Isotopes of fermium

Fermium (Fm) is a synthetic element, and thus a standard atomic mass 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.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[1][n 1]	daughter isotope(s)	nuclear spin
	excitation energy
^241Fm	100	141	241.07421(32)#	730(60) µs	SF(>78%)	(various)	5/2#+
					α (<14%)	^237Cf
^242Fm	100	142	242.07343(43)#	0.8(2) ms	SF	(various)	0+
					α (rare)	^238Cf
^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+
					α (1%)	^240Cf
^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+)
					β^+ (<50%)	^247Es
^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+)#
					α (15%)	^245Cf
^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−)
					α (1.8%)	^247Cf
^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)+
					α (12%)	^249Cf
^254Fm	100	154	254.0868544(30)	3.240(2) h	α (99.94%)	^250Cf	0+
					SF (.0592%)	(various)
^255Fm	100	155	255.089964(5)	20.07(7) h	α	^251Cf	7/2+
					SF (2.4×10^−5%)	(various)
^256Fm	100	156	256.091774(8)	157.6(13) min	SF (91.9%)	(various)	0+
					α (8.1%)	^252Cf
^257Fm[n 2]	100	157	257.095106(7)	100.5(2) d	α (99.79%)	^253Cf	(9/2+)
					SF (.21%)	(various)
^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+

1. Abbreviations:
   EC: Electron capture
   IT: Isomeric transition
   SF: Spontaneous fission
2. Heaviest nuclide produced via neutron capture
3. Discovery of this isotope is unconfirmed
4. 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

1. "Universal Nuclide Chart". nucleonica. (registration required (help)).
2. 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.
• 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. doi:10.1088/1674-1137/36/12/001.
  • 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.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. Check date values in: |access-date= (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". arXiv:1203.2105.