CHEMISTRY
OF NOBLE GAS
Noble gases are a special group
because it is the most stable group in the periodic table. Noble gases have
electron valence configuration s2p6, except helium with
electron valence configuration s2. This structure cause noble gases
are unreactive. Although noble gases are stable and unreactive but, it could
form compounds with some mechanisms. Beside that, noble gases are useful in
human life. In this essay will explain briefly the properties of noble gases,
their uses, as well as some compound of noble gases and their ways of
preparation.
In periodic table, group of VIIIA
also called noble gases group. In one period, the noble gas has smallest atomic
radii. The elements of the noble gas group are not very reactive because they are stable (have eight
electrons in the valence shell). The noble gases are, in fact, comparatively unreactive,
but some (especially xenon and krypton) are not “inert” even though that label
was once applied to the noble gas.
Elements belonging to noble gases are He, Ne, Ar, Kr, Xe, and Rn (James
E. House, 2008). Some properties of noble gases can be seen in the following
table
(James E. House, 2008)
Elements belonging to noble gases
are He, Ne, Ar, Kr, Xe, and Rn. First is helium (He), this element was named
helium the Greek word helios, its
mean “sun”. (James E. House, 2008). In 1968 Pierre Jansen found evidence of He
during the total solar eclipse when he detected a new line in the solar
spectrum. In 1895, Ramsay discovered He in the uranium mineral cleveite. At the
same time, the Swedish chemists Cleve and Langlet discovered He in cleveite. In
1907 Rutherford and Royds demonstrated that alpha particles are He nuclei
(Yulianto Mohsin, 2005). It has long been related to nuclear chemistry because
of the formation of alpha particles (
4He2+) during the decay
of heavy nuclei.
4He2+) during the decay
of heavy nuclei.
Second is neon (Ne), in 1898 Ne
was discovered by William Ramsay and Morris Travers. This element was named
neon the Greek word neos, its mean
“new”. In nature, it was produced by some of the stars from the nuclear
reactions that occur in the star. Ne is naturally present in the form of
isotopes. There are three stable isotopes of Ne are Ne-20, Ne-21 and Ne-22.
Third is argon (Ar), Ar was
discovered in 1885 by Sir William Ramsay as a constituents in the residual gas
after O2 and N2 were removed from air. The name comes
from the Greek argos, which means
“inactive”. Argon is generated by the electron capture decay of 40K .
Fourth is krypton (Kr) and xenon
(Xe), in 1898 Ramsay and Travers discovered a new substance that is Kr and Xe.
Kr and Xe were founded in the residue left after almost all of liquid air
evaporating. This element was named Kr the Greek word kryptos, its mean “hidden” and Xe the Greek word xeneus its means “stranger”.
The last is radon (Rn), in 1900 Rn
was by discovered Friedrich Dorn, called emanation (emission) of radium. In
1908, Ramsay and Gray, named niton,
isolate these elements and determine its density, and then it was known that
this element is the heaviest gas of all the elements that have been found at
that time. Rn is inert and occupies the last position in the group of noble
gases in the periodic table. Rn was produced in the three naturally occurring
decay series of 235U, 238U, 232Th. Each of
that series consists of numerous steps before stable nuclide results, but the
final product is Rn in each case. All of the isotopes are radioactive and decay
by 
-emission to produce isotopes
of polonium.
-emission to produce isotopes
of polonium.
All noble gases present in the atmosphere
and used in human life. Here are the benefits of each element of the noble gas.
He was used to fill the balloon, as a mixture of gas for sea dives in lieu of N2.
He also used to substitute N2 as heliox gas mixture (He-O2)
in vitro oxygen to divers, because it is unreactive, very light weight and
solubility in body tissues less than N2. Liquid He was used for
cooling the metal coil in the scanner, because a low boiling point. It was also
used for heat transfer medium in nuclear reactors.
Neon was used as billboards and
lights on the runway. Ne produces high-intensity light when an electric current
pass. This light can be seen from a far and through the mist. Red is the
characteristics color of Re. However, the use of fluorescent tubes with powder
can produce other colors. Ne also used in TV
tubes, gas lasers, high-voltage indicators and liquid it was used as a coolant.
