Radon
Radon is a noble gas. It is not to be confused with radium, an alkali earth metal. The atomic weight of Radon-222, the most common isotope, is 222.0175777.
Radon is a radioactive gas which comes from the natural decay of uranium in the ground. The EPA claims that it is a health hazard and causes cancer in people and animals exposed to it, though many conservative commentators and some industrial hygienists[1] state that the dangers are significantly exaggerated.