Argon

From Free net encyclopedia

Argon is the chemical element in the periodic table that has the symbol Ar and atomic number 18. The third noble gas, in group 18, argon is present in the Earth's atmosphere at slightly less than 1 %. This makes it the most common noble gas on Earth.

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Compounds
6 Isotopes
7 References
8 External links

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Notable characteristics

Argon is 2.5 times as soluble in water as nitrogen which is approximately the same solubility as oxygen. This highly stable chemical element is colorless and odorless in both its liquid and gaseous forms. There are few known true chemical compounds that contain argon, which is one of the reasons it was formerly called an inert gas. The creation of argon hydrofluoride (HArF), a highly unstable compound of argon with fluorine, was reported by researchers at the University of Helsinki in 2000, but has not been confirmed as of yet.

Although no chemical compounds of argon are presently confirmed, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Theoretical calculations on computers have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

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Applications

It is used in incandescent lighting since it will not react with the filament in a lightbulb even under high temperatures and other cases where diatomic nitrogen is an unsuitable (semi-)inert gas. Other uses:

Argon is used as an inert gas shield in many forms of welding, including metal inert gas welding and tungsten inert gas welding.
as a non-reactive blanket in the manufacture of titanium and other reactive elements.
as a protective atmosphere for growing silicon and germanium crystals.
as a gas for use in plasma globes.
as a gas for thermal insulation in energy efficient windows.
Argon-39 has been used for a number of applications, primarily ice coring. It has also been used for ground water dating.
Cryosurgery procedures such as cryoablation use liquified argon to destroy cancer cells.
Liquid argon is used in calorimetry in experimental particle physics.
Argon is used in technical SCUBA diving to inflate the dry suit, because it is inert and has low thermal conductivity.

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History

Argon (Greek argos meaning "inactive") was suspected to be present in air by Henry Cavendish in 1785 but was not discovered until 1894 by Lord Rayleigh and Sir William Ramsay in an experiment in which they removed all of the oxygen and nitrogen from the air.

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Occurrence

This gas is isolated through liquid air fractionation since the atmosphere contains only 0.934% volume of argon (1.29% mass). The Martian atmosphere in contrast contains 1.6% of argon-40 and 5 ppm of argon-36. In 2005, the Huygens probe also discovered the presence of Ar-40 on Titan, the largest moon of Saturn [1].

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Compounds

Before 1962, argon and the other noble gases were generally considered to be chemically inert and not able to form compounds. However, since then, scientists have been able to force the heavier noble gases to form compounds. In 2000, the first argon compounds were formed by researchers at the University of Helsinki. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, they were able to form argon hydrofluoride (HArF): see http://pubs.acs.org/cen/80th/noblegases.html in its paragraph starting "Many recent findings". It is stable up to 40 kelvin.

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Isotopes

The main isotopes of argon found on Earth are ⁴⁰Ar, ³⁶Ar, and ³⁸Ar. Naturally occurring ⁴⁰K with a half-life of 1.250 x 10⁹ years, decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and also transforms to stable ⁴⁰Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.

In the Earth's atmosphere, ³⁹Ar is made by cosmic ray activity, primarily with ⁴⁰Ar. In the subsurface environment, it is also produced through neutron capture by ³⁹K or alpha emission by calcium. ³⁷Ar is created from the decay of ⁴⁰Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.

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