by Alison Bowler

Created on: January 05, 2009   Last Updated: April 23, 2011

Argon

Symbol: Ar

Atomic Number: 18

Atomic Mass: 39.984 amu (atomic mass units)

Melting Point: minus 189.35°C (83.80K, minus 308.83°F)

Boiling Point: minus 185.85°C (87.30K, minus 302.23°F)

Number of Protons: 18

Number of Electrons: 18

Number of Neutrons: 22

Classification: Noble Gas

Crystal Structure: Cubic

Density @ 293 K: 0.0017837 grams per cubic centimeter

Color: Colorless

Argon, one of the noble gases, was discovered by the Scottish chemist Sir William Ramsay and the English scientist Lord Rayleigh in 1884 while working together in Scotland. They isolated the gas by removing oxygen, carbon dioxide, nitrogen and water from liquid air and then examining the residue. The name argon comes from the Greek "Argos" which means inactive. Until 1957 the element was given the symbol A but since that year it has had the symbol Ar.

Earth's atmosphere contains less than one percent argon. Spectrographic examination of the Martian atmosphere has been carried out and argon's characteristic lines at the red end of the spectrum have been found. It is estimated that Mars has just under two percent argon present in its atmosphere.

Argon was believed to be a completely inert element and as such unable to form compounds with any other element. In 2000 a group of researchers reported the production of the compound argon fluorohydride. This compound is only stable at extremely low temperatures and will decompose at temperatures above minus 245°C (28 K, minus 409°F). Because it can exist only at such low temperatures argon fluorohydride is of use only within the field of scientific research.

There are three naturally occurring isotopes of argon. The most common isotope, making up 99.6003% of the total, is argon-40. The other two stable isotopes are argon-36 and argon-38. Twenty one unstable isotopes of the element are known with half lives ranging from less than 20 nanoseconds to 269 years.

Argon is used industrially where an inert atmosphere is required. It is ideally suited to this function as argon is heavier than air as well as being less reactive than nitrogen.

* It is used in incandescent and fluorescent light bulbs where it protects the hot filament from oxygen which could cause corrosion. The bulbs are filled to a pressure of about 3mm.

* Acts as an inert gas shield for arc welding and cutting.

* Can form a blanket protecting titanium and other reactive elements during production processes.

* Forms a protective atmosphere for the growing of semiconductor crystals such as germanium and silicon.

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