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Argon                             

Standard Sate: solid at 298k

Color: Colorless                                                                                                                   

Argon is colourless and odourless. Argon is very inert and is not known to form true chemical compounds. It makes a good atmosphere for working with air-sensitive materials since it is heavier than air and less reactive than N₂.

(AR), chemical element, inert gas of Group 0 (noble gases) of the
periodic table, terrestrially the most abundant and industrially the most
frequently used of the noble gases. Colourless, odourless, and
tasteless, argon gas was isolated (1894) from air by the British
scientists Lord Rayleigh and Sir William Ramsay. Henry Cavendish,
while investigating atmospheric nitrogen ("phlogisticated air"), had
concluded in 1785 that not more than 1/120 part of air might be some
inert constituent. His work was forgotten until Lord Rayleigh, more
than a century later, found that nitrogen prepared by removing oxygen
from air is always about 0.5 percent more dense than nitrogen derived
from chemical sources such as ammonia. The heavier gas remaining
after both oxygen and nitrogen had been removed from air was the
first of the noble gases to be discovered on Earth and was named
argon because of its chemical inertness. (Helium had been
spectroscopically detected in the Sun in 1868.)

Argon constitutes 1.3 percent of the atmosphere by weight and 0.94
percent by volume and is found occluded in rocks. A major portion of
terrestrial argon has been produced, since the Earth's formation, in
potassium-containing minerals by decay of the rare, naturally
radioactive isotope potassium-40. The gas slowly leaks into the
atmosphere from the rocks in which it is still being formed. The
production of argon-40 from potassium-40 decay is utilized as a
means of determining the Earth's age (potassium-argon dating). On
Earth, naturally occurring argon is a mixture of three stable isotopes:
argon-36 (0.34 percent), argon-38 (0.06 percent), and argon-40
(99.60 percent).

Argon is isolated on a large scale by the fractional distillation of liquid
air. It is used in gas-filled electric light bulbs, radio tubes, and Geiger
counters. It also is widely utilized as an inert atmosphere for
arc-welding metals, such as aluminum and stainless steel; for the
production and fabrication of metals, such as titanium, zirconium, and
uranium; and for growing crystals of semiconductors, such as silicon
and germanium.

Argon gas condenses to a colourless liquid at -185.8 C (-302.4 F)
and to a crystalline solid at -189.4 C (-308.9 F). The gas cannot be
liquefied by pressure above a temperature of -122.3 C (-188.1 F),
and at this point a pressure of at least 48 atmospheres is required to
make it liquefy. At 12 C (53.6 F), 3.94 volumes of argon gas
dissolve in 100 volumes of water. An electric discharge through
argon at low pressure appears pale red and at high pressure, steely
blue. (See thermal fusion.)

The outermost (valence) shell of argon has eight electrons, making it
exceedingly stable and, thus, chemically inert. Argon atoms do not
combine with one another; nor have they been observed to combine
chemically with atoms of any other element. Argon atoms have been
trapped mechanically in cagelike cavities among molecules of other
substances, as in crystals of ice or the organic compound
hydroquinone (called argon clathrates). atomic number 18 atomic
weight 39.948 melting point -189.2 C (-308.6 F) boiling point -185.7
C (-302.3 F) density (1 atm, 0 C) 1.784 g/litre valence 0 electronic
config. 2-8-8 or 1s22s22p63s23p6

"argon" Encyclop�dia Britannica Online.

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