Argon was used in light bulbs
replace the O2 because it is unreactive so that the filaments are
not easily broken. The use of Ar enables heating the filament at a higher
temperature thus obtained is more white light. Ar was used as an inert
atmosphere in welding, metal production in industry and in laboratory
experiments. This is because Ar was unreactive that metal will not oxidize as
if the process takes place in the open air. Ar used for laser to produce a
variety of light with blue-green waves. Lasers can be used for live
entertainment and medical purposes, such as eye surgery and hardening dental
fillings.
Krypton (Kr) was used in light aircraft on the runway, the
lighthouse, the light high-speed photography, fluorescence and laser for
treating eye retina. Kr-85 is the isotope, Kr was used for industry to control
the thickness of the paper industry, because it is radioactive, it can remove
particles of β (Beta).
Xenon (Xe) was used to flash (flash gun) and vacuum tubes. In
a vacuum tube, it was produced a very bright white light. Xenon is the only
noble gas that is anesthesia / drugged at atmospheric pressure. Xe was
considered as an ideal anesthetic among other anesthetics. It was also use in
nuclear reactor.
Radon (Rn) was used for cancer therapy. Rn that is
radioactive put in a small tube sealed and placed near the tissue affected by
cancer. It also used for earthquake warning system. Rn comes from the decay of
U and Ra in rocks. As the plates move, Rn levels will increase due to the
release of Rn of rocks. Rn levels are an indicator of the earthquake.
That’s all about the properties and function of noble
gas elements. Although elements of the noble gas group are stable, there was a
noble gas elements are capable forming compounds or bonded with other elements.
For example compound that consists of xenon such as XeF2, XeF4,
XeF6 and other noble gas
chemistry such as KrF2.
Linus
Pauling (1933) speculated about the reaction of O2 with PF6
mentioned earlier suggested that a similar reaction with xenon might be
successful in light of the similarity in ionization potentials. It should be
apparent that if Xe is to react, it should be with an extremely strong
oxidizing agent, and F2 is a suitable candidate. By this means, the
difluoride and tetrafluoride of xenon were prepared as a mixture of the
elements was heated or subjected to electromagnetic radiation.
Xenon
difluorides is made by interaction of Xe with a deficiency of F2 at
high pressure. The deficiency of F2 insures exclusive formation of
the difluoride.
Xe(g)
+ F2(g) → XeF2(s)
It dissolves in
water to give solutions with a pungent odor of XeF2. Hydrolysis is
slow in acid solution, but rapid in the pressure of bases, due to
XeF2 + 2 OH- →Xe + 
O2 + 2 F- + H2O
O2 + 2 F- + H2O
Xenon tetrafluorides is easy to prepare. When preparing XeF4, a mixture containing a 5 : 1
ratio of fluorine to xenon was heated at 4000C and subjected to
several atmospheres pressure about 6 atm.
XeF2(g) + F2(g) → XeF4(s)
Xenon heksafluorides at a higher ratio of fluorine to xenon was used, XeF6
can be obtained. Xenon heksafluoride (XeF6) is obtained by the
interaction of XeF4 and F2 at temperature above 2500C
and pressure greater than 50 atm.
XeF4(g) + F2(g) → XeF6(s)
Monomeric XeF6
in the liquid or the vapor has a distorted octahedral structure because of a
lone of electrons at Xe.
Reactions of Xenon Fluorides and
Oxyfluorides, many of the reactions that xenon fluorides undergo are similar in
some ways to those of interhalogens. However, the xenon halides differ markedly
in terms of their reactivity with XeF2 being much less reactive than
either XeF4 or XeF6. The difluoride reacts slowly with
water,
2 XeF2(s) + 2 H2O(l) → 2 Xe(g) + O2(g) + 4 HF(aq)
But in basic
solution a different reaction takes place rapidly, which can be shown as
2 XeF2 + 4 OH- → 2Xe + O2
+ 2 H2O + 4 F-
Xenon
tetrafluoride reacts rapidly with water by undergoing disproportionation,
6 XeF4 + 12 H2O → 2 XeO3
+ 4 Xe + 3 O2 + 24 HF
As xenon +4 is
converted into xenon +6 and Xe. The oxide is an explosive compound that is also
produced by the hydrolysis of XeF6.
