The radioactive noble gas first isolated from the decay of radium is radon. It is a colorless, odorless element with the symbol Rn and atomic number 86. Radon forms naturally when radium decays, so it can seep from uranium-bearing soil and rocks into buildings.
The gas was initially called “radium emanation.” Friedrich Ernst Dorn reported this emanation from radium compounds in 1900, while Ernest Rutherford and Robert B. Owens studied related radioactive gases. In 1909, William Ramsay and Robert Whytlaw-Gray isolated radium emanation and measured its properties, helping establish that it was a new member of the noble-gas family.
Radon-222, the naturally important isotope, has a half-life of about 3.8 days. It decays by emitting an alpha particle and produces short-lived radioactive “radon daughters,” several of which can attach to airborne dust. Inhaling them is the main health concern associated with indoor radon exposure.
Radon is chemically unreactive because its outer electron shell is full, like those of helium, neon, argon, krypton, and xenon. Unlike those lighter noble gases, however, all radon isotopes are radioactive.