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= Noble_gas =
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Introduction
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↓ Period
1
2
3
4
5
6
7
colspan="2" ---- 'Legend' {
|Primordial}}; background:" | primordial element
|from decay}}; padding:0 2px; background:" | element by radioactive
decay
|Synthetic}}; background:;" | synthetic
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The noble gases (historically the inert gases, sometimes referred to
as aerogens) are the members of group 18 of the periodic table: helium
(He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and,
in some cases, oganesson (Og). Under standard conditions, the first
six of these elements are odorless, colorless, monatomic gases with
very low chemical reactivity and cryogenic boiling points. The
properties of oganesson are uncertain.
The intermolecular force between noble gas atoms is the very weak
London dispersion force, so their boiling points are all cryogenic,
below 165 K.
The noble gases' inertness, or tendency not to react with other
chemical substances, results from their electron configuration: their
outer shell of valence electrons is "full", giving them little
tendency to participate in chemical reactions. Only a few hundred
noble gas compounds are known to exist. The inertness of noble gases
makes them useful whenever chemical reactions are unwanted. For
example, argon is used as a shielding gas in welding and as a filler
gas in incandescent light bulbs. Helium is used to provide buoyancy in
blimps and balloons. Helium and neon are also used as refrigerants due
to their low boiling points. Industrial quantities of the noble gases,
except for radon, are obtained by separating them from air using the
methods of liquefaction of gases and fractional distillation. Helium
is also a byproduct of the mining of natural gas. Radon is usually
isolated from the radioactive decay of dissolved radium, thorium, or
uranium compounds.
The seventh member of group 18 is oganesson, an unstable synthetic
element whose chemistry is still uncertain because only five very
short-lived atoms (t1/2 = 0.69 ms) have ever been synthesized ().
IUPAC uses the term "noble gas" interchangeably with "group 18" and
thus includes oganesson; however, due to relativistic effects,
oganesson is predicted to be a solid under standard conditions and
reactive enough not to qualify functionally as "noble".
History
======================================================================
'Noble gas' is translated from the German noun , first used in 1900 by
Hugo Erdmann to indicate their extremely low level of reactivity. The
name makes an analogy to the term "noble metals", which also have low
reactivity. The noble gases have also been referred to as 'inert
gases', but this label is deprecated as many noble gas compounds are
now known. 'Rare gases' is another term that was used, but this is
also inaccurate because argon forms a fairly considerable part (0.94%
by volume, 1.3% by mass) of the Earth's atmosphere due to decay of
radioactive potassium-40.
Pierre Janssen and Joseph Norman Lockyer had discovered a new element
on 18 August 1868 while looking at the chromosphere of the Sun, and
named it helium after the Greek word for the Sun, (). No chemical
analysis was possible at the time, but helium was later found to be a
noble gas. Before them, in 1784, the English chemist and physicist
Henry Cavendish had discovered that air contains a small proportion of
a substance less reactive than nitrogen. A century later, in 1895,
Lord Rayleigh discovered that samples of nitrogen from the air were of
a different density than nitrogen resulting from chemical reactions.
Along with Scottish scientist William Ramsay at University College,
London, Lord Rayleigh theorized that the nitrogen extracted from air
was mixed with another gas, leading to an experiment that successfully
isolated a new element, argon, from the Greek word (, "idle" or
"lazy"). With this discovery, they realized an entire class of gases
was missing from the periodic table. During his search for argon,
Ramsay also managed to isolate helium for the first time while heating
cleveite, a mineral. In 1902, having accepted the evidence for the
elements helium and argon, Dmitri Mendeleev included these noble gases
as group 0 in his arrangement of the elements, which would later
become the periodic table.
Ramsay continued his search for these gases using the method of
fractional distillation to separate liquid air into several
components. In 1898, he discovered the elements krypton, neon, and
xenon, and named them after the Greek words (, "hidden"), (, "new"),
and (, "stranger"), respectively. Radon was first identified in 1898
by Friedrich Ernst Dorn, and was named 'radium emanation', but was not
considered a noble gas until 1904 when its characteristics were found
to be similar to those of other noble gases. Rayleigh and Ramsay
received the 1904 Nobel Prizes in Physics and in Chemistry,
respectively, for their discovery of the noble gases; in the words of
J. E. Cederblom, then president of the Royal Swedish Academy of
Sciences, "the discovery of an entirely new group of elements, of
which no single representative had been known with any certainty, is
something utterly unique in the history of chemistry, being
intrinsically an advance in science of peculiar significance".
