xenon

xenon (Xe), chemical element, a heavy and extremely rare gas of Group 18 (noble gases) of the periodic table. It was the first noble gas found to form true chemical compounds. More than 4.5 times heavier than air, xenon is colorless, odorless, and tasteless. Solid xenon belongs to the face-centered cubic crystal system, which implies that its molecules, which consist of single atoms, behave as spheres packed together as closely as possible. The name xenon is derived from the Greek word xenos, “strange” or “foreign.”

Properties of the element

Xenon occurs in slight traces in gases within Earth and is present to an extent of about 0.0000086 percent, or about 1 part in 10 million by volume of dry air. Like several other noble gases, xenon is present in meteorites. Xenon is manufactured on a small scale by the fractional distillation of liquid air. It is the least volatile (boiling point, −108.0 °C [−162.4 °F]) of the noble gases obtainable from the air. The British chemists Sir William Ramsay and Morris W. Travers isolated the element in 1898 by repeated fractional distillation of the noble gas krypton, which they had discovered six weeks previously.

The element xenon is used in lamps that produce extremely short and intense flashes of light, such as stroboscopes and lights for high-speed photography. When a charge of electricity is passed through the gas at low pressure, it emits a flash of bluish-white light; at higher pressures, white light resembling daylight is emitted. Xenon flashlamps are used to activate ruby lasers.

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Natural xenon is a mixture of nine stable isotopes in the following percentages: xenon-124 (0.096), xenon-126 (0.090), xenon-128 (1.92), xenon-129 (26.44), xenon-130 (4.08), xenon-131 (21.18), xenon-132 (26.89), xenon-134 (10.44), and xenon-136 (8.87). The mass numbers of the known isotopes of xenon range from 118 to 144. The xenon found in some stony meteorites shows a large proportion of xenon-129, believed to be a product of radioactive decay of iodine-129, whose half-life is 17,000,000 years. Measuring the xenon-129 content of meteorites casts light on the history of the solar system. More than a dozen radioactive xenon isotopes produced by fission of uranium and other nuclear reactions are known. For example, xenon-135 (9.2-hour half-life) is produced by uranium fission in nuclear reactors, where it is troublesome because it absorbs fission-producing neutrons. Xenon-129 is of particular importance because this isotope can be observed by nuclear magnetic resonance spectroscopy, which makes it useful for the structural characterization of xenon compounds. The xenon isotopes produced in the greatest amount by nuclear fission are xenon-131, -132, -134, and -136, which are stable, and xenon-133, which is radioactive, with a half-life of 5.27 days.

Compounds

Noble gases were thought to be chemically inert until 1962, when British chemist Neil Bartlett produced the first noble-gas compound, a yellow-orange solid that can best be formulated as a mixture of [XeF⁺][PtF₆⁻], [XeF⁺][Pt₂F₁₁⁻], and PtF₅. Xenon has the most extensive chemistry in Group 18 and exhibits the oxidation states +1/2, +2, +4, +6, and +8 in the compounds it forms. Since the discovery of noble-gas reactivity, xenon compounds, including halides, oxides, oxofluorides, oxo salts, and numerous covalent derivatives with a number of compounds covalently bonded to other polyatomic ligands, have been synthesized and structurally characterized. As might be predicted from the position of xenon in the periodic table, xenon compounds are poorer oxidizing agents than krypton compounds. Hence, much of currently known xenon chemistry involves its fluorides and oxofluorides in their reactions with strong Lewis acid acceptors and fluoride-ion donors to form a variety of fluoro- and oxofluorocations and anions, respectively. Examples of xenon covalently bonded to fluorine, oxygen, nitrogen, and carbon are now known.

Three fluorides of xenon are known, XeF₂ (the easiest to prepare), XeF₄, and XeF₆. They are stable, colorless, crystalline solids that can be sublimed under vacuum at 25 °C (77 °F). Like KrF₂, XeF₂ is a linear symmetric molecule. Xenon tetrafluoride (XeF₄) is a square planar molecule, and XeF₆ in the gas phase is a distorted octahedral molecule arising from the presence of an “extra” pair of nonbonding electrons in the xenon valence shell. Higher halides such as XeCl₂, XeClF, XeBr₂, and XeCl₄ are thermodynamically unstable and have been detected only in small amounts. The unstable and short-lived monohalides XeF, XeCl, XeBr, and XeI have been produced in the gas phase and are of considerable importance as light-emitting species in gas lasers.

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Two oxides of xenon are known: xenon trioxide (XeO₃) and xenon tetroxide (XeO₄), and both are unstable, highly explosive solids that must be handled with the greatest care. The oxide fluorides XeO₃F₂, XeO₂F₄, XeOF₄, XeO₂F₂, and XeOF₂ are known and, with the exception of XeOF₄, all are thermodynamically unstable.

Xenon difluoride behaves as a simple fluoride-ion donor toward many metal pentafluorides to form complex salts containing the XeF⁺ and Xe₂F₃⁺[F(XeF)₂]⁺ cations by analogy with KrF₂ (see krypton: compounds). Mixtures of xenon and fluorine gases react spontaneously with liquid antimony pentafluoride in the dark to form solutions of XeF⁺Sb₂ F₁₁⁻, in which Xe₂⁺ is formed as an intermediate product that is subsequently oxidized by fluorine to the XeF⁺ cation. The bright emerald green, paramagnetic dixenon cation, Xe₂⁺, is the only example of xenon in a fractional oxidation state, +1/2.

