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Isotopes of fermium

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Main isotopes^[1]			Decay
Isotope	abundance	half-life(t_1/2)	mode	product
²⁵²Fm	synth	25.39 h	α	²⁴⁸Cf
²⁵²Fm	synth	25.39 h	SF	–
²⁵³Fm	synth	3 d	ε	²⁵³Es
²⁵³Fm	synth	3 d	α	²⁴⁹Cf
²⁵⁵Fm	synth	20.07 h	α	²⁵¹Cf
²⁵⁵Fm	synth	20.07 h	SF	–
²⁵⁷Fm	synth	100.5 d	α	²⁵³Cf
²⁵⁷Fm	synth	100.5 d	SF	–

Fermium (₁₀₀Fm) is asynthetic element, and thus astandard atomic weight cannot be given. Like all artificial elements, it has nostable isotopes. The firstisotope to be discovered (innuclear fallout from theIvy Mike H-bomb test) was²⁵⁵Fm in 1952.²⁵⁰Fm was independently synthesized (to establish priority if the former result had to remain classified) shortly after the discovery of²⁵⁵Fm.

There are 20 knownradioisotopes ranging inatomic mass from²⁴¹Fm to²⁶⁰Fm (²⁶⁰Fm is unconfirmed), and 5nuclear isomers. The longest-lived isotope is²⁵⁷Fm with ahalf-life of 100.5 days, and the longest-lived isomer is^247mFm with a half-life of 5.1 seconds.