The discovery of the noble gases aided in the development of a general
understanding of atomic structure. In 1895, French chemist Henri
Moissan attempted to form a reaction between fluorine, the most
electronegative element, and argon, one of the noble gases, but
failed. Scientists were unable to prepare compounds of argon until the
end of the 20th century, but these attempts helped to develop new
theories of atomic structure. Learning from these experiments, Danish
physicist Niels Bohr proposed in 1913 that the electrons in atoms are
arranged in shells surrounding the nucleus, and that for all noble
gases except helium the outermost shell always contains eight
electrons. In 1916, Gilbert N. Lewis formulated the 'octet rule',
which concluded an octet of electrons in the outer shell was the most
stable arrangement for any atom; this arrangement caused them to be
unreactive with other elements since they did not require any more
electrons to complete their outer shell.
In 1962, Neil Bartlett discovered the first chemical compound of a
noble gas, xenon hexafluoroplatinate. Compounds of other noble gases
were discovered soon after: in 1962 for radon, radon difluoride (),
which was identified by radiotracer techniques and in 1963 for
krypton, krypton difluoride (). The first stable compound of argon was
reported in 2000 when argon fluorohydride (HArF) was formed at a
temperature of 40 K.
In October 2006, scientists from the Joint Institute for Nuclear
Research and Lawrence Livermore National Laboratory successfully
created synthetically oganesson, the seventh element in group 18, by
bombarding californium with calcium.
Physical and atomic properties
======================================================================
Property Helium Neon Argon Krypton Xenon Radon Oganesson
|align="left"|Density (g/dm3) 0.1786 0.9002 1.7818 3.708
5.851 9.97 7200 (predicted)
|align="left"|Boiling point (K) 4.4 27.3 87.4 121.5 166.6
211.5 450±10 (predicted)
|align="left"|Melting point (K) - 24.7 83.6 115.8 161.7
202.2 325±15 (predicted)
|align="left"|Enthalpy of vaporization (kJ/mol) 0.08 1.74 6.52
9.05 12.65 18.1 -
|align="left"|Solubility in water at 20 °C (cm3/kg) 8.61 10.5
33.6 59.4 108.1 230 -
|align="left"| Atomic number 2 10 18 36 54 86 118
|align="left"|Atomic radius (calculated) (pm) 31 38 71 88
108 120 -
|align="left"|Ionization energy (kJ/mol) 2372 2080 1520 1351
1170 1037 839 (predicted)
|align="left"|Electronegativity 4.16 4.79 3.24 2.97 2.58 2.60
2.59
The noble gases have weak interatomic force, and consequently have
very low melting and boiling points. They are all monatomic gases
under standard conditions, including the elements with larger atomic
masses than many normally solid elements. Helium has several unique
qualities when compared with other elements: its boiling point at 1
atm is lower than those of any other known substance; it is the only
element known to exhibit superfluidity; and, it is the only element
that cannot be solidified by cooling at atmospheric pressure (an
effect explained by quantum mechanics as its zero point energy is too
high to permit freezing) - a pressure of 25 atm must be applied at a
temperature of 0.95 K to convert it to a solid while a pressure of
about is required at room temperature. The noble gases up to xenon
have multiple stable isotopes; krypton and xenon also have naturally
occurring radioisotopes, namely 78Kr, 124Xe, and 136Xe, all have very
long lives (> 1021 years) and can undergo double electron capture
or double beta decay. Radon has no stable isotopes; its longest-lived
isotope, 222Rn, has a half-life of 3.8 days and decays to form helium
and polonium, which ultimately decays to lead. Oganesson also has no
stable isotopes, and its only known isotope 294Og is very short-lived
(half-life 0.7 ms). Melting and boiling points increase going down the
group.
