Radon (symbol Rn) is a chemical element with atomic number 86 and atomic weight 222. It is a radioactive, colourless, odourless and tasteless noble gas. It has a density of 9.73 kg/m³ which makes Radon one of the densest substances that remains a gas under normal conditions.
Radon occurs naturally, being formed as one intermediate step (namely it being decay product of radium) in the normal radioactive decay chains through which thorium and uranium slowly decay into lead. Thorium and uranium are the two most common radioactive elements on Earth with their naturally occurring isotopes having very long half-lives (many billions of years). As radon itself decays, it produces other radioactive elements called radon progeny (formerly called daughters) or decay products. Its most stable isotope, 222Rn, has a half-life of 3.8 days.