Fermium-255

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

Nuclides with atomic number of 100 but with different mass numbers

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 4 nuclear isomers, ^247mFm, ^250mFm, ^251mFm, and ^253mFm. The longest-lived isotope is ²⁵⁷Fm with a half-life of 100.5 days, and the longest-lived isomer is ^247mFm with a half-life of 5.1 seconds.

Quick Facts Main isotopes, Decay ...

List of isotopes

More information Nuclide, Z ...

Nuclide ^{[n 1]}	Z	N	Isotopic mass (Da) ^{[n 2]}^{[n 3]}	Half-life	Decay mode ^{[n 4]}	Daughter isotope	Spin and parity ^{[n 5]}^{[n 6]}
Nuclide ^{[n 1]}	Excitation energy			Half-life	Decay mode ^{[n 4]}	Daughter isotope	Spin and parity ^{[n 5]}^{[n 6]}
²⁴¹Fm	100	141	241.07421(32)#	730(60) μs	SF	(various)	5/2+#
²⁴¹Fm	100	141	241.07421(32)#	730(60) μs	α (<14%)	²³⁷Cf	5/2+#
²⁴²Fm	100	142	242.07343(43)#	<4 μs^[2]	SF	(various)	0+
²⁴³Fm^[3]	100	143	243.07447(23)#	231(9) ms	α (91%)	²³⁹Cf	(7/2−)
					SF (9%)	(various)
					β⁺ (<10%)	²⁴³Es
²⁴⁴Fm^[2]	100	144	244.07404(22)#	3.12(8) ms	SF	(various)	0+
²⁴⁴Fm^[2]	100	144	244.07404(22)#	3.12(8) ms	α (<1%)	²⁴⁰Cf	0+
²⁴⁵Fm^[4]	100	145	245.07535(21)#	5.5(7) s	α (88.5%)	²⁴¹Cf	(1/2+)
					β⁺ (11.5%)	²⁴⁵Es
					SF (<0.3%)^[3]	(various)
²⁴⁶Fm^[5]	100	146	246.075350(17)	1.54(4) s	α (93.2%)	²⁴²Cf	0+
					SF (6.8%)	(various)
					β⁺ (<1.3%)	²⁴⁶Es
²⁴⁷Fm^[6]	100	147	247.07695(12)#	31(1) s	α (64%)	²⁴³Cf	(7/2+)
²⁴⁷Fm^[6]	100	147	247.07695(12)#	31(1) s	β⁺ (36%)	²⁴⁷Es	(7/2+)
^247mFm	45(7) keV			5.1(2) s	α (88%)	²⁴³Cf	(1/2+)
					IT (12%)	²⁴⁷Fm
					β⁺?	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⁺ (84.4%)	²⁴⁹Es	(7/2+)
²⁴⁹Fm	100	149	249.078928(7)	1.6(1) min	α (15.6%)^[7]	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	IT	²⁵¹Fm	(5/2+)
²⁵²Fm	100	152	252.082467(6)	25.39(4) h	α (99.99%)^{[n 7]}	²⁴⁸Cf	0+
²⁵²Fm	100	152	252.082467(6)	25.39(4) h	SF (.0023%)	(various)	0+
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)+
^253mFm^[8]	~351 keV			0.56(6) μs	IT	²⁵³Fm	(11/2-)
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 8]}	100	157	257.095106(7)	100.5(2) d	α (99.79%)	²⁵³Cf	(9/2+)
²⁵⁷Fm^{[n 8]}	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 9]}^{[n 10]}	100	160	260.10281(55)#	4 ms	SF	(various)	0+
This table header & footer: view

[n 1]
^mFm – Excited nuclear isomer.
[n 2]
( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
[n 3]
# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
[n 4]
Modes of decay:

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission
[n 5]
( ) spin value – Indicates spin with weak assignment arguments.
[n 6]
# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
[n 7]
Theoretically capable of β⁺β⁺ decay to ²⁵²Cf^[1]
[n 8]
Heaviest nuclide produced via neutron capture
[n 9]
Discovery of this isotope is unconfirmed
[N 10]
Not directly synthesized, occurs as decay product of ²⁶⁰Md

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[2] [n 1]
^mFm – Excited nuclear isomer.

[3] [n 2]
( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-4] [n 3]
# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[5] [n 4]
Modes of decay:

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission

[6] [n 5]
( ) spin value – Indicates spin with weak assignment arguments.

[TNN-7] [n 6]
# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

[14] [n 7]
Theoretically capable of β⁺β⁺ decay to ²⁵²Cf^[1]

[16] [n 8]
Heaviest nuclide produced via neutron capture

[uc-17] [n 9]
Discovery of this isotope is unconfirmed

[18] [N 10]
Not directly synthesized, occurs as decay product of ²⁶⁰Md

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[:1-8] [2]
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[:0-9] [3]
Khuyagbaatar, J.; Heßberger, F. P.; Hofmann, S.; Ackermann, D.; Burkhard, H. G.; Heinz, S.; Kindler, B.; Kojouharov, I.; Lommel, B.; Mann, R.; Maurer, J.; Nishio, K. (12 October 2020). "α decay of Fm 243 143 and Fm 245 145 , and of their daughter nuclei". Physical Review C. 102 (4): 044312. doi:10.1103/PhysRevC.102.044312. ISSN 2469-9985. S2CID 241259726. Retrieved 24 June 2023.

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[11] [5]
Venhart, M.; Heßberger, F. P.; Ackermann, D.; Antalic, S.; Gray-Jones, C.; Greenlees, P. T.; Heinz, S.; Herzberg, R. -D.; Hofmann, S.; Ketelhut, S.; Kindler, B.; Kojouharov, I.; Leino, M.; Lommel, B.; Mann, R.; Papadakis, P.; Rostron, D.; Rudolph, D.; Šáro, Š.; Sulignano, B. (4 February 2011). "Decay study of 246Fm at SHIP" (PDF). The European Physical Journal A. 47 (2): 20. Bibcode:2011EPJA...47...20V. doi:10.1140/epja/i2011-11020-9. ISSN 1434-601X. S2CID 119353521. Retrieved 24 June 2023.

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Antalic, S.; Heßberger, F. P.; Ackermann, D.; Heinz, S.; Hofmann, S.; Kalaninová, Z.; Kindler, B.; Khuyagbaatar, J.; Kojouharov, I.; Kuusiniemi, P.; Leino, M.; Lommel, B.; Mann, R.; Nishio, K.; Šáro, Š.; Streicher, B.; Sulignano, B.; Venhart, M. (20 May 2011). "Isomeric states in 253No and 253Fm" (PDF). The European Physical Journal A. 47 (5): 62. Bibcode:2011EPJA...47...62A. doi:10.1140/epja/i2011-11062-y. ISSN 1434-601X. S2CID 121080273. Retrieved 25 June 2023.

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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 of ²⁶⁰Md)

Fermium-255

Isotopes of fermium

List of isotopes

Chronology of isotope discovery

References

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