Xenon tetrafluoride is a much weaker fluoride-ion donor than XeF₂ and only forms stable complex salts with the strongest fluoride-ion acceptors to give compounds such as [XeF₃⁺][SbF₆⁻] and [XeF₃⁺][Sb₂F₁₁⁻]. Xenon tetrafluoride has also been shown to behave as a weak fluoride-ion acceptor toward the fluoride ion to give salts of the pentagonal planar XeF₅⁻ anion. Xenon oxide difluoride is also a fluoride-ion acceptor, forming the only other anion containing xenon in the +4 oxidation state, the XeOF₃⁻ anion in Cs⁺XeOF₃⁻.

Xenon hexafluoride is both a strong fluoride-ion donor and a strong fluoride-ion acceptor. Examples of salts containing the XeF₅⁺ cation are numerous, with counter anions such as PtF₆⁻ and AuF₆⁻. Examples of salts containing the fluoride bridged Xe₂F₁₁⁺ cation are also known. Xenon hexafluoride behaves as a fluoride-ion acceptor, reacting with alkali metal fluorides to form salts that contain the XeF₇⁻ and XeF₈²⁻ anions. Several nonalkali metal salts have been shown to contain the anions XeF₇⁻ and XeF₈²⁻ and include [NF₄⁺][XeF₇⁻] and [NO⁺]₂[XeF₈²⁻].

The oxofluorides of xenon +6, XeOF₄ and XeO₂F₂, exhibit analogous fluoride-ion donor and acceptor properties. Salts of both the XeOF₃⁺ and XeO₂F⁺ cations, as well as a salt of the fluoride-bridged cation Xe₂O₄F₃⁺, are known. These include [XeOF₃⁺][SbF₆⁻] and [Xe₂O₄F₃⁺][AsF₆⁻]. Several alkali metal fluoride complexes with XeOF₄ are known, such as 3KF∙XeOF₄ and CsF∙3XeOF₄. Structural studies show that the CsF and N(CH₃)₄F complexes are best formulated as [Cs⁺][XeOF₅⁻], [N(CH₃)₄⁺][XeOF₅⁻], and [Cs⁺][(XeOF₄)₃F⁻]. In these compounds, XeOF₄ behaves as a fluoride acceptor. The only complexes between XeO₂F₂ and a strong fluoride-ion donor are the salts [Cs⁺][XeO₂F₃⁻] and [NO₂⁺][XeO₂F₃∙XeO₂F₂⁻].

When XeF₆ is hydrolyzed in a strongly alkaline solution, part of the xenon is lost as gas (reduced to the 0 oxidation state), but a large fraction precipitates as a perxenate (XeO₆⁴⁻) salt in which xenon is in the +8 oxidation state. The salts are kinetically very stable and lose water gradually when heated; for example, Na₄XeO₆∙6H₂O becomes anhydrous at 100 °C (212 °F) and decomposes at 360 °C (680 °F).

Alkali metal xenates of composition MHXeO₄∙1.5H₂O, where M is sodium, potassium, rubidium, or cesium and xenon is in the +6 oxidation state, have been prepared. The xenates are unstable explosive solids. Alkali metal fluoroxenates [K⁺][XeO₃F⁻], [Rb⁺][XeO₃F⁻], [Cs⁺][XeO₃F⁻] (which decomposes above 200 °C [392 °F]), and the chloroxenate [Cs⁺][XeO₃Cl⁻] (which decomposes above 150 °C [302 °F]) have been prepared by evaporating aqueous solutions of XeO₃ and the corresponding alkali metal fluorides and chlorides. The alkali metal fluoroxenates are the most stable solid oxygen compounds of xenon(+6) known. However, CsXeO₃Br is unstable even at room temperature.

A number of polyatomic ligands of high effective group electronegativities form compounds with xenon. The greatest variety of polyatomic ligand groups bonded to xenon occurs for xenon in its +2 oxidation state, and those groups bonded through oxygen are most plentiful. Both mono- and disubstituted derivatives having the formulations FXeL and XeL₂ are known where L = OTeF₅ and OSeF₅, for example.

The highly electronegative OTeF₅ ⁻ group closely mimics the ability of F⁻ to stabilize the oxidation states of xenon, with stable OTeF₅⁻ derivatives also existing for the +4 and +6 oxidation states of xenon. Cations that contain the (OTeF₅)⁺ group also are known.

Several ligand groups form compounds containing xenon-nitrogen bonds. Among the first xenon-nitrogen bonded compounds to be prepared were FXe[N(SO₂F)₂] and Xe[N(SO₂F)₂]₂. Like XeF₂ and KrF₂, FXe[N(SO₂F)₂] is a fluoride-ion donor toward AsF₅, forming [XeN(SO₂F)₂⁺][AsF₆⁻]. Like KrF⁺, the XeF⁺ cation behaves as a electron pair acceptor toward nitrogen Lewis bases, but because XeF⁺ is not as powerful an oxidant as KrF⁺, the range of ligands that can be coordinated to XeF⁺ is more extensive. These include HCN and (CH₃)₃CCN, which interact with XeF⁺ to form the HCNXeF⁺ and (CH₃)₃CCNXeF⁺ cations, respectively.

A number of compounds containing Xe-C bonds are known. These compounds are salts of cations containing xenon(+2) coordinated to carbon and include cations such as (C₆F₅)Xe⁺ and (m-CF₃C₆H₄)Xe⁺. An example of xenon(+4) bonded to carbon is also known. The (C₆F₅)XeF₂⁺ cation has been prepared as the BF₄⁻ salt.