Radon is a radioactive element. Two of its isotopes (radon-220 and radon-222) are progeny in two decay chains that begin with naturally occurring thorium and uranium, respectively, in rock, soil, water, and air. 1  Because radon is a noble gas, it is colorless, odorless, tasteless, and imperceptible to the senses.adon (Rn) is a radioactive gas (Lewis 2001) that naturally occurs in different forms known as isotopes. Radon is a chemically and biologically inert noble gas. Its nucleus is heavily neutron-rich, making it radioactive. 1  Radon's half-life is 3.8 days. 2  Radon is present in air, water, and soil.