Radon (/ËreÉªdÉn/ RAY-don) is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. It is one of the densest substances that remains a gas under normal conditions, and is considered to be a health hazard due to its radioactivity.adon is formed as part of the normal radioactive decay chain of uranium into 206Pb. Uranium has been present since the earth was formed and its most common isotope has a very long half-life (4.5 billion years), which is the amount of time required for one-half of uranium to break down.