Descendants of thorium
Naturally occurring thorium 232 is also the patriarch of a chain of radioactive elements which decay into each other, ending with the formation of a stable isotope of lead, lead 208. The genealogy of thorium also contains a gaseous element, radon 220, historically known as thoron. This form of radon is less dangerous than that released by uranium, however, as the half-life of radon 220 is only 55 seconds, too short for it to be able to escape from the rock.