The noble gas atoms, like atoms in most groups, increase steadily in
atomic radius from one period to the next due to the increasing number
of electrons. The size of the atom is related to several properties.
For example, the ionization potential decreases with an increasing
radius because the valence electrons in the larger noble gases are
farther away from the nucleus and are therefore not held as tightly
together by the atom. Noble gases have the largest ionization
potential among the elements of each period, which reflects the
stability of their electron configuration and is related to their
relative lack of chemical reactivity. Some of the heavier noble gases,
however, have ionization potentials small enough to be comparable to
those of other elements and molecules. It was the insight that xenon
has an ionization potential similar to that of the oxygen molecule
that led Bartlett to attempt oxidizing xenon using platinum
hexafluoride, an oxidizing agent known to be strong enough to react
with oxygen. Noble gases cannot accept an electron to form stable
anions; that is, they have a negative electron affinity.
The macroscopic physical properties of the noble gases are dominated
by the weak van der Waals forces between the atoms. The attractive
force increases with the size of the atom as a result of the increase
in polarizability and the decrease in ionization potential. This
results in systematic group trends: as one goes down group 18, the
atomic radius increases, and with it the interatomic forces increase,
resulting in an increasing melting point, boiling point, enthalpy of
vaporization, and solubility. The increase in density is due to the
increase in atomic mass.
The noble gases are nearly ideal gases under standard conditions, but
their deviations from the ideal gas law provided important clues for
the study of intermolecular interactions. The Lennard-Jones potential,
often used to model intermolecular interactions, was deduced in 1924
by John Lennard-Jones from experimental data on argon before the
development of quantum mechanics provided the tools for understanding
intermolecular forces from first principles. The theoretical analysis
of these interactions became tractable because the noble gases are
monatomic and the atoms spherical, which means that the interaction
between the atoms is independent of direction, or isotropic.
Chemical properties
======================================================================
The noble gases are colorless, odorless, tasteless, and nonflammable
under standard conditions. They were once labeled 'group 0' in the
periodic table because it was believed they had a valence of zero,
meaning their atoms cannot combine with those of other elements to
form compounds. However, it was later discovered some do indeed form
compounds, causing this label to fall into disuse.
Electron configuration
========================
Like other groups, the members of this family show patterns in its
electron configuration, especially the outermost shells resulting in
trends in chemical behavior:
!'Z' !! Element !! Electrons per shell
2 helium 2
10 neon 2, 8
18 argon 2, 8, 8
36 krypton 2, 8, 18, 8
54 xenon 2, 8, 18, 18, 8
86 radon 2, 8, 18, 32, 18, 8
118 oganesson 2, 8, 18, 32, 32, 18, 8 (predicted)
The noble gases have full valence electron shells. Valence electrons
are the outermost electrons of an atom and are normally the only
electrons that participate in chemical bonding. Atoms with full
valence electron shells are extremely stable and therefore do not tend
to form chemical bonds and have little tendency to gain or lose
electrons. However, heavier noble gases such as radon are held less
firmly together by electromagnetic force than lighter noble gases such
as helium, making it easier to remove outer electrons from heavy noble
gases.
As a result of a full shell, the noble gases can be used in
conjunction with the electron configuration notation to form the
'noble gas notation'. To do this, the nearest noble gas that precedes
the element in question is written first, and then the electron
configuration is continued from that point forward. For example, the
electron notation of
phosphorus is , while the noble gas notation is . This more compact
notation makes it easier to identify elements, and is shorter than
writing out the full notation of atomic orbitals.
The noble gases cross the boundary between blocks--helium is an
s-element whereas the rest of members are p-elements--which is unusual
among the IUPAC groups. All other IUPAC groups contain elements from
'one' block each. This causes some inconsistencies in trends across
the table, and on those grounds some chemists have proposed that
helium should be moved to group 2 to be with other s2 elements, but
this change has not generally been adopted.
Compounds
===========
The noble gases show extremely low chemical reactivity; consequently,
only a few hundred noble gas compounds have been formed. Neutral
compounds in which helium and neon are involved in chemical bonds have
not been formed (although some helium-containing ions exist and there
is some theoretical evidence for a few neutral helium-containing
ones), while xenon, krypton, and argon have shown only minor
reactivity.
