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Isotopes of lead

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Lead (82Pb) has four observationally stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. 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 205Tl. The three series terminating in lead represent the decay chain products of long-lived primordial 238U, 235U, and 232Th. 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. (See lead–lead dating and uranium–lead dating.)

Quick Facts Main isotopes, Decay ...

The longest-lived radioisotopes, both decaying by electron capture, are 205Pb with a half-life of 17.3 million years and 202Pb with a half-life of 52,500 years. A shorter-lived naturally occurring radioisotope, 210Pb 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.[5]

The heaviest stable isotope, 208Pb, belongs to this element. (The more massive 209Bi, long considered to be stable, actually has a half-life of 2.01×1019 years.) 208Pb is also a doubly magic isotope, as it has 82 protons and 126 neutrons.[6] It is the heaviest doubly magic nuclide known.

The four primordial isotopes of lead are all observationally stable, meaning that they are predicted to undergo radioactive decay but no decay has been observed yet. These four isotopes are predicted to undergo alpha decay and become isotopes of mercury which are themselves radioactive or observationally stable.

There are trace quantities existing of the radioactive isotopes 209-214. The largest and most important is lead-210 as it has by far the longest half-life (22.2 years) and occurs in the abundant uranium decay series. Lead-211, −212, and −214 are present in the decay chains of uranium-235, thorium-232, and uranium-238, further, making these three lead isotopes also detectable in natural sources. The more minute traces of lead-209 arise from three rare decay branches: by frequency, the rare beta-minus-neutron decay of thallium-210 (in the uranium series), the last step of the neptunium series, traces of which are produced by neutron capture in uranium ores, and the very rare cluster decay of radium-223 (yielding also carbon-14). Lead-213 also occurs in a branch of the neptunium series. Lead-210 is particularly useful for helping to identify the ages of samples by measuring its ratio to lead-206 (both isotopes are present in a single decay chain).[7]

In total, 43 lead isotopes have been synthesized, from 178Pb to 220Pb.

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List of isotopes

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More information Nuclide, Historic name ...
  1. [n 1]
    mPb – Excited nuclear isomer.
  2. [n 2]
    ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. [n 3]
    # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. [n 4]
    Modes of decay:
    EC:Electron capture
    IT:Isomeric transition
  5. [n 5]
    Bold italics symbol as daughter – Daughter product is nearly stable.
  6. [n 6]
    Bold symbol as daughter – Daughter product is stable.
  7. [n 7]
    ( ) spin value – Indicates spin with weak assignment arguments.
  8. [n 8]
    # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  9. [n 9]
    Order of ground state and isomer is uncertain.
  10. [N 10]
    Used in lead–lead dating
  11. [N 11]
    Believed to undergo α decay to 200Hg with a half-life over 1.4×1020 years; the theoretical lifetime is around ~1035–37 years.[9]
  12. [N 12]
    Final decay product of 4n+2 decay chain (the Radium or Uranium series)
  13. [N 13]
    Believed to undergo α decay to 202Hg with a half-life over 2.5×1021 years; the theoretical lifetime is ~1065–68 years.[9]
  14. [N 14]
    Final decay product of 4n+3 decay chain (the Actinium series)
  15. [N 15]
    Believed to undergo α decay to 203Hg with a half-life over 1.9×1021 years; the theoretical lifetime is ~10152–189 years.[9]
  16. [N 16]
    Heaviest observationally stable nuclide; final decay product of 4n decay chain (the Thorium series)
  17. [N 17]
    Believed to undergo α decay to 204Hg with a half-life over 2.6×1021 years; the theoretical lifetime is ~10124–132 years.[9]
  18. [N 18]
    Intermediate decay product of 237Np
  19. [N 19]
    Intermediate decay product of 238U
  20. [N 20]
    Intermediate decay product of 235U
  21. [N 21]
    Intermediate decay product of 232Th
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Lead-206

See also: Decay chain

206Pb is the final step in the decay chain of 238U, the "radium series" or "uranium series". In a closed system, over time, a given mass of 238U will decay in a sequence of steps culminating in 206Pb. The production of intermediate products eventually reaches an equilibrium (though this takes a long time, as the half-life of 234U is 245,500 years). Once this stabilized system is reached, the ratio of 238U to 206Pb will steadily decrease, while the ratios of the other intermediate products to each other remain constant.

Like most radioisotopes found in the radium series, 206Pb was initially named as a variation of radium, specifically radium G. It is the decay product of both 210Po (historically called radium F) by alpha decay, and the much rarer 206Tl (radium EII) by beta decay.

Lead-206 has been proposed for use in fast breeder nuclear fission reactor coolant over the use of natural lead mixture (which also includes other stable lead isotopes) as a mechanism to improve neutron economy and greatly suppress unwanted production of highly radioactive byproducts.[12]

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Lead-204, -207, and -208

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204Pb is entirely primordial, and is thus useful for estimating the fraction of the other lead isotopes in a given sample that are also primordial, since the relative fractions of the various primordial lead isotopes is constant everywhere.[13] Any excess lead-206, -207, and -208 is thus assumed to be radiogenic in origin,[13] allowing various uranium and thorium dating schemes to be used to estimate the age of rocks (time since their formation) based on the relative abundance of lead-204 to other isotopes. 207Pb is the end of the actinium series from 235U.

208Pb is the end of the thorium series from 232Th. While it only makes up approximately half of the composition of lead in most places on Earth, it can be found naturally enriched up to around 90% in thorium ores.[14] 208Pb is the heaviest known stable nuclide and also the heaviest known doubly magic nucleus, as Z = 82 and N = 126 correspond to closed nuclear shells.[15] As a consequence of this particularly stable configuration, its neutron capture cross section is very low (even lower than that of deuterium in the thermal spectrum), making it of interest for lead-cooled fast reactors.

In 2025 a published study suggested that the nucleus of 208Pb is not perfectly spherical as previously believed, but rather is a "prolate spheroid", more commonly described as the shape of a rugby ball.[16]

Lead-212

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  • Thumb
    Pb-212 (27.2 mCi)
  • Thumb
    Pb-212 Separation from Ra-224 (19.3 mCi)

Lead-212 (212Pb) is a radioactive isotope of lead that has gained significant attention in nuclear medicine, particularly in targeted alpha therapy (TAT).[17] This isotope is part of the thorium decay series and serves as an important intermediate in various radioactive decay chains.[18] 212Pb is produced through the decay of radon-220 (220Rn), an intermediate product of thorium-228 (228Th) decay.[17] It undergoes radioactive decay through beta emission to form bismuth-212 (212Bi), which further decays to emit alpha particles.[19] This decay chain is particularly important in medical applications, as it is an in-vivo generator system of alpha particles, that can be utilized for therapeutic purposes, particularly TAT, by delivering potent, localized radiation to cancer cells.

The isotope is part of the thorium decay series, which begins with natural thorium-232. Its beta decay (10.627 hours) results in the formation of bismuth-212 (212Bi), which then emits alpha particles (6.1 MeV), crucial for the effectiveness of TAT in cancer treatment.[20]

While in aqueous solutions, free Pb2+ tends to hydrolyze under physiological pH conditions to form species like Pb(OH)+, which can impact its biodistribution if not properly chelated,[21] chelator-modified complexes have demonstrated high stability in saline and serum environments for extended periods (e.g., 24–72 hours), which is critical for therapeutic applications.[22]

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