Ar - Argon
(Gr. argos, inactive) Its presence in air was suspected by Cavendish in 1785,discovered by Lord Rayleigh and Sir William Ramsay in 1894.
The gas is prepared by fractionation of liquid air because the atmosphere contains0.94% argon. The atmosphere of Mars contains 1.6% of 40Ar and 5 p.p.m. of 36Ar.