In 1933, Linus Pauling predicted that the heavier noble gases could
form compounds with fluorine and oxygen. He predicted the existence of
krypton hexafluoride () and xenon hexafluoride () and speculated that
xenon octafluoride () might exist as an unstable compound, and
suggested that xenic acid could form perxenate salts. These
predictions were shown to be generally accurate, except that is now
thought to be both thermodynamically and kinetically unstable.
Xenon compounds are the most numerous of the noble gas compounds that
have been formed. Most of them have the xenon atom in the oxidation
state of +2, +4, +6, or +8 bonded to highly electronegative atoms such
as fluorine or oxygen, as in xenon difluoride (), xenon tetrafluoride
(), xenon hexafluoride (), xenon tetroxide (), and sodium perxenate
(). Xenon reacts with fluorine to form numerous xenon fluorides
according to the following equations:
::Xe + F2 → XeF2
::Xe + 2F2 → XeF4
::Xe + 3F2 → XeF6
Some of these compounds have found use in chemical synthesis as
oxidizing agents; , in particular, is commercially available and can
be used as a fluorinating agent. As of 2007, about five hundred
compounds of xenon bonded to other elements have been identified,
including organoxenon compounds (containing xenon bonded to carbon),
and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon
itself. Compounds of xenon bound to boron, hydrogen, bromine, iodine,
beryllium, sulphur, titanium, copper, and silver have also been
observed but only at low temperatures in noble gas matrices, or in
supersonic noble gas jets.
Radon is more reactive than xenon, and forms chemical bonds more
easily than xenon does. However, due to the high radioactivity and
short half-life of radon isotopes, only a few fluorides and oxides of
radon have been formed in practice. Radon goes further towards
metallic behavior than xenon; the difluoride RnF2 is highly ionic, and
cationic Rn2+ is formed in halogen fluoride solutions. For this
reason, kinetic hindrance makes it difficult to oxidize radon beyond
the +2 state. Only tracer experiments appear to have succeeded in
doing so, probably forming RnF4, RnF6, and RnO3.
Krypton is less reactive than xenon, but several compounds have been
reported with krypton in the oxidation state of +2. Krypton difluoride
is the most notable and easily characterized. Under extreme
conditions, krypton reacts with fluorine to form KrF2 according to the
following equation:
::Kr + F2 → KrF2
Compounds in which krypton forms a single bond to nitrogen and oxygen
have also been characterized, but are only stable below -60 C and -90
C respectively.
Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine,
carbon) as well as some late transition metals (copper, silver, gold)
have also been observed, but only either at low temperatures in noble
gas matrices, or in supersonic noble gas jets. Similar conditions were
used to obtain the first few compounds of argon in 2000, such as argon
fluorohydride (HArF), and some bound to the late transition metals
copper, silver, and gold. As of 2007, no stable neutral molecules
involving covalently bound helium or neon are known.
Extrapolation from periodic trends predict that oganesson should be
the most reactive of the noble gases; more sophisticated theoretical
treatments indicate greater reactivity than such extrapolations
suggest, to the point where the applicability of the descriptor "noble
gas" has been questioned.Translated into English by W. E. Russey and
published in three parts in ChemViews Magazine: Oganesson is
expected to be rather like silicon or tin in group 14: a reactive
element with a common +4 and a less common +2 state, which at room
temperature and pressure is not a gas but rather a solid
semiconductor. Empirical / experimental testing will be required to
validate these predictions. (On the other hand, flerovium, despite
being in group 14, is predicted to be unusually volatile, which
suggests noble gas-like properties.)
The noble gases--including helium--can form stable molecular ions in
the gas phase. The simplest is the helium hydride molecular ion, HeH+,
discovered in 1925. Because it is composed of the two most abundant
elements in the universe, hydrogen and helium, it was believed to
occur naturally in the interstellar medium, and it was finally
detected in April 2019 using the airborne SOFIA telescope. In addition
to these ions, there are many known neutral excimers of the noble
gases. These are compounds such as ArF and KrF that are stable only
when in an excited electronic state; some of them find application in
excimer lasers.
