Argon - An element that is a member of the 8A group (Noble Gases) of the periodic table. Argon is represented by the atomic symbol Ar, has an atomic number of 18, and an atomic weight of 39.948. It is colorless, odorless, and a very inert gas. It comprises about 1% of the Earth's atmosphere.
[Chemical & Engineering News; v73n36; 8; 1995.]
[Chemistry The Central Science; Brown, Theodore; Lemay, H. Eugene; Bursten, Bruce; p. 846; 1994; Prentice Hall; Englewood Cliffs, New Jersey.] 
Atmospheric Chemistry Glossary

History

(Gr. argos, inactive) Its presence in air was suspected by Cavendish in 1785, discovered by Lord Raleigh and Sir William Ramsay in 1894.

Sources

The gas is prepared by fractionation of liquid air because the atmosphere contains 0.94% argon. The atmosphere of Mars contains 1.6% of  40Ar and 5 p.p.m. of  36Ar.

Properties

Argon is two and one half times as soluble in water as nitrogen, having about the same solubility as oxygen. Argon is colorless and odorless, both as a gas and liquid. Argon is considered to be a very inert gas and is not known to form true chemical compounds, as do krypton, xenon, and radon. 
A Periodic Table of the Elements at Los Alamos National Laboratory