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Fermium (₁₀₀Fm) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in fallout from nuclear testing) was Fm in 1952. Fm was independently synthesized shortly after the discovery of Fm. There are 20 known radioisotopes ranging in atomic mass from Fm to Fm (Fm is unconfirmed), and 4 nuclear isomers, Fm, Fm, Fm, and Fm. The longest-lived isotope is Fm with a half-life of 100.5 days, and the longest-lived isomer is Fm with a half-life of 5.1 seconds.

List of isotopes

1. Fm – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
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. Modes of decay:

   EC:	Electron capture
   IT:	Isomeric transition
   SF:	Spontaneous fission

5. ( ) spin value – Indicates spin with weak assignment arguments.
6. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
7. Theoretically capable of ββ decay to Cf
8. Heaviest nuclide produced via neutron capture
9. Discovery of this isotope is unconfirmed
10. Not directly synthesized, occurs as decay product of Md

Chronology of isotope discovery

Fm was not confirmed in 1997.

References

1. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
2. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
3. ^ Khuyagbaatar, J.; Heßberger, F. P.; Hofmann, S.; Ackermann, D.; Burkhard, H. G.; Heinz, S.; Kindler, B.; Kojouharov, I.; Lommel, B.; Mann, R.; Maurer, J.; Nishio, K. (12 October 2020). "α decay of Fm 243 143 and Fm 245 145 , and of their daughter nuclei". Physical Review C. 102 (4): 044312. doi:10.1103/PhysRevC.102.044312. ISSN 2469-9985. S2CID 241259726. Retrieved 24 June 2023.
4. Tezekbayeva, M. S.; Yeremin, A. V.; Svirikhin, A. I.; Lopez-Martens, A.; Chelnokov, M. L.; Chepigin, V. I.; Isaev, A. V.; Izosimov, I. N.; Karpov, A. V.; Kuznetsova, A. A.; Malyshev, O. N.; Mukhin, R. S.; Popeko, A. G.; Popov, Yu. A.; Rachkov, V. A.; Sailaubekov, B. S.; Sokol, E. A.; Hauschild, K.; Jacob, H.; Chakma, R.; Dorvaux, O.; Forge, M.; Gall, B.; Kessaci, K.; Andel, B.; Antalic, S.; Bronis, A.; Mosat, P. (24 March 2022). "Study of the production and decay properties of neutron-deficient nobelium isotopes". The European Physical Journal A. 58 (3): 52. arXiv:2203.15659. Bibcode:2022EPJA...58...52T. doi:10.1140/epja/s10050-022-00707-9. ISSN 1434-601X. S2CID 247720708. Retrieved 24 June 2023.

• Isotope masses from:
  • M. Wang; G. Audi; A. H. Wapstra; F. G. Kondev; M. MacCormick; X. Xu; et al. (2012). "The AME2012 atomic mass evaluation (II). Tables, graphs and references" (PDF). Chinese Physics C. 36 (12): 1603–2014. Bibcode:2012ChPhC..36....3M. doi:10.1088/1674-1137/36/12/003. S2CID 250839471.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, G.; Kondev, F. G.; Wang, M.; Pfeiffer, B.; Sun, X.; Blachot, J.; MacCormick, M. (2012). "The NUBASE2012 evaluation of nuclear properties" (PDF). Chinese Physics C. 36 (12): 1157–1286. Bibcode:2012ChPhC..36....1A. doi:10.1088/1674-1137/36/12/001. Archived from the original (PDF) on 2014-02-22.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.
  • M. Thoennessen (2012). "Discovery of Isotopes of Elements with Z≥100". Atomic Data and Nuclear Data Tables. 99 (3): 312–344. arXiv:1203.2105. Bibcode:2013ADNDT..99..312T. doi:10.1016/j.adt.2012.03.003. S2CID 94982856.

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Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

• Isotopes of fermium
• Fermium
• Lists of isotopes by element