CH226.29
•The Noble gases (18 / VIII)
The Noble gases (group 18/ VIII)
Members of group VIII have the electronic configuration ns2np6.
He, helium (The name is derived from the Greek helios, sun). A colorless, odorless
gas that is totally unreactive. It is extracted from natural gas wells, some of
which contain up to 7% He. It is present in the Earth's atmosphere in about 1
part in 200,000. Liquid He (b.p. 4.2 K) is an coolant for superconducting
devices. Helium is used as a heat-transfer agent in nuclear reactors.
Ne, neon (The name is derived from the Greek neos, meaning new). Neon is a
colorless, odorless gas present in the atmosphere to the extent of 1 part in
65,000 of air. In a vacuum discharge tube neon glows a reddish orange
color, and is therefore used in making the ubiquitous neon advertising
signs, which accounts for its largest use.
Ar, argon (From the Greek argos meaning inactive). Ar is the 3rd most abundant gas in
air, making up 1 % of the atmosphere. Argon is a colorless, odorless gas that
is totally inert to other substances, and for this reason it is ideal in light bulbs
and chemical experiments with highly reactive compounds.
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The Noble gases (group 18/ VIII)
Kr, krypton (the name is derived from the Greek kryptos, meaning hidden). A
colorless, odorless gas that is inert to everything but fluorine gas. Krypton is
one of the rarest gases in the Earth’s atmosphere, accounting for only 1 part
per million by volume.
Xe, xenon (the name is derived from the Greek xenos, meaning strange). A colorless,
odorless gas that makes up 0.086 parts per million of the atmosphere. About
500 kg a year is produced from liquid air and used for research purposes.
Xenon is inert towards most other chemicals but reacts with fluorine gas to
form xenon fluorides. Xenon oxides, acids and salts are also known.
Rn, radon (the name is derived from radium). Radon was first discovered as the gas
produced from radium as it decayed in sealed ampoules. It is colorless and
odorless, and is chemically inert, but it is dangerous because it gives off rays. Radon is a natural product of radioactive decay of some elements.
Radon is heavy and can build up in basement rooms in places where it
escapes from the ground (particularly where there is much granite). Radon is
now thought to account for about 10% of cases of lung cancer.
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Testing for radon
Almost every home in Canada has some radon. But the levels vary from one
house to another, even if they are next door to each other.
The only way to know if you have a radon problem is to test your home. It is
simple and inexpensive. You have two options for testing:
Hire a certified radon measurement professional.
Do it yourself. Home radon test kits cost between $30 and $60 and can be
purchased from some hardware stores or ordered by phone or online. Kits include
a radon detector that is meant to be exposed to the air inside your home for a
period of time and then sent to a lab for analysis.
Health Canada recommends using a long-term test device for at least three
months. The best time to test is between September and April, when your
windows are mostly closed.
For more information, see Health Canada's web section on radon
http://www.hc-sc.gc.ca/ewh-semt/radiation/radon/index-eng.php
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The discovery of Noble gases
W. Ramsay won the Nobel Prize for Chemistry in 1904 for the discovery of the Noble
gases. Ramsay passed a sample of air through P2O5, NaOH & Ca(OH)2, heated Cu,
and then repeatedly over heated magnesium (3Mg + N2  Mg3N2)
As the volume of the residual gas decreased, its density increased. When all the O2
and N2 had apparently been absorbed, the volume was only about 1/100th of the
parent sample of air, and the density had risen to 1.780 g/l. The resulting gas was
chemically inactive: it was unaffected when heated even with the most reactive
substances, including fluorine, so when Ramsay announced its discovery in 1894, it
was named argon.
The appearance of a new element challenged Mendeleev's Periodic Law, because
this law implies that elements come in groups. Ramsay, however, had a bold answer
to this challenge. He suggested that an entire family of new elements awaited
discovery. In 1895, Ramsay found helium in a uranium mineral. Then, in 1898, by
liquefying samples of his “argon” and carefully distilling the liquid, he obtained not
just a purer argon, but, in quick succession, neon, krypton and xenon, which were
present as very minor contaminants. Thus, the Periodic Law not only survived the
argon challenge; it emerged from it with enhanced prestige.
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Discovery of compounds of Noble gases
In 1962, Neil Bartlett was working with the very reactive
red gas, platinum hexafluoride, PtF6. In the course of the
work, some air accidentally leaked into the apparatus,
and an orange solid formed, which contained PtF6– and
O2+ ions:
PtF6(g) + O2(g) = O2+[PtF6]-(s)
Here PtF6 acted as an oxidizing agent (platinum was reduced to platinum(V)) and an
oxygen molecule was oxidized to O2+. This is remarkable because the ionization
energy of O2 is very high (1,175 kJ/mol). Even more remarkable consequences
followed when Bartlett noticed that the ionization energy of O2 was very similar to that
of xenon (1,177 kJ/mol).
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Discovery of compounds of Noble gases
When Bartlett passed xenon gas into red PtF6 vapor, the first noble gas compound
appeared instantly as an orange-yellow solid on the walls of the containing vessel.
He published his discovery under the title “Xenon hexafluoroplatinate(V), Xe+[PtF6]–”
Subsequent investigations have failed to confirm this formula: the product appears to
be a mixture that includes the compound [XeF+]PtF6-.
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Fluorides of xenon
The most stable compounds are the colorless XeF2 (available from Sigma-Aldrich,
$140 per gram), XeF4 and XeF6. It is impossible to prepare XeF4 free from XeF2
and/or XeF6. Similarly XeF6 always forms with contamination by the lower fluorides.
Xe + F2  XeF2
Xe + (excess) F2  XeF4 at 400 °C
Xe-F 2.00 Å
m.p. 129 °C
Xe-F 1.95 Å
m.p. 117 °C
Xe-F 1.89 Å
m.p. 49 °C
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Reactions of Xenon fluorides
Many reactions are similar to those of interhalogens.
Xenon fluorides are strong oxidizing agents:
2 XeF2 + 2H2O  2 Xe + 2HF + O2
XeF4 + Pt  Xe + PtF4
An important reaction of XeF6 is the double-displacement with oxides:
2 XeF6 + 3 SiO2  2 XeO3 + 3 SiF4
Xenon trioxide, XeO3, is a highly explosive white solid.
Xenon fluorides react with strong Lewis acids and bases:
XeF2 + SbF5 (strong L.A.) [XeF+][SbF6–]
XeF4 + [NMe4]F (strong L.B.)  [NMe4][XeF5−]
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