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Argon (Ar)

a gas whose name is the Greek word for "inactive"



atomic number 18
atomic mass 39.948 amu

melting point -189.35 C (-308.83 F)
boiling point -185.7 C (
-302.53 F)

isotopes 6

Argon is colorless and odorless in both liquid and gaseous forms.

Occurence and Sources

Making up about 1% of the atmosphere, argon is the third most abundant gas on Earth. The vast majority of argon on Earth comes from the radioactive decay of potassium-40, which produces stable argon-40.

Not found in any compounds, pure argon is isolated through liquid air fractionation. Over 750 tons of argon are produced every year.


Because argon is chemically inert, it is used in a number of devices and applications that require a non-reactive atmosphere. It is the gas that protects the filament inside most lightbulbs from premature failure, and, because it does not conduct heat or cold very well, also fills the gap(s) between panes in double- and triple-pane windows. Its non-reactive property makes it ideally suited as an inert gas shield in arc welding and cutting, as a non-reactive blanket in the manufacture of titanium and other reactive elements, and as a protective atmosphere for growing silicon and germanium crystals.

Most people are familiar with carbon dating, which uses the decay of the radioactive carbon-14 isotope to find the ages of things that were once alive. But, because carbon-14's half-life is about 5,730 years, the technique is not useful for material more than about 60 thousand years old. Potassium-argon dating allow us to date rocks that are much older than this. Potassium-40 decays to argon-40 and calcium-40, with a half-life of 1.25 billion years. The ratio of potassium-40 to argon-40 trapped in rock can be used to determine how long it has been since the rock has solidified.


The first Noble Gas to be discovered, argon's presence was first suspected by Henry Cavendish in 1785, but it was not isolated until Lord Rayleigh and Sir William Ramsay did so in 1894.

Henry Cavendish

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This page was last updated on 04/14/2017.