In addition to the compounds where a noble gas atom is involved in a
covalent bond, noble gases also form non-covalent compounds. The
clathrates, first described in 1949, consist of a noble gas atom
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 must be of appropriate size to fit in
the cavities of the host crystal lattice. For instance, argon,
krypton, and xenon form clathrates with hydroquinone, but helium and
neon do not because they are too small or insufficiently polarizable
to be retained. Neon, argon, krypton, and xenon also form clathrate
hydrates, where the noble gas is trapped in ice.
Noble gases can form endohedral fullerene compounds, in which the
noble gas atom is trapped inside a fullerene molecule. In 1993, it was
discovered that when , a spherical molecule consisting of 60 carbon
atoms, is exposed to noble gases at high pressure, complexes such as
can be formed (the '@' notation indicates He is contained inside but
not covalently bound to it). As of 2008, endohedral complexes with
helium, neon, argon, krypton, and xenon have been created. These
compounds have found use in the study of the structure and reactivity
of fullerenes by means of the nuclear magnetic resonance of the noble
gas atom.
Noble gas compounds such as xenon difluoride () are considered to be
hypervalent because they violate the octet rule. Bonding in such
compounds can be explained using a three-center four-electron bond
model. This model, first proposed in 1951, considers bonding of three
collinear atoms. For example, bonding in is described by a set of
three molecular orbitals (MOs) derived from p-orbitals on each atom.
Bonding results from the combination of a filled p-orbital from Xe
with one half-filled p-orbital from each F atom, resulting in a filled
bonding orbital, a filled non-bonding orbital, and an empty
antibonding orbital. The highest occupied molecular orbital is
localized on the two terminal atoms. This represents a localization of
charge that is facilitated by the high electronegativity of fluorine.
The chemistry of the heavier noble gases, krypton and xenon, are well
established. The chemistry of the lighter ones, argon and helium, is
still at an early stage, while a neon compound is yet to be
identified.
Occurrence
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The abundances of the noble gases in the universe decrease as their
atomic numbers increase. Helium is the most common element in the
universe after hydrogen, with a mass fraction of about 24%. Most of
the helium in the universe was formed during Big Bang nucleosynthesis,
but the amount of helium is steadily increasing due to the fusion of
hydrogen in stellar nucleosynthesis (and, to a very slight degree, the
alpha decay of heavy elements).
Abundances on Earth follow different trends; for example, helium is
only the third most abundant noble gas in the atmosphere. The reason
is that there is no primordial helium in the atmosphere; due to the
small mass of the atom, helium cannot be retained by the Earth's
gravitational field. Helium on Earth comes from the alpha decay of
heavy elements such as uranium and thorium found in the Earth's crust,
and tends to accumulate in natural gas deposits. The abundance of
argon, on the other hand, is increased as a result of the beta decay
of potassium-40, also found in the Earth's crust, to form argon-40,
which is the most abundant isotope of argon on Earth despite being
relatively rare in the Solar System. This process is the basis for the
potassium-argon dating method.
Xenon has an unexpectedly low abundance in the atmosphere, in what has
been called the 'missing xenon problem'; one theory is that the
missing xenon may be trapped in minerals inside the Earth's crust.
Radon is formed in the lithosphere by the alpha decay of radium. It
can seep into buildings through cracks in their foundation and
accumulate in areas that are not well ventilated. Due to its high
radioactivity, radon presents a significant health hazard; it is
implicated in an estimated 21,000 lung cancer deaths per year in the
United States alone. Oganesson does not occur in nature and is instead
created manually by scientists.
Abundance Helium Neon Argon Krypton Xenon Radon
|align="left"|Solar System (for each atom of silicon) 2343 2.148
0.1025 5.515 × 10−5 5.391 × 10−6 -
|align="left"|Earth's atmosphere (volume fraction in ppm) 5.20
18.20 9340.00 1.10 0.09 (0.06-18) × 10−19
|align="left"|Igneous rock (mass fraction in ppm) 3 × 10−3 7 ×
10−5 4 × 10−2 - - 1.7 × 10−10
Gas 2004 price (USD/m3)
|align=left| Helium (industrial grade) 4.20-4.90
|align=left| Helium (laboratory grade) 22.30-44.90
|align=left| Argon 2.70-8.50
|align=left| Neon 60-120
|align=left| Krypton 400-500
|align=left| Xenon 4000-5000
For large-scale use, helium is extracted by fractional distillation
from natural gas, which can contain up to 7% helium.
