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Bismuth-209, 209Bi
Namesbismuth-209, Bi-209
Protons (Z)83
Neutrons (N)126
Nuclide data
Natural abundance100%
Half-life (t1/2)2.01×1019 years[1]
Isotope mass208.9803987 Da
Excess energy−18258.461±2.4 keV
Binding energy7847.987±1.7 keV
Parent isotopes209Pb (β)
209Po (β+)
213At (α)
Decay products205Tl
Decay modes
Decay modeDecay energy (MeV)
Alpha emission3.1373
Isotopes of bismuth
Complete table of nuclides

Bismuth-209 (209Bi) is the isotope of bismuth with the longest known half-life of any radioisotope that undergoes α-decay (alpha decay). It has 83 protons and a magic number of 126 neutrons, and an atomic mass of 208.9803987 amu (atomic mass units). Primordial bismuth consists entirely of this isotope.

Decay properties[edit]

Bismuth-209 was long thought to have the heaviest stable nucleus of any element, but in 2003, a research team at the Institut d’Astrophysique Spatiale in Orsay, France, discovered that 209Bi undergoes alpha decay with a half-life of approximately 19 exayears (1.9×1019, approximately 19 quintillion years), over a billion times longer than the current estimated age of the universe. The heaviest nucleus considered to be stable is now lead-208 and the heaviest stable monoisotopic element is gold as the 197Au isotope.

Theory had previously predicted a half-life of 4.6×1019 years. The decay event produces a 3.14 MeV alpha particle and converts the atom to thallium-205.[2][3]

Bismuth-209 will eventually form 205Tl if unperturbed:

+ 4

If perturbed, it would join in lead-bismuth neutron capture cycle from lead-206/207/208 to bismuth-209, despite low capture cross sections. Even in thallium-205 case above, once fully ionized, again reverts to lead.

Due to its extraordinarily long half-life, for nearly all applications 209Bi can still be treated as if it were non-radioactive. Its radioactivity is much slighter than that of human flesh, so it poses no meaningful hazard from radiation. Although 209Bi holds the half-life record for alpha decay, bismuth does not have the longest half-life of any radionuclide to be found experimentally—this distinction belongs to tellurium-128 (128Te) with a half-life estimated at 7.7 × 1024 years by double β-decay (double beta decay).[5]

The half-life of bismuth-209 was confirmed in 2012 by an Italian team in Gran Sasso who reported (2.01±0.08)×1019 years. They also reported an even longer half-life for alpha decay of bismuth-209 to the first excited state of thallium-205 (at 204 keV), was estimated to be 1.66×1021 years.[6] Even though this value is shorter than the measured half-life of tellurium-128, both alpha decays of bismuth-209 hold the record of the thinnest natural line widths of any measurable physical excitation, estimated respectively at ΔΕ~5.5×10−43 eV and ΔΕ~1.3×10−44 eV in application of the uncertainty principle of Heisenberg[7] (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).


210Po can be manufactured by bombarding 209Bi with neutrons in a nuclear reactor. Only some 100 grams of 210Po are produced each year.[8] Astatine can also be produced by bombarding 209Bi with alpha particles.


In the red giant stars of the asymptotic giant branch, the s-process (slow process) is ongoing to produce bismuth-209 and polonium-210 by neutron capture as the heaviest elements to be formed, and the latter quickly decays. All elements heavier than it are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovas.[9]

See also[edit]


Bismuth-209 is an
isotope of bismuth
Decay product of:
astatine-213 (α)
polonium-209 (β+)
lead-209 (β)
Decay chain
of bismuth-209
Decays to:
thallium-205 (α)


  1. ^ Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  2. ^ Dumé, Belle (2003-04-23). "Bismuth breaks half-life record for alpha decay". Physicsweb.
  3. ^ Marcillac, Pierre de; Noël Coron; Gérard Dambier; Jacques Leblanc; Jean-Pierre Moalic (April 2003). "Experimental detection of α-particles from the radioactive decay of natural bismuth". Nature. 422 (6934): 876–878. Bibcode:2003Natur.422..876D. doi:10.1038/nature01541. PMID 12712201. S2CID 4415582.
  4. ^ "Isotope data for americium-241 in the Periodic Table".
  5. ^ "Noble Gas Research". Archived from the original on 2011-09-28. Retrieved 2013-01-10. Tellurium-128 information and half-life. Accessed July 14, 2009.
  6. ^ J.W. Beeman; et al. (2012). "First Measurement of the Partial Widths of 209Bi Decay to the Ground and to the First Excited States". Physical Review Letters. 108 (6): 062501. arXiv:1110.3138. doi:10.1103/PhysRevLett.108.062501. PMID 22401058. S2CID 118686992.
  7. ^ "Particle lifetimes from the uncertainty principle".
  8. ^ "Swiss study: Polonium found in Arafat's bones". Al Jazeera. Retrieved 2013-11-07.
  9. ^ Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. San Francisco: Pearson Education, 2008.