Among all the available radioactive elements,only Radon (Rn) is radioactive.
The six noble gases that occur naturally are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn).
In 1900, while studying radium's decay chain, Friedrich Earns Dorn discovered the last noble gas in group 18; Radon. Through some experiments, Dorn noticed that radium compounds emanated radioactive gas. This radioactive gas was originally named niton after the Latin word "nitens". By 1923, the International Committee for Chemical Elements and International Union of Pure Applied Chemistry (IUPAC) decided to name the element as Radon. All isotopes of radon are radioactive. Radon-222 has the longest half-life - less than 4 days, and is an alpha-decay product of Radium-226, part of the U-238 to Pb-206 radioactive decay chain.