Extraction
======================================================================
Neon, argon, krypton, and xenon are obtained from air using the
methods of liquefaction of gases, to convert elements to a liquid
state, and fractional distillation, to separate mixtures into
component parts. Helium is typically produced by separating it from
natural gas, and radon is isolated from the radioactive decay of
radium compounds. The prices of the noble gases are influenced by
their natural abundance, with argon being the cheapest and xenon the
most expensive. As an example, the adjacent table lists the 2004
prices in the United States for laboratory quantities of each gas.
Biological chemistry
======================================================================
None of the elements in this group has any biological importance.
Applications
======================================================================
Noble gases have very low boiling and melting points, which makes them
useful as cryogenic refrigerants. In particular, liquid helium, which
boils at 4.2 K, is used for superconducting magnets, such as those
needed in nuclear magnetic resonance imaging and nuclear magnetic
resonance. Liquid neon, although it does not reach temperatures as low
as liquid helium, also finds use in cryogenics because it has over 40
times more refrigerating capacity than liquid helium and over three
times more than liquid hydrogen.
Helium is used as a component of breathing gases to replace nitrogen,
due its low solubility in fluids, especially in lipids. Gases are
absorbed by the blood and body tissues when under pressure like in
scuba diving, which causes an anesthetic effect known as nitrogen
narcosis. Due to its reduced solubility, little helium is taken into
cell membranes, and when helium is used to replace part of the
breathing mixtures, such as in trimix or heliox, a decrease in the
narcotic effect of the gas at depth is obtained. Helium's reduced
solubility offers further advantages for the condition known as
decompression sickness, or 'the bends'. The reduced amount of
dissolved gas in the body means that fewer gas bubbles form during the
decrease in pressure of the ascent. Another noble gas, argon, is
considered the best option for use as a drysuit inflation gas for
scuba diving. Helium is also used as filling gas in nuclear fuel rods
for nuclear reactors.
Since the 'Hindenburg' disaster in 1937, helium has replaced hydrogen
as a lifting gas in blimps and balloons: despite an 8.6% decrease in
buoyancy compared to hydrogen, helium is not combustible.
In many applications, the noble gases are used to provide an inert
atmosphere. Argon is used in the synthesis of air-sensitive compounds
that are sensitive to nitrogen. Solid argon is also used for the study
of very unstable compounds, such as reactive intermediates, by
trapping them in an inert matrix at very low temperatures. Helium is
used as the carrier medium in gas chromatography, as a filler gas for
thermometers, and in devices for measuring radiation, such as the
Geiger counter and the bubble chamber. Helium and argon are both
commonly used to shield welding arcs and the surrounding base metal
from the atmosphere during welding and cutting, as well as in other
metallurgical processes and in the production of silicon for the
semiconductor industry.
Noble gases are commonly used in lighting because of their lack of
chemical reactivity. Argon, mixed with nitrogen, is used as a filler
gas for incandescent light bulbs. Krypton is used in high-performance
light bulbs, which have higher color temperatures and greater
efficiency, because it reduces the rate of evaporation of the filament
more than argon; halogen lamps, in particular, use krypton mixed with
small amounts of compounds of iodine or bromine. The noble gases glow
in distinctive colors when used inside gas-discharge lamps, such as
"neon lights". These lights are called after neon but often contain
other gases and phosphors, which add various hues to the orange-red
color of neon. Xenon is commonly used in xenon arc lamps, which, due
to their nearly continuous spectrum that resembles daylight, find
application in film projectors and as automobile headlamps.
