chemical element, element - any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter

Ar, argon, atomic number 18 - a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere

atomic number 2, He, helium - 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)

atomic number 36, Kr, krypton - a colorless element that is one of the six inert gasses; occurs in trace amounts in air

atomic number 10, Ne, neon - 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

atomic number 86, radon, Rn - 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

atomic number 54, Xe, xenon - a colorless odorless inert gaseous element occurring in the earth's atmosphere in trace amounts

gas - a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely