Radon is a chemical element with symbol Rn and atomic number 86. It 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. Its most stable isotope, 222Rn, has a half-life of 3.8 days. Due to its high radioactivity, it has been less well-studied by chemists, but a few compounds are known.