List of isotopes

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Nuclide ^{[n 1]}	Z	N	Isotopic mass(Da)^[2] ^{[n 2]}^{[n 3]}	Half-life^[1]	Decay mode^[1] ^{[n 4]}	Daughter isotope	Spin and parity^[1] ^{[n 5]}^{[n 6]}
Nuclide ^{[n 1]}	Excitation energy			Half-life^[1]	Decay mode^[1] ^{[n 4]}	Daughter isotope	Spin and parity^[1] ^{[n 5]}^{[n 6]}
²⁴¹Fm	100	141	241.07431(32)#	730(60) μs	SF	(various)	5/2+#
					α (<14%)	²³⁷Cf
					β⁺ (<12%)	²⁴¹Es
²⁴²Fm	100	142	242.07343(43)#	0.8(2) ms	SF	(various)	0+
²⁴³Fm	100	143	243.07441(14)#	231(9) ms	α (91%)	²³⁹Cf	7/2−#
²⁴³Fm	100	143	243.07441(14)#	231(9) ms	SF (9%)	(various)	7/2−#
²⁴⁴Fm	100	144	244.07404(22)#	3.12(8) ms	SF (>97%)	(various)	0+
					β⁺ (<2%)	²⁴⁴Es
					α (<1%)	²⁴⁰Cf
²⁴⁵Fm	100	145	245.07535(21)#	4.2(13) s	α (88.5%)	²⁴¹Cf	1/2+#
					β⁺ (11.5%)^[3]	²⁴⁵Es
					SF (<0.3%)	(various)
²⁴⁶Fm	100	146	246.075353(15)	1.54(4) s	α (93.2%)	²⁴²Cf	0+
					SF (6.8%)	(various)
					EC (<1.3%)	²⁴⁶Es
²⁴⁷Fm	100	147	247.07694(19)#	31(1) s	α (~64%)	²⁴³Cf	(7/2+)
²⁴⁷Fm	100	147	247.07694(19)#	31(1) s	β⁺? (~36%)	²⁴⁷Es	(7/2+)
^247mFm	49(8) keV			5.1(2) s	α (88%)	²⁴³Cf	(1/2+)
^247mFm	49(8) keV			5.1(2) s	IT? (12%)	²⁴⁷Fm	(1/2+)
²⁴⁸Fm	100	148	248.077185(9)	34.5(12) s	α (99.9%)	²⁴⁴Cf	0+
²⁴⁸Fm	100	148	248.077185(9)	34.5(12) s	SF (0.1%)	(various)	0+
^248mFm	1200(100)# keV			10.1(6) ms	IT?	²⁴⁸Fm	6+#
					α?	²⁴⁴Cf
					β⁺?	²⁴⁸Es
²⁴⁹Fm	100	149	249.078926(7)	1.6(1) min	β⁺? (67%)	²⁴⁹Es	7/2+
²⁴⁹Fm	100	149	249.078926(7)	1.6(1) min	α (33%)	²⁴⁵Cf	7/2+
²⁵⁰Fm	100	150	250.079520(8)	31.0(11) min	α (99.99%)	²⁴⁶Cf	0+
²⁵⁰Fm	100	150	250.079520(8)	31.0(11) min	SF (6.9×10⁻³%)	(various)	0+
^250mFm	1199.2(10) keV			1.92(5) s	IT	²⁵⁰Fm	(8−)
²⁵¹Fm	100	151	251.081545(15)	5.30(8) h	β⁺ (98.20%)	²⁵¹Es	9/2−
²⁵¹Fm	100	151	251.081545(15)	5.30(8) h	α (1.80%)	²⁴⁷Cf	9/2−
^251mFm	200.0(1) keV			21.8(8) μs	IT	²⁵¹Fm	5/2+
²⁵²Fm	100	152	252.082466(6)	25.39(4) h	α (99.99%)^{[n 7]}	²⁴⁸Cf	0+
²⁵²Fm	100	152	252.082466(6)	25.39(4) h	SF (0.0023%)	(various)	0+
²⁵³Fm	100	153	253.0851809(17)	3.00(12) d	EC (88%)	²⁵³Es	1/2+
²⁵³Fm	100	153	253.0851809(17)	3.00(12) d	α (12%)	²⁴⁹Cf	1/2+
^253mFm	351(6) keV			0.56(6) μs	IT	²⁵³Fm	11/2−#
²⁵⁴Fm	100	154	254.0868524(20)	3.240(2) h	α (99.94%)	²⁵⁰Cf	0+
²⁵⁴Fm	100	154	254.0868524(20)	3.240(2) h	SF (0.0592%)	(various)	0+
²⁵⁵Fm	100	155	255.089963(4)	20.07(7) h	α	²⁵¹Cf	7/2+
²⁵⁵Fm	100	155	255.089963(4)	20.07(7) h	SF (2.4×10⁻⁵%)	(various)	7/2+
²⁵⁶Fm	100	156	256.091772(3)	157.1(13) min	SF (91.9%)	(various)	0+
²⁵⁶Fm	100	156	256.091772(3)	157.1(13) min	α (8.1%)	²⁵²Cf	0+
²⁵⁷Fm^{[n 8]}	100	157	257.095105(5)	100.5(2) d	α (99.79%)	²⁵³Cf	9/2+
²⁵⁷Fm^{[n 8]}	100	157	257.095105(5)	100.5(2) d	SF (0.210%)	(various)	9/2+
²⁵⁸Fm	100	158	258.09708(22)#	370(14) μs	SF	(various)	0+
²⁵⁹Fm	100	159	259.10060(30)#	1.5(2) s	SF	(various)
²⁶⁰Fm^{[n 9]}^{[n 10]}	100	160	260.10281(47)#	1# min	SF	(various)	0+
This table header & footer: view

^^mFm – Excitednuclear isomer.
^( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
^# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
^Modes of decay:
EC: Electron capture

IT: Isomeric transition
SF: Spontaneous fission
^( ) spin value – Indicates spin with weak assignment arguments.
^# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
^Theoretically capable of β⁺β⁺ decay to²⁵²Cf
^Heaviest nuclide produced vianeutron capture
^Discovery of this isotope is unconfirmed
^Not directly synthesized, occurs as decay product of²⁶⁰Md

Chronology of isotope discovery

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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 of²⁶⁰Md)

²⁶⁰Fm was not confirmed in 1997.

References

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^^a ^b ^c ^dKondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021)."The NUBASE2020 evaluation of nuclear properties"(PDF).Chinese Physics C.45 (3) 030001.doi:10.1088/1674-1137/abddae.
^Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*".Chinese Physics C.45 (3) 030003.doi:10.1088/1674-1137/abddaf.
^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. (24 March 2022)."Study of the production and decay properties of neutron-deficient nobelium isotopes".The European Physical Journal A.58 (3): 52.arXiv:2203.15659.Bibcode:2022EPJA...58...52T.doi:10.1140/epja/s10050-022-00707-9.ISSN 1434-601X.S2CID 247720708. Retrieved24 June 2023.

