Isotopes of fermium explained

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 ²⁵⁵Fm in 1952. ²⁵⁰Fm was independently synthesized shortly after the discovery of ²⁵⁵Fm. There are 20 known radioisotopes ranging in atomic mass from ²⁴¹Fm to ²⁶⁰Fm (²⁶⁰Fm is unconfirmed), and 4 nuclear isomers, ^247mFm, ^250mFm, ^251mFm, and ^253mFm. The longest-lived isotope is ²⁵⁷Fm with a half-life of 100.5 days, and the longest-lived isomer is ^247mFm with a half-life of 5.1 seconds.

List of isotopes

|-| rowspan=2|²⁴¹Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 141| rowspan=2|241.07421(32)#| rowspan=2|730(60) μs| SF| (various)| rowspan=2|5/2+#|-| α (<14%)| ²³⁷Cf|-| ²⁴²Fm| style="text-align:right" | 100| style="text-align:right" | 142| 242.07343(43)#| <4 μs^[1] | SF| (various)| 0+|-| rowspan=3|²⁴³Fm^[2] | rowspan=3 style="text-align:right" | 100| rowspan=3 style="text-align:right" | 143| rowspan=3|243.07447(23)#| rowspan=3|231(9) ms| α (91%)| ²³⁹Cf| rowspan=3|(7/2−)|-| SF (9%)| (various)|-| β⁺ (<10%)| ²⁴³Es|-| rowspan=2|²⁴⁴Fm^[1] | rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 144| rowspan=2|244.07404(22)#| rowspan=2|3.12(8) ms| SF| (various)| rowspan=2|0+|-| α (<1%)| ²⁴⁰Cf|-| rowspan=3|²⁴⁵Fm^[3] | rowspan=3 style="text-align:right" | 100| rowspan=3 style="text-align:right" | 145| rowspan=3|245.07535(21)#| rowspan=3|5.5(7) s| α (88.5%)| ²⁴¹Cf| rowspan=3|(1/2+)|-| β⁺ (11.5%)| ²⁴⁵Es|-| SF (<0.3%)^[2] | (various)|-| rowspan=3|²⁴⁶Fm^[4] | rowspan=3 style="text-align:right" | 100| rowspan=3 style="text-align:right" | 146| rowspan=3|246.075350(17)| rowspan=3|1.54(4) s| α (93.2%)| ²⁴²Cf| rowspan=3|0+|-| SF (6.8%)| (various)|-| β⁺ (<1.3%)| ²⁴⁶Es|-| rowspan=2|²⁴⁷Fm^[5] | rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 147| rowspan=2|247.07695(12)#| rowspan=2|31(1) s| α (64%)| ²⁴³Cf| rowspan=2|(7/2+)|-| β⁺ (36%)| ²⁴⁷Es|-| rowspan=3 style="text-indent:1em" | ^247mFm| rowspan=3 colspan="3" style="text-indent:2em" | 45(7) keV| rowspan=3| 5.1(2) s| α (88%)| ²⁴³Cf| rowspan=3|(1/2+)|-| IT (12%)| ²⁴⁷Fm|-| β⁺?| ²⁴⁷Es|-| rowspan=3|²⁴⁸Fm| rowspan=3 style="text-align:right" | 100| rowspan=3 style="text-align:right" | 148| rowspan=3|248.077186(9)| rowspan=3|35.1(8) s| α (93%)| ²⁴⁴Cf| rowspan=3|0+|-| β⁺ (7%)| ²⁴⁸Es|-| SF (.10%)| (various)|-| rowspan=2|²⁴⁹Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 149| rowspan=2|249.078928(7)| rowspan=2|1.6(1) min| β⁺ (84.4%)| ²⁴⁹Es| rowspan=2|(7/2+)|-| α (15.6%)^[6] | ²⁴⁵Cf|-| rowspan=3|²⁵⁰Fm| rowspan=3 style="text-align:right" | 100| rowspan=3 style="text-align:right" | 150| rowspan=3|250.079521(9)| rowspan=3|30.4(15) min| α (90%)| ²⁴⁶Cf| rowspan=3|0+|-| EC (10%)| ²⁵⁰Es|-| SF (6.9×10⁻³%)| (various)|-| style="text-indent:1em" | ^250mFm| colspan="3" style="text-indent:2em" | 1199.2(10) keV| 1.92(5) s| IT| ²⁵⁰Fm| (8−)|-| rowspan=2|²⁵¹Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 151| rowspan=2|251.081540(16)| rowspan=2|5.30(8) h| β⁺ (98.2%)| ²⁵¹Es| rowspan=2|(9/2−)|-| α (1.8%)| ²⁴⁷Cf|-| style="text-indent:1em" | ^251mFm| colspan="3" style="text-indent:2em" | 200.09(11) keV| 21.1(16) μs| IT| ²⁵¹Fm| (5/2+)|-| rowspan=2|²⁵²Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 152| rowspan=2|252.082467(6)| rowspan=2|25.39(4) h| α (99.99%)| ²⁴⁸Cf| rowspan=2|0+|-| SF (.0023%)| (various)|-| rowspan=2|²⁵³Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 153| rowspan=2|253.085185(4)| rowspan=2|3.00(12) d| EC (88%)| ²⁵³Es| rowspan=2|(1/2)+|-| α (12%)| ²⁴⁹Cf|-| style="text-indent:1em" | ^253mFm^[7] | colspan="3" style="text-indent:2em" | ~351 keV| 0.56(6) μs| IT| ²⁵³Fm| (11/2-)|-| rowspan=2|²⁵⁴Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 154| rowspan=2|254.0868544(30)| rowspan=2|3.240(2) h| α (99.94%)| ²⁵⁰Cf| rowspan=2|0+|-| SF (.0592%)| (various)|-| rowspan=2|²⁵⁵Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 155| rowspan=2|255.089964(5)| rowspan=2|20.07(7) h| α| ²⁵¹Cf| rowspan=2|7/2+|-| SF (2.4×10⁻⁵%)| (various)|-| rowspan=2|²⁵⁶Fm| rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 156| rowspan=2|256.091774(8)| rowspan=2|157.6(13) min| SF (91.9%)| (various)| rowspan=2|0+|-| α (8.1%)| ²⁵²Cf|-| rowspan=2|²⁵⁷Fm^[8] | rowspan=2 style="text-align:right" | 100| rowspan=2 style="text-align:right" | 157| rowspan=2|257.095106(7)| rowspan=2|100.5(2) d| α (99.79%)| ²⁵³Cf| rowspan=2|(9/2+)|-| SF (.21%)| (various)|-| ²⁵⁸Fm| style="text-align:right" | 100| style="text-align:right" | 158| 258.09708(22)#| 370(14) μs| SF| (various)| 0+|-| ²⁵⁹Fm| style="text-align:right" | 100| style="text-align:right" | 159| 259.1006(3)#| 1.5(3) s| SF| (various)| 3/2+#|-| ²⁶⁰Fm^{[9] [10]} | style="text-align:right" | 100| style="text-align:right" | 160| 260.10281(55)#| 4 ms| SF| (various)| 0+

