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There is more information available on this subject at Argon on the English Wikipedia.
A diagram of Argon.

Argon (pronounced /ˈɑrgɒn/) is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). It is present in the Earth's atmosphere at 0.93%, making it the most abundant noble gas on Earth. Its full outer shell, a characteristic common to all noble gases, makes it highly resistant to bonding with other elements. Although argon is not poisonous, the pure element cannot be breathed by humans and will eventually cause death. Argon has no color, odor, or taste, so it is difficult to find its presence. Sometimes argon is used in aerosol cans or to displace Oxygen. The gas is also used in fire containment systems, such as the one in New Mombasa's underground data center.[1]

A significant number of inhabited planets in the Orion Arm have argon present in their atmospheres, including Earth.