- M. Thoennessen (2012). "Discovery of Isotopes of Elements with Z≥100".Atomic Data and Nuclear Data Tables.99 (3):312–344.arXiv:1203.2105.Bibcode:2013ADNDT..99..312T.doi:10.1016/j.adt.2012.03.003.S2CID 94982856.

Isotopes of thechemical elements

Group	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Period	Hydrogen and alkali metals	Alkaline earth metals													Pnictogens	Chalcogens	Halogens	Noble gases
①	Isotopes § List H 1																	Isotopes § List He 2
②	Isotopes § List Li 3	Isotopes § List Be 4											Isotopes § List B 5	Isotopes § List C 6	Isotopes § List N 7	Isotopes § List O 8	Isotopes § List F 9	Isotopes § List Ne 10
③	Isotopes § List Na 11	Isotopes § List Mg 12											Isotopes § List Al 13	Isotopes § List Si 14	Isotopes § List P 15	Isotopes § List S 16	Isotopes § List Cl 17	Isotopes § List Ar 18
④	Isotopes § List K 19	Isotopes § List Ca 20	Isotopes § List Sc 21	Isotopes § List Ti 22	Isotopes § List V 23	Isotopes § List Cr 24	Isotopes § List Mn 25	Isotopes § List Fe 26	Isotopes § List Co 27	Isotopes § List Ni 28	Isotopes § List Cu 29	Isotopes § List Zn 30	Isotopes § List Ga 31	Isotopes § List Ge 32	Isotopes § List As 33	Isotopes § List Se 34	Isotopes § List Br 35	Isotopes § List Kr 36
⑤	Isotopes § List Rb 37	Isotopes § List Sr 38	Isotopes § List Y 39	Isotopes § List Zr 40	Isotopes § List Nb 41	Isotopes § List Mo 42	Isotopes § List Tc 43	Isotopes § List Ru 44	Isotopes § List Rh 45	Isotopes § List Pd 46	Isotopes § List Ag 47	Isotopes § List Cd 48	Isotopes § List In 49	Isotopes § List Sn 50	Isotopes § List Sb 51	Isotopes § List Te 52	Isotopes § List I 53	Isotopes § List Xe 54
⑥	Isotopes § List Cs 55	Isotopes § List Ba 56	Isotopes § List Lu 71	Isotopes § List Hf 72	Isotopes § List Ta 73	Isotopes § List W 74	Isotopes § List Re 75	Isotopes § List Os 76	Isotopes § List Ir 77	Isotopes § List Pt 78	Isotopes § List Au 79	Isotopes § List Hg 80	Isotopes § List Tl 81	Isotopes § List Pb 82	Isotopes § List Bi 83	Isotopes § List Po 84	Isotopes § List At 85	Isotopes § List Rn 86
⑦	Isotopes § List Fr 87	Isotopes § List Ra 88	Isotopes § List Lr 103	Isotopes § List Rf 104	Isotopes § List Db 105	Isotopes § List Sg 106	Isotopes § List Bh 107	Isotopes § List Hs 108	Isotopes § List Mt 109	Isotopes § List Ds 110	Isotopes § List Rg 111	Isotopes § List Cn 112	Isotopes § List Nh 113	Isotopes § List Fl 114	Isotopes § List Mc 115	Isotopes § List Lv 116	Isotopes § List Ts 117	Isotopes § List Og 118
⑧	Isotopes § List Uue 119	Isotopes § List Ubn 120

			Isotopes § List La 57	Isotopes § List Ce 58	Isotopes § List Pr 59	Isotopes § List Nd 60	Isotopes § List Pm 61	Isotopes § List Sm 62	Isotopes § List Eu 63	Isotopes § List Gd 64	Isotopes § List Tb 65	Isotopes § List Dy 66	Isotopes § List Ho 67	Isotopes § List Er 68	Isotopes § List Tm 69	Isotopes § List Yb 70
			Isotopes § List Ac 89	Isotopes § List Th 90	Isotopes § List Pa 91	Isotopes § List U 92	Isotopes § List Np 93	Isotopes § List Pu 94	Isotopes § List Am 95	Isotopes § List Cm 96	Isotopes § List Bk 97	Isotopes § List Cf 98	Isotopes § List Es 99	Isotopes § List Fm 100	Isotopes § List Md 101	Isotopes § List No 102

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EC:	Electron capture
IT:	Isomeric transition
SF:	Spontaneous fission