Chronology of isotope discovery

Isotope	Discovered	Reaction
^241Fm	2008	^204Pb(^40Ar,3n)
^242Fm	1975	^204Pb(^40Ar,2n), ^206Pb(^40Ar,4n)
^243Fm	1981	^206Pb(^40Ar,3n)
^244Fm	1967	^233U(^16O,5n)
^245Fm	1967	^233U(^16O,4n)
^246Fm	1966	^235U(^16O,5n)
^247Fm	1967	^239Pu(^12C,4n)
^248Fm	1958	^240Pu(^12C,4n)
^249Fm	1960	^238U(^16O,5n)
^250Fm	1954	^238U(^16O,4n)
^251Fm	1957	^249Cf(α,2n)
^252Fm	1956	^249Cf(α,n)
^253Fm	1957	^252Cf(α,3n)
^254Fm	1954	Neutron capture
^255Fm	1954	Neutron capture
^256Fm	1955	Neutron capture
^257Fm	1964	Neutron capture
^258Fm	1971	^257Fm(d,p)
^259Fm	1980	^257Fm(t,p)
^260Fm?	1992?	^254Es+^18O, ^22Ne — transfer (EC of ^260Md)

²⁶⁰Fm was not confirmed in 1997.

References

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2. 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. . α decay of Fm 243 143 and Fm 245 145, and of their daughter nuclei . Physical Review C . 12 October 2020 . 102 . 4 . 044312 . 10.1103/PhysRevC.102.044312 . 241259726 . 24 June 2023 . en . 2469-9985.
3. 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. . Study of the production and decay properties of neutron-deficient nobelium isotopes . The European Physical Journal A . 24 March 2022 . 58 . 3 . 52 . 10.1140/epja/s10050-022-00707-9 . 2203.15659 . 2022EPJA...58...52T . 247720708 . 24 June 2023 . en . 1434-601X.
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5. Heßberger . F. P. . Hofmann . S. . Ackermann . D. . Antalic . S. . Kindler . B. . Kojouharov . I. . Kuusiniemi . P. . Leino . M. . Lommel . B. . Mann . R. . Nishio . K. . Popeko . A. G. . Sulignano . B. . Saro . S. . Streicher . B. . Venhart . M. . Yeremin . A. V. . Alpha-gamma decay studies of 255Rf, 251No and 247Fm . The European Physical Journal A - Hadrons and Nuclei . 1 December 2006 . 30 . 3 . 561–569 . 10.1140/epja/i2006-10137-2 . 2006EPJA...30..561H . 123871572 . 24 June 2023 . en . 1434-601X.
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8. Heaviest nuclide produced via neutron capture
9. Discovery of this isotope is unconfirmed
10. Not directly synthesized, occurs as decay product of ²⁶⁰Md

• Isotope masses from:
  • M. Wang . G. Audi . A. H. Wapstra . F. G. Kondev . M. MacCormick . X. Xu. 2012 . The AME2012 atomic mass evaluation (II). Tables, graphs and references. . Chinese Physics C . 36 . 12 . 1603–2014 . 10.1088/1674-1137/36/12/003. 2012ChPhC..36....3M . 250839471 . etal.
    • Half-life, spin, and isomer data selected from the following sources.
      • • • • • • • M. Thoennessen . 2012 . Discovery of Isotopes of Elements with Z≥100 . 1203.2105 . 10.1016/j.adt.2012.03.003 . 99 . 3 . Atomic Data and Nuclear Data Tables . 312–344. 2013ADNDT..99..312T . 94982856 .