XeF6 + 3 H2O → XeO3 +
6 HF
Xenon trioxide
is formed in the hydrolysis of XeF4 and XeF6. The hydrolysis
may take place by the formation of XeOF4 as an intermediate (Albert
F Cotton, 1930).
XeF6 + H2O → XeOF4 +
2 HF
XeOF4 + 2 H2O → XeO3
+ 4 HF
Some oxides of xenon are known, and like most other compounds of xenon,
they are usually obtained from the fluorides. The heat of formation of XeO3
is approximately + 400 kJ/mol so, it should be not surprise that the compound
is a very sensitive material. On evaporation of water, XeO3 is
obtained as a white deliquescent solid that is dangerously explosive. In basic solution, a reaction between OH- and XeO3 occurs
and xenate ion (HXeO4-) is formed, which can be shown as
XeO3 + OH- → HXeO4-
A reaction of
HXeO4- in basic solution is very similar, and it can be
shown as
2 HXeO4- + 2 OH- →
XeO64- + Xe + O2 + 2 H2O
Which result in the production of the perxenate ion, XeO64-.
Several solids containing perxenate ion have been isolated, and the ion is the
conjugated base of weak acid, H4XeO6. Therefore, the
salts hydrolyze to produce basic solutions.
XeO64- + H2O ⇌
HXeO63- + OH-
HXeO63- + H2O ⇌ H2XeO62- + OH-
With the oxidation state of Xe in perxenates being +8, they are as
expected, very strong oxidizing agents. In an analogous reaction, xenon
oxyfluorides are produced by the reactions
XeF6 + 2 XeO3 → 3 XeO2F2
2XeF6 + XeO3 → 3 XeOF4
Krypton difluoride, as mentioned
earlier krypton is known to form several compounds, but they are fewer and less
well characterized than the compounds of xenon. The difluoride has been
obtained by electric discharge through a mixture of Kr and F2 at low
temperature. Krypton difluoride is obtained when an electric discharge is
passed through a mixture of Kr and F2 at 1800C. As in the case of xenon difluoride, a cation
is produced in the reaction with a strong Lewis acid such as SbF5.
KrF2 + SbF5 → KrF+SbF6-
It resembles XeF2 being volatile white solid constructed of
linear FKrF molecules, but differs in that it is thermodynamically unstable.
KrF2(g) → Kr(g) + F2(g) ∆H0 = -63 kJ/mol
XeF2(g) → Xe(g) + F2(g) ∆H0 = 105 kJ/mol
Krypton difluoride has been prepared from the elements, but only at low
temperature using electric discharge. When irradiated with ultraviolet light, a
mixture of liquid krypton and fluorine reacts to produce KrF2.
The chemistry of krypton is well established, but is still much less
extensive than xenon. Although a rather extensive chemistry of the noble gases
has developed, the vast majority of the studies have dealt with the xenon
compounds. As expected, radon difluoride can be obtained, but because all
isotopes of radon undergo rapid decay, there is not much interest in the
compound (James E. House, 2008).
Based on explanation above, it can be conclude than noble gases are a
special group because it is the most stable group in the periodic table. The
elements of the noble gas group are not very
reactive because they are stable (have eight electrons in the valence
shell). Noble gas was useful in daily life such as in industry and health.
Although it is stable and very unreactive noble gases could still form
compounds with some mechanisms, for example the chemistry of xenon such as XeF2,
XeF4, XeF6 and
other noble gas chemistry such as KrF2.
References
Cotton, F. Albert dkk. 1930. Basic Inorganic Chemistry. Canada: John
Wiley & Sons, Inc.
Cotton and Wilkinson. 1989. Kimia Anorganik Dasar. Jakarta:
Universitas Indonesia.
House,
E James. 2008. Inorganic Chemistry.
Canada: Academic Press.
Siregar, Manimpan and Sudria. 1999. Kimia Anorganik I. Singaraja: Jurdik
Kimia MIPA STKIP Singaraja.
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