Isotopes of fermium

Main isotopes of fermium (₁₀₀Fm)
α	248Cf
α	249Cf
α	251Cf
SF	–

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 2 nuclear isomers, ^250mFm and ^251mFm. The longest-lived isotope is ²⁵⁷Fm with a half-life of 100.5 days, and the longest-lived isomer is ^250mFm with a half-life of 1.8 seconds.

List of isotopes

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)^[1]^{[n 1]}	daughter isotope(s)	nuclear spin and parity
nuclide symbol	excitation energy			half-life	decay mode(s)^[1]^{[n 1]}	daughter isotope(s)	nuclear spin and parity
²⁴¹Fm	100	141	241.07421(32)#	730(60) µs	SF(>78%)	(various)	5/2#+
²⁴¹Fm	100	141	241.07421(32)#	730(60) µs	α (<14%)	²³⁷Cf	5/2#+
²⁴²Fm	100	142	242.07343(43)#	0.8(2) ms	SF	(various)	0+
²⁴²Fm	100	142	242.07343(43)#	0.8(2) ms	α (rare)	²³⁸Cf	0+
²⁴³Fm	100	143	243.07447(23)#	231(9) ms	α (91%)	²³⁹Cf	7/2−#
					SF (9%)	(various)
					β⁺ (rare)	²⁴³Es
²⁴⁴Fm	100	144	244.07404(22)#	3.12(8) ms	SF (99%)	(various)	0+
²⁴⁴Fm	100	144	244.07404(22)#	3.12(8) ms	α (1%)	²⁴⁰Cf	0+
²⁴⁵Fm	100	145	245.07535(21)#	4.2(13) s	α (95.7%)	²⁴¹Cf	1/2+#
					β⁺ (4.2%)	²⁴⁵Es
					SF (.13%)	(various)
²⁴⁶Fm	100	146	246.075350(17)	1.54(4) s	α (85%)	²⁴²Cf	0+
					β⁺ (10%)	²⁴⁶Es
					β⁺, SF (10%)	(various)
					SF (4.5%)	(various)
²⁴⁷Fm	100	147	247.07695(12)#	31(1) s	α (>50%)	²⁴³Cf	(7/2+)
²⁴⁷Fm	100	147	247.07695(12)#	31(1) s	β⁺ (<50%)	²⁴⁷Es	(7/2+)
²⁴⁸Fm	100	148	248.077186(9)	35.1(8) s	α (93%)	²⁴⁴Cf	0+
					β⁺ (7%)	²⁴⁸Es
					SF (.10%)	(various)
²⁴⁹Fm	100	149	249.078928(7)	1.6(1) min	β⁺ (85%)	²⁴⁹Es	(7/2+)#
²⁴⁹Fm	100	149	249.078928(7)	1.6(1) min	α (15%)	²⁴⁵Cf	(7/2+)#
²⁵⁰Fm	100	150	250.079521(9)	30.4(15) min	α (90%)	²⁴⁶Cf	0+
					EC (10%)	²⁵⁰Es
					SF (6.9×10⁻³%)	(various)
^250mFm	1199.2(10) keV			1.92(5) s	IT	²⁵⁰Fm	(8−)
²⁵¹Fm	100	151	251.081540(16)	5.30(8) h	β⁺ (98.2%)	²⁵¹Es	(9/2−)
²⁵¹Fm	100	151	251.081540(16)	5.30(8) h	α (1.8%)	²⁴⁷Cf	(9/2−)
^251mFm	200.09(11) keV			21.1(16) µs			(5/2+)
²⁵²Fm	100	152	252.082467(6)	25.39(4) h	α (99.99%)	²⁴⁸Cf	0+
					SF (.0023%)	(various)
					β⁺β⁺ (rare)	²⁵²Cf
²⁵³Fm	100	153	253.085185(4)	3.00(12) d	EC (88%)	²⁵³Es	(1/2)+
²⁵³Fm	100	153	253.085185(4)	3.00(12) d	α (12%)	²⁴⁹Cf	(1/2)+
²⁵⁴Fm	100	154	254.0868544(30)	3.240(2) h	α (99.94%)	²⁵⁰Cf	0+
²⁵⁴Fm	100	154	254.0868544(30)	3.240(2) h	SF (.0592%)	(various)	0+
²⁵⁵Fm	100	155	255.089964(5)	20.07(7) h	α	²⁵¹Cf	7/2+
²⁵⁵Fm	100	155	255.089964(5)	20.07(7) h	SF (2.4×10⁻⁵%)	(various)	7/2+
²⁵⁶Fm	100	156	256.091774(8)	157.6(13) min	SF (91.9%)	(various)	0+
²⁵⁶Fm	100	156	256.091774(8)	157.6(13) min	α (8.1%)	²⁵²Cf	0+
²⁵⁷Fm^{[n 2]}	100	157	257.095106(7)	100.5(2) d	α (99.79%)	²⁵³Cf	(9/2+)
²⁵⁷Fm^{[n 2]}	100	157	257.095106(7)	100.5(2) d	SF (.21%)	(various)	(9/2+)
²⁵⁸Fm	100	158	258.09708(22)#	370(14) µs	SF	(various)	0+
²⁵⁹Fm	100	159	259.1006(3)#	1.5(3) s	SF	(various)	3/2+#
²⁶⁰Fm^{[n 3]}^{[n 4]}	100	160	260.10281(55)#	4 ms	SF	(various)	0+

↑ Abbreviations:
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission
↑ Heaviest nuclide produced via neutron capture
↑ Discovery of this isotope is unconfirmed
↑ Not directly synthesized, occurs as decay product of ²⁶⁰Md

Notes

Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.
The isotopes denoted by an isotope number with an "m" are metastable isomers.

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)^[2]

²⁶⁰Fm? was not confirmed in 1997.

References

↑ "Universal Nuclide Chart". nucleonica. (Registration required (help)).
↑ see mendelevium

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.
- G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 2008-09-23.
Isotopic compositions and standard atomic masses from:
- J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
- M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary.
Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
- G. Audi; F. G. Kondev; M. Wang; B. Pfeiffer; X. Sun; J. Blachot; M. MacCormick (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.
- G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 2008-09-23.
- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. Check date values in: |accessdate= (help)
- N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.
- M. Thoennessen (2012). "Discovery of Isotopes of Elements with Z≥100". Atomic Data and Nuclear Data Tables. 99: 312–344. arXiv:1203.2105. Bibcode:2013ADNDT..99..312T. doi:10.1016/j.adt.2012.03.003.

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[2] Abbreviations:
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission

[3] Heaviest nuclide produced via neutron capture

[uc-4] Discovery of this isotope is unconfirmed

[5] Not directly synthesized, occurs as decay product of ²⁶⁰Md

[1] "Universal Nuclide Chart". nucleonica. (Registration required (help)).

[6] see mendelevium