A radioactive gas, symbol An; an inert gaseous element that is an isotope of radon and thoron. Actinon is a member of the uranium–actinium family of radioactive elements. Its radioactive half-life is extremely short, only 3. 92 s, and its parent atoms (actinium x) are comparatively rare in the earth's crust, so actinon is considerably less important as an agency of atmospheric ionization than are radon and thoron.