Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. Thallium tends to oxidize to the +3 state resembles that of the other elements in Group 13 (boron, aluminum, gallium, indium). It is also common in the +1 state, unlike its smaller homologs.
Thallium through polonium form a band in which elements transition from being mainly of chemical interest to being mainly of nuclear interest. Thallium is rare in both the solar system and earth's crust, and appears to be somewhat depleted in the crust. It heavier isotopes also "reflect" actinide decay chains back toward polonium. These chains arrive in this band at the end of long series of alpha decays. They arrive neutron-rich. Between polonium's alpha decays and thallium's beta decays, actinide chains oscillate back and forth until they arrive at lead or bismuth (chain almost extinct).
NUCLEAR PROPERTIES
At least 93 isotopes, ranging from 265Tl to 172Tl, have been predicted; of which 41, ranging from 216Tl to 176Tl, have been observed. Thallium has two effectively stable isotopes, 205Tl and 203Tl. The odd-Z, odd-N isotope 204Tl decays by beta emission to 204Pb [ BR(b-) = 0.971. ] and by EC (electron capture) to 204Hg. [ BR(EC) = 0.029. ] Its half-life is 2.78 yrs, making it the only Tl isotope whose half-life exceeds 2 weeks.
Positive beta decay has been observed between 202Tl and 180Tl. Alpha decay is also observed at 194Tl (very weak) and between 187Tl and 177Tl. Below 179Tl, proton emission becomes an active decay mode. 42 isomers have also been observed, most of which have half-lives comparable to their corresponding ground-state nuclide and decay independently in qualitatively the same way (although branch ratios differ).
Five thallium isotopes, 206Tl through 210Tl, form as a result of thorium or uranium decay. Concentration of these, relative to their chain heads, as the product of (overall branch ratio between the isotope in question and its chain head) times (half-life of isotope)/(half-life of chain head). If desired, total quantity of the isotope in question can be computed from known chain head abundances. This gives [206Tl]/[U] = 3.4E-20, [207Tl]/[U] = 1.3E-14 [208Tl]/[Th] = 1.5E-16, [209Tl]/[U] = 7.7E-44, and [210Tl]/[U] = 1.2E-19; which corresponds to n(207Tl) = 42 mol/planet, n(206Tl) = 0.015 mol/planet, n(208Tl) = 210 mol/planet, n(209Tl) = 3.5E-26 mol/planet, and n(210Tl) = 0.053 mol/planet.
The role of 210Tl's (b+n) decay mode in linking the 238U and 237Np decay chains was addressed in the article "Lead" in this wiki.
Thallium has one more very strange isotope. 180Tl has a positron emission decay mode (BR = 0.25) to 180Hg. From context, this decay goes to an excited state rather 180Hg's ground state. 180mHg then decays very quickly by what can either be called cluster decay or fission. Unlike normal cluster decay, the ejected particle has A not much smaller than 90, half the mass number of 180mHg. That's normally in the size range of fission fragments. Yet, unlike fission, 180mHg decays to a specific pair of nuclides - 100Ru and 80Kr. No other nuclide is observed or predicted to decay like this.
ATOMIC PROPERTIES
Thallium is a Group 13 metal, in the same family as Al, Ga, and In. Its whole-earth abundance can be estimated from its relatively well known solar abundance by assuming that metals concentration in earth as a whole has the same abundance pattern as for the solar system. Absolute abundances can be obtained by multiplying by [Si]earth/[Si]sun. On this basis, , [Tl] = [U], making it one of the rarest elements. Its crustal abundance is enhanced, not surprising for a Group 13 element.
(02-28-22)