The noble gases are used in excimer lasers, which are based on
short-lived electronically excited molecules known as excimers. The
excimers used for lasers may be noble gas dimers such as Ar2, Kr2 or
Xe2, or more commonly, the noble gas is combined with a halogen in
excimers such as ArF, KrF, XeF, or XeCl. These lasers produce
ultraviolet light, which, due to its short wavelength (193 nm for ArF
and 248 nm for KrF), allows for high-precision imaging. Excimer lasers
have many industrial, medical, and scientific applications. They are
used for microlithography and microfabrication, which are essential
for integrated circuit manufacture, and for laser surgery, including
laser angioplasty and eye surgery.
Some noble gases have direct application in medicine. Helium is
sometimes used to improve the ease of breathing of people with asthma.
Xenon is used as an anesthetic because of its high solubility in
lipids, which makes it more potent than the usual nitrous oxide, and
because it is readily eliminated from the body, resulting in faster
recovery. Xenon finds application in medical imaging of the lungs
through hyperpolarized MRI. Radon, which is highly radioactive and is
only available in minute amounts, is used in radiotherapy.
Noble gases, particularly xenon, are predominantly used in ion engines
due to their inertness. Since ion engines are not driven by chemical
reactions, chemically inert fuels are desired to prevent unwanted
reaction between the fuel and anything else on the engine.
Oganesson is too unstable to work with and has no known application
other than research.
Noble gases in Earth sciences application
===========================================
The relative isotopic abundances of noble gases serve as an important
geochemical tracing tool in earth science. They can unravel the
Earth's degassing history and its effects to the surrounding
environment (i.e., atmosphere composition). Due to their inert nature
and low abundances, change in the noble gas concentration and their
isotopic ratios can be used to resolve and quantify the processes
influencing their current signatures across geological settings.
Helium
========
Helium has two abundant isotopes: helium-3, which is primordial with
high abundance in earth's core and mantle, and helium-4, which
originates from decay of radionuclides (232Th, 235,238U) abundant in
the earth's crust. Isotopic ratios of helium are represented by RA
value, a value relative to air measurement (3He/4He = 1.39*10−6).
Volatiles that originate from the earth's crust have a 0.02-0.05 RA,
which indicate an enrichment of helium-4. Volatiles that originate
from deeper sources such as subcontinental lithospheric mantle (SCLM),
have a 6.1± 0.9 RA and mid-oceanic ridge basalts (MORB) display higher
values (8 ± 1 RA). Mantle plume samples have even higher values than
> 8 RA. Solar wind, which represent an unmodified primordial
signature is reported to have ~ 330 RA.
Neon
======
Neon has three main stable isotopes:20Ne, 21Ne and 22Ne, with 20Ne
produced by cosmic nucleogenic reactions, causing high abundance in
the atmosphere. 21Ne and 22Ne are produced in the earth's crust as a
result of interactions between alpha and neutron particles with light
elements; 18O, 19F and 24,25Mg. The neon ratios (20Ne/22Ne and
21Ne/22Ne) are systematically used to discern the heterogeneity in the
Earth's mantle and volatile sources. Complimenting He isotope data,
neon isotope data additionally provide insight to thermal evolution of
Earth's systems.
!20Ne/22Ne !21Ne/22Ne !Endmember
|9.8 |0.029 |Air
|12.5 |0.0677 |MORB
|13.81 |0.0330 |Solar Wind
|0 |3.30±0.2 |Archean Crust
|0 |0.47 |Precambrian Crust
Argon
=======
Argon has three stable isotopes: 36Ar, 38Ar and 40Ar. 36Ar and 38Ar
are primordial, with their inventory on the earth's crust dependent on
the equilibration of meteoric water with the crustal fluids. This
explains huge inventory of 36Ar in the atmosphere. Production of these
two isotopes (36Ar and 38Ar) is negligible within the earth's crust,
only limited concentrations of 38Ar can be produced by interaction
between alpha particles from decay of 235,238U and 232Th and light
elements (37Cl and 41K). While 36Ar is continuously being produced by
Beta-decay of 36Cl. 40Ar is a product of radiogenic decay of 40K.
Different endmembers values for 40Ar/36Ar have been reported; Air =
295.5, MORB = 40,000, and crust = 3000.
