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)	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	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	100	160	260.10281(55)#	4 ms			0+

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)

²⁶⁰Fm? was not confirmed in 1997.

References

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Isotopes of einsteinium	Isotopes of fermium	Isotopes of mendelevium
Table of nuclides