iso	NA	half-life	DM	DE M eV	DP
³⁶Ar	0.336%	Ar is stable with 18 neutrons
³⁸Ar	0.063%	Ar is stable with 20 neutrons
³⁹Ar	{syn.}	269 y	Beta^-	0.565	³⁹K
⁴⁰Ar	99.6%	Ar is stable with 22 neutrons
⁴²Ar	{syn}	32.9 y	Beta^-	0.600	⁴²K

SI units & STP are used except where noted.

Argon is a chemical element in the periodic table that has the symbol Ar and atomic number 18. The third noble gas, in period 8, argon makes up about 1% of the Earth's atmosphere.

Table of contents

1 Notable Characteristics
2 Applications
3 History
4 Occurrence
5 Isotopes
6 External Links

Notable Characteristics

Argon is 2.5 times as soluble in water as nitrogen which is approximately the same solubility as oxygen. This chemically inert element is colorless and odorless in both its liquid and gaseous forms. There are no known true chemical compounds that contain argon.

Applications

It is used in 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;

Used as an inert gas shield in arc welding and cutting,
as a non-reactive blanket in the manufacture of titanium and other reactive elements,
as a protective atmosphere for growing silicon and germanium crystals.
Argon-39 has been used for a number of applications, primarily ice coring. It has also been used for ground water dating

Argon is also used in technical SCUBA diving to inflate the drysuit, due to its nonreactive, heat isolating effect.

History

Argon (Greek argos meaning "lazy") was suspected to be present in air by Henry Cavendish in 1785 but wasn't discovered until 1894 by Lord Rayleigh and Sir William Ramsay.

Occurrence

This gas is isolated through liquid air fractionation since the atmosphere contains only 0.94% argon. The Martian atmosphere in contrast contains 1.6% of Ar-40 and 5 ppm Ar-36.

Isotopes

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

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