Argon is a chemical element wi seembol Ar an atomic nummer 18. It is in group 18 o the periodic table an is a noble gas. Argon is the third maist common gas in the Yird's atmosphere, at 0.93% (9,300 ppm), makin it approximately 23.8 times as abundant as the next maist common atmospheric gas, carbon dioxide (390 ppm), an mair nor 500 times as abundant as the next maist common noble gas, neon (18 ppm). Near aw o this argon is radiogenicargon-40 derived frae the decay o potassium-40 in the Yird's crust. In the universe, argon-36 is bi far the maist common argon isotope, bein the preferred argon isotope produced bi stellar nucleosynthesis in supernovas.