Element Radioactive is a term used to describe a type of element that has an unstable nucleus and emits radiation. RADON is an example of an Element Radioactive, which is a naturally occurring, colorless, odorless, tasteless noble gas. It is the heaviest known gas and is produced from the radioactive decay of radium and uranium. RADON is considered to be a health hazard due to its radioactivity and is the second leading cause of lung cancer in the United States.