Argon
From Halopedia, the Halo wiki
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). Argon is present in the Earth's atmosphere at 0.93%, making it the most abundant noble gas on Earth. Its full outer shell makes argon stable and 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 hard to find its presence. Sometimes argon is used in aerosol cans or to displace oxygen.
A significant number of inhabited planets in the Orion Arm have Argon present in their atmospheres, including Earth.
