Argon is a chemical ELEMENT with the symbol Ar and atomic number 18. It is a noble GAS and is the third most abundant GAS in the Earth's atmosphere, making up about 0.934% of the AIR. Argon is a colorless, odorless, non-toxic GAS that is inert and does not react with other elements. It is classified as a GAS at standard temperature and pressure and is the most common of the noble gases. As an ELEMENT, it is found in nature as a component of AIR and is used in many industrial applications.