/2001/robson/raregascompounds.htm
- http://www.webelements.com/webelements/elements/