Isotopes of fermium

Fermium (Fm) is an artificial 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 19 known radioisotopes ranging in atomic mass from ²⁴²Fm to ²⁶⁰Fm, 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
nuclide symbol	excitation energy			half-life	decay mode(s)^[1]^{[n 1]}	daughter isotope(s)	nuclear spin
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	243.07435(23)#	210(60) ms [0.18(+8-4) s]	α (60%)	²³⁹Cf	7/2-#
					β⁺ (40%)	²⁴³Es
					SF (.57%)	(various)
²⁴⁴Fm	100	144	244.07408(30)#	3.3(5) ms	SF (99%)	(various)	0+
²⁴⁴Fm	100	144	244.07408(30)#	3.3(5) ms	α (1%)	²⁴⁰Cf	0+
²⁴⁵Fm	100	145	245.07539(30)#	4.2(13) s	α (95.7%)	²⁴¹Cf	1/2+#
					β⁺ (4.2%)	²⁴⁵Es
					SF (.13%)	(various)
²⁴⁶Fm	100	146	246.07530(4)	1.1(2) s	α (85%)	²⁴²Cf	0+
					β⁺ (10%)	²⁴⁶Es
					β⁺, SF (10%)	(various)
					SF (4.5%)	(various)
²⁴⁷Fm	100	147	247.07685(15)#	29(1) s	α (50%)	²⁴³Cf	(7/2+)
²⁴⁷Fm	100	147	247.07685(15)#	29(1) s	β⁺ (50%)	²⁴⁷Es	(7/2+)
²⁴⁸Fm	100	148	248.077195(13)	36(2) s	α (99%)	²⁴⁴Cf	0+
					β⁺ (1%)	²⁴⁸Es
					SF (.05%)	(various)
²⁴⁹Fm	100	149	249.07903(11)#	2.6(7) min	β⁺ (85%)	²⁴⁹Es	(7/2+)#
²⁴⁹Fm	100	149	249.07903(11)#	2.6(7) min	α (15%)	²⁴⁵Cf	(7/2+)#
²⁵⁰Fm	100	150	250.079521(13)	30(3) min	α (90%)	²⁴⁶Cf	0+
					EC (10%)	²⁵⁰Es
					SF (6×10⁻⁴%)	(various)
^250mFm	1500(300)# keV			1.8(1) s	IT	²⁵⁰Fm	7,8#
²⁵¹Fm	100	151	251.081575(9)	5.30(8) h	β⁺ (98.2%)	²⁵¹Es	(9/2-)
²⁵¹Fm	100	151	251.081575(9)	5.30(8) h	α (1.8%)	²⁴⁷Cf	(9/2-)
^251mFm	191(2) keV			15.2(23) 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	254.0868542(30)	3.240(2) h	α (99.94%)	²⁵⁰Cf	0+
²⁵⁴Fm	100	154	254.0868542(30)	3.240(2) h	SF (.06%)	(various)	0+
²⁵⁵Fm	100	155	255.089962(5)	20.07(7) h	α	²⁵¹Cf	7/2+
²⁵⁵Fm	100	155	255.089962(5)	20.07(7) h	SF (2×10⁻⁵%)	(various)	7/2+
²⁵⁶Fm	100	156	256.091773(8)	157.6(13) min	SF (91.9%)	(various)	0+
²⁵⁶Fm	100	156	256.091773(8)	157.6(13) min	α (8.1%)	²⁵²Cf	0+
²⁵⁷Fm^{[n 2]}	100	157	257.095105(7)	100.5(2) d	α (99.79%)	²⁵³Cf	(9/2+)
²⁵⁷Fm^{[n 2]}	100	157	257.095105(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	100	160	260.10268(54)#	1# min			0+

^ Abbreviations:
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission
^ Heaviest nuclide produced via neutron capture

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.

References

^ http://www.nucleonica.net/unc.aspx

Isotope masses from:
- G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf.
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 and 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. http://www.iupac.org/publications/pac/75/6/0683/pdf/.
- 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. http://iupac.org/publications/pac/78/11/2051/pdf/. Lay summary.
Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
- G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf.
- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. Retrieved September 2005.
- 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 978-0849304859.

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