* The noble gases occupy the rightmost column of the periodic table, column 18. They all have completely filled outer electron shells, making them inert, and they are in the form of colorless, odorless, and nontoxic monatomic gases at room temperature. They generally remain gases to fairly low temperatures, since their lack of reactivity makes them reluctant to condense. They are usually obtained by fractional distillation of air -- that is, cooling air to a liquid and then warming it up, with the different elements being driven off as gas at their boiling points, then collected.
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HELIUM / He / 2
Nearly all helium is He<4/2>, but there are traces of the stable
He<3/2> isotope at a millionth of the concentration of He<4/2>.
There are six known radioactive isotopes of helium. Helium can't
be solidified at all unless it is placed under about 26 atmospheres
of pressure at just above absolute zero.
atomic weight: 4.0026
abundance: 71st
density: 0.1785 gm/l
boiling point: -269 C
valence: (inert)
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Helium is a fairly useful element, being used as the primary lifting gas in modern balloons and other lighter-than-air vehicles. Since its boiling point is so low, it is commonly used as a deep-cryogenic cooling liquid. Helium is also used for "heliarc" welding, in which a metal such as magnesium or aluminum is welded in an inert helium atmosphere to prevent oxidation that might weaken the weld.
Another use is as a "buffer gas" in deep-sea diving. Pure oxygen under high pressure tends to overload a person breathing it, and it also creates an incredible fire hazard: almost anything that can burn will burn in such an environment, and generally will burn very energetically. In our lives on the surface world, nitrogen performs such buffering as necessary, but if nitrogen is used as a buffer gas at high pressure and a person breathing the mix is brought to lower pressure too quickly, nitrogen bubbles will form in the blood, blocking blood vessels and causing excruciating pain or injury. This affliction is known as "the bends". It will not occur in a helium-oxygen ("heliox") mixture, though the lighter gas has a higher speed of sound, and so the voices of those breathing it become high-pitched and cartoonish.
Helium is actually the second most common element in the Universe, but since it is so light, as with hydrogen it tends to "outgas" into space, making it fairly scarce here on Earth. There are traces of helium in the atmosphere and it can be obtained from liquefied air like the other noble gases, but most helium is commercially obtained as a byproduct of oil drilling.
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NEON / Ne / 10
Three neon isotopes are found in nature:
Ne<20/10> / 90.5%
Ne<22/10> / 9.2%
Ne<21/10> / 0.3%.
All are stable.
atomic weight: 20.1797
abundance: 82nd
density: 0.9 gm/l
melting point: -249 C
boiling point: -246 C
valence: (inert)
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Neon, though rare in general, has reasonably high concentration in the atmosphere, and it is obtained by distillation of liquid air. Its primary use is in gas-discharge lamps; it emits a bright red color when excited by an electric current. Not all "neon lamps" actually contain only neon, however, since different colors can be obtained with different gas mixes. A mixture of helium and neon is used in "gas-filled lasers" that emit green or red light; the very first gas laser was a helium-neon laser. Neon is also sometimes used as a cryogenic cooling liquid.
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ARGON / Ar / 18
Three isotopes of argon are found in nature:
Ar<40/18> / 99.6%.
Ar<36/18> / 0.34%
Ar<38/18> / 0.06%.
All are stable.
atomic weight: 39.948
abundance: 56th
density: 1.7837 gm/l
melting point: -189 C
boiling point: -186 C
valence: (inert)
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Argon is also fairly common in the air, and it is an important industrial gas, being obtained from the distillation of air at a level of many hundreds of thousands of tonnes per year for use as a thermal insulating filler in double-pane windows; for an inert welding atmosphere; and as a "blowing agent" for stirring iron during steel production. Argon is used in gas-discharge lamps that emit a blue color, and similarly as the lasing medium in gas lasers that produce blue-green light.
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KRYPTON / Kr / 36
Krypton has a total of six isotopes found in nature:
Kr<84/36> / 57%
Kr<86/36> / 17%
Kr<82/36> / 12%
Kr<83/36> / 12%
Kr<80/36> / 2%
Kr<78/36> / 0.4%.
All are stable. There are radioactive isotopes, such as Kr<85/36>
with a half-life of 11 years; Kr<85/36> is released by nuclear power
plants, but isn't regarded as much of a threat because of its lack
of reactivity.
atomic weight: 83.80
abundance: 83rd
density: 3.7 gm/l
melting point: -157 C
boiling point: -153 C
valence: (inert)
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In contrast to argon, krypton is very rare, though it is used a bit in gas-discharge lamps, emitting a bright orange color.
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XENON / Xe / 54
Xenon has an unusual total of nine isotopes found in nature:
Xe<132/54> / 27%
Xe<129/54> / 26.5%
Xe<131/54> / 21%.
Xe<134/54> / 10.5%
Xe<136/54> / 9%
Xe<130/54> / 4%
Xe<128/54> / 2%
Xe<124/54> / 0.1%
Xe<126/54> / 0.1%.
All are stable, except for Xe<136/54>, with a half-life of 9.3E19
years. There are a number of transient radioactive isotopes.
atomic weight: 131.29
abundance: 85th
density: 5.887 gm/l
melting point: -112 C
boiling point: -107 C
valence: (inert)
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Xenon is even rarer than krypton. It is used in various types of high brightness blue discharge lamps. It is also a preferred propellant in the "xenon ion rocket engine", in which the xenon atoms are ionized and then accelerated by high-voltage electric grids to provide fuel efficient propulsion for space probes. Cesium and mercury have also been used as propellants for ion engines, but cesium is corrosive and mercury is toxic.
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RADON / Rn / 86
There are over 30 radon isotopes, but even the most long-lived,
Rn<222/86>, has a half-life of only 3.8 days. Radon can't
accumulate in quantity, but it is so radioactive that it is a
nuisance even at low concentrations.
atomic weight: ~222
abundance: negligible
density: 9.73 gm/l
melting point: -71 C
boiling point: -62 C
valence: (inert)
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Radon is essentially a nuisance. Radon is emitted by some radioactive decay processes, and it tends to accumulate in houses and other structures, where its radioactivity presents a health threat. Some claim the worries over household radon are grossly exaggerated, but a relatively high concentration of radon is still enough to block the sale of a house. Fortunately, the lack of stable isotopes means that radon's existence is transitory, and so it is very rare.
* It is not true that the noble gases are completely unreactive. Some have been tortured into reacting with fluorine, the most reactive of all the elements, under extreme conditions, resulting in highly unstable compounds. However, these compounds are essentially little more than lab freaks and have absolutely no practical applications.