Krypton
=========
Krypton has several isotopes, with 78, 80, 82Kr being primordial,
while 83,84, 86Kr results from spontaneous fission of 244Pu and
radiogenic decay of 238U. Krypton's isotopes geochemical signature in
mantle reservoirs resembling the modern atmosphere. preserves the
solar-like primordial signature. Krypton isotopes have been used to
decipher the mechanism of volatiles delivery to earth's system, which
had great implication to evolution of earth (nitrogen, oxygen, and
oxygen) and emergence of life. This is largely due to a clear
distinction of krypton isotope signature from various sources such as
chondritic material, solar wind and cometary.
Xenon
=======
Xenon has nine isotopes, most of which are produced by the radiogenic
decay. Krypton and xenon noble gases requires pristine, robust
geochemical sampling protocol to avoid atmospheric contamination.
Furthermore, sophisticated instrumentation is required to resolve mass
peaks among many isotopes with narrow mass difference during analysis.
!129Xe/130Xe !Endmember
|6,496 |Air
|7.7 |MORB
|6.7 |OIB Galapagos
|6.8 |OIB Icelands
Sampling of noble gases
=========================
Noble gas measurements can be obtained from sources like volcanic
vents, springs, and geothermal wells following specific sampling
protocols. The classic specific sampling protocol include the
following.
* Copper tubes - These are standard refrigeration copper tubes, cut to
~10 cm³ with a 3/8” outer diameter, and are used for sampling volatile
discharges by connecting an inverted funnel to the tube via TygonⓇ
tubing, ensuring one-way inflow and preventing air contamination.
Their malleability allows for cold welding or pinching off to seal the
ends after sufficient flushing with the sample.
** Sampling of noble gases using a Giggenbach bottle, a funnel is
placed on top of the hot spring to focus the stream of sample towards
the bottle via the Tygon tube. A geochemist is controlling the flow of
the sample inlet using a Teflon valve. Note the condensation process
inside the evacuated Giggenbach bottle.Giggenbach bottles - Giggenbach
bottles are evacuated glass flasks with a Teflon stopcock, used for
sampling and storing gases. They require pre-evacuation before
sampling, as noble gases accumulate in the headspace. These bottles
were first invented and distributed by a Werner F. Giggenbach, a
German chemist.
Analysis of noble gases
=========================
Noble gases have numerous isotopes and subtle mass variation that
requires high-precision detection systems. Originally, scientists
used magnetic sector mass spectrometry, which is time-consuming and
has low sensitivity due to "peak jumping mode". Multiple-collector
mass spectrometers, like Quadrupole mass spectrometers (QMS), enable
simultaneous detection of isotopes, improving sensitivity and
throughput. Before analysis, sample preparation is essential due to
the low abundance of noble gases, involving extraction, purification
system. Extraction allows liberation of noble gases from their carrier
(major phase; fluid or solid) while purification remove impurities and
improve concentration per unit sample volume. Cryogenic traps are used
for sequential analysis without peak interference by stepwise
temperature raise.
Research labs have successfully developed miniaturized field-based
mass spectrometers, such as the portable mass spectrometer
([
https://gasometrix.com/products/ miniRuedi]), which can analyze
noble gases with an analytical uncertainty of 1-3% using low-cost
vacuum systems and quadrupole mass analyzers. Extraction and
purification (clean up) mass spectrometer line.
Discharge color
======================================================================
Colors and spectra (bottom row) of electric discharge in noble gases;
only the second row represents pure gases. width="20%"|120px
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|Helium |Neon |Argon |Krypton |Xenon
The color of gas discharge emission depends on several factors,
including the following:
* discharge parameters (local value of current density and electric
field, temperature, etc. - note the color variation along the
discharge in the top row);
* gas purity (even small fraction of certain gases can affect color);
* material of the discharge tube envelope - note suppression of the UV
and blue components in the bottom-row tubes made of thick household
glass.
See also
======================================================================
*Noble gas (data page), for extended tables of physical properties.
*Noble metal, for metals that are resistant to corrosion or oxidation.
*Inert gas, for any gas that is not reactive under normal
circumstances.
*Industrial gas
*Octet rule
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Noble_gas
