atomic number 18
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|Noun||1.||atomic number 18 - a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere|
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
argonon, inert gas, noble gas - any of the chemically inert gaseous elements of the helium group in the periodic table
air - a mixture of gases (especially oxygen) required for breathing; the stuff that the wind consists of; "air pollution"; "a smell of chemicals in the air"; "open a window and let in some air"; "I need some fresh air"