Page 341 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN010K-480 July 16, 2001 17:22

Noble-Gas Chemistry 451

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as “Xe PtF .” Within a very short time, a number of other II. PREPARATIVE METHODS FOR
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xenon compounds, a krypton fluoride, and a radon fluo- THE BINARY FLUORIDES
ride, were reported.
The heavy noble gases—krypton, xenon, and radon— The only way to “fix” krypton, xenon, and radon gases is
have been shown to react with fluorine and other power- through reaction with fluorine or a reactive fluoride so that
ful oxidants to form a number of stable products. Xenon their chemistries are ultimately derived from the binary
has the most extensive chemistry in this group and ex- fluorides.
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hibits the oxidation states + , +2, +4, +6, and +8in Amounts of the binary xenon fluorides suitable for syn-
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the compounds it forms. Since the discovery of noble-gas thetic work are generally prepared by heating mixtures
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reactivity, xenon compounds, including halides, oxides, of xenon and fluorine to 250–400 C in nickel or Monel
oxofluorides, oxosalts, and numerous covalent derivatives vessels. Although all three fluorides coexist in equilib-
in which xenon is covalently bonded to other polyatomic rium, suitable adjustments of the temperature, pressure,
ligands, have been prepared. Additionally, the fluorides and xenon/fluorine ratio can be made to yield primarily
and oxofluorides of xenon form a variety of fluoro- and the difluoride, tetrafluoride, or hexafluoride. Xenon diflu-
oxofluorocations and anions in their reactions with strong oride can be prepared photochemically by exposing xenon
Lewis acid acceptors and fluoride ion donors, respectively. and fluorine, contained in a Pyrex flask, to either direct
Examples of xenon covalently bonded to fluorine, oxygen, sunlight or to ultraviolet light from a mercury arc lamp.
nitrogen, carbon, and gold are now known. The chemistry Other methods, including electric discharges, proton and
of krypton is far less extensive than that of xenon. Kryp- electron beams, and γ -rays, have been employed for the
ton has been shown to form a difluoride and a series of preparation of xenon fluorides, but are rarely used today.
complex salts derived from krypton difluoride. Several ex- Xenon difluoride can also be prepared by the interaction
amples of krypton bonded to nitrogen are now known, and of Xe and F 2 in the dark when the reaction takes place in
a single compound containing krypton bonded to oxygen anhydrous HF.
has been reported. Both XeF 2 and XeF 4 can be manipulated in glass
Radon lies on the diagonal of the Periodic Table be- vacuum systems, but XeF 6 must be handled in either
tween the true metals and nonmetals and is classed as a fluorine-passivated metal or fluoroplastic (e.g., Teflon,
metalloid. As the heaviest and most metallic of the natu- Kel-F, FEP) vacuum systems, since Pyrex and quartz are
rally occurring noble gases, radon has the lowest ioniza- attacked to form initially XeOF 4 , which can react further
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tion energy of the group (1030 kJ mol ); consequently, to form XeO 2 F 2 and treacherously explosive XeO 3 .
it is expected to be the most reactive. The chemistry of Safety measures, such as the use of protective glasses,
radon is, however, less extensive than the chemistries of face and apparatus shields, and other personal protective
krypton and xenon and is rendered considerably more dif- covering, are essential for work involving the tetrafluoride
ficult because no stable isotopes of this element exist. The and hexafluoride, owing to the adventitious formation
inherent radiation hazard that accompanies the intense ra- of highly explosive XeO 3 by inadvertent exposure of
dioactivity of radon requires tracer level experimentation. the compounds to moisture (the tetrafluoride dispropor-
Nevertheless, evidence has been obtained that radon forms tionates in water according to the reaction, 6 XeF 4s + 12
adifluoride and several complex salts. H 2 O l → 2 XeO 3s + 4Xe g + 3O 2g + 24 HF aq ).
Thus far, no stable bulk compounds of the lighter noble Krypton difluoride cannot be synthesized by the stan-
gases, helium, neon, and argon, have been found, although dard high-pressure, high-temperature means used to pre-
HArF has been observed spectroscopically. Element-118, pare xenon fluorides because of the low thermal stability
or ununoctium, a synthetic (transuranium) element, was of KrF 2 . There are three low-temperature methods which
first made in 1999 in a cyclotron by colliding krypton- have proven practical for the preparation of gram and
86 ions with a lead-208 target at an energy just sufficient larger amounts of KrF 2 . High-voltage electric discharges
to fuse their nuclei together and to loose one neutron. through a 1:1 mixture of krypton and fluorine at −183 C
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Only a few atoms of the isotope having the mass number over a period of several hours result in product deposi-
293 have been produced by this method. Ununoctium-293 tion on the cold walls of the cell. The second method in-
decays within less than 1 msec after creation into another volves the resistive heating of a nickel filament, inside in a
transuranium element, element-116, by emitting an alpha metal vessel (usually stainless steel, copper, or aluminum),
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particle. Although no physical and chemical properties of cooled to −196 C and containing solid krypton condensed
element-118 can be directly determined at this time, it has on its walls and fluorine in the gas phase. Fluorine atoms
been proposed that element-118 may be a solid at room formed by thermal dissociation of F 2 at the hot sur-
temperature. face of the nickel filament diffuse to the reactor walls,