S: (n) argon, Ar, atomic number 18 (a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere)
S: (n) helium, He, atomic number 2 (a very light colorless element that is one of the six inert gasses; the most difficult gas to liquefy; occurs in economically extractable amounts in certain natural gases (as those found in Texas and Kansas))
S: (n) neon, Ne, atomic number 10 (a colorless odorless gaseous element that give a red glow in a vacuum tube; one of the six inert gasses; occurs in the air in small amounts)
S: (n) radon, Rn, atomic number 86 (a radioactive gaseous element formed by the disintegration of radium; the heaviest of the inert gasses; occurs naturally (especially in areas over granite) and is considered a hazard to health)
S: (n) xenon, Xe, atomic number 54 (a colorless odorless inert gaseous element occurring in the earth's atmosphere in trace amounts)