Radon is the chemical element with the symbol Rn, and atomic number 86. It is named after the element radium as it is a decay product of radium. It is one of two elements named after another element, the other one being protactinium. It is a radioactive gas, the only one known to occur naturally. All radon in the atmosphere is produced by the decay of radioactive elements, especially uranium and thorium. Given that uranium is somewhat a common element, radon is considered a hazard in many parts of the world. It is mostly produced urban-wise via granite, and the elements that decay in it. Radon's longest lived isotope, Rn-222, lasts 3 days. Radon is a noble gas, and its chemical properties are probably similar to those of other noble gases. Radon probably forms RnF4 with fluorine, given that xenon creates a similar compound.