Thorium series in the context of Stable nuclide

Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to be released from the soil and rock where it is generated. Radon isotopes are the immediate decay products of radium isotopes.

The instability of Rn, its most stable isotope, makes radon one of the rarest elements. Radon will be present on Earth for several billion more years despite its short half-life, because it is constantly being produced as a step in the decay chains of U and Th, both of which are abundant radioactive nuclides with half-lives of at least several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Rn occurs in significant quantities as a step in the normal radioactive decay chain of U, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into stable Pb. Rn occurs in minute quantities as an intermediate step in the decay chain of Th, also known as the thorium series, which eventually decays into stable Pb.

Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

All known thorium isotopes are unstable. The most stable isotope, Th, has a half-life of 14.0 billion years, or about the age of the universe; it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable Pb. On Earth, thorium and uranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as primordial elements. Thorium is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth elements.

Lead (₈₂Pb) has four observationally stable isotopes: Pb, Pb, Pb, Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium series (or radium series), the actinium series, and the thorium series, respectively; a fourth decay chain, the neptunium series, terminates with the thallium isotope Tl. The three series terminating in lead represent the decay chain products of long-lived primordial U, U, and Th. Each isotope also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. This is the basis for lead–lead dating and uranium–lead dating.

The longest-lived radioisotopes, both decaying by electron capture, are Pb with a half-life of 17.0 million years and Pb with a half-life of 52,500 years. A shorter-lived naturally occurring radioisotope, Pb with a half-life of 22.2 years, is useful for studying the sedimentation chronology of environmental samples on time scales shorter than 100 years.

There are 39 known isotopes of radon (₈₆Rn), from Rn to Rn; all are radioactive. The most stable isotope is Rn with a half-life of 3.8215 days, which decays into
Po.

Six isotopes of radon, Rn, occur in trace quantities in nature as decay products of, respectively, At, At, Ra, Ra, Ra, and Ra. Rn and Rn are produced in rare branches in the decay chain of trace quantities of Np; Rn (and also Rn in a rare branch) is an intermediate step in the decay chain of U; Rn is an intermediate step in the decay chain of U; and Rn occurs in the decay chain of Th.