Fermium (₁₀₀Fm) is a

synthetic element A synthetic element is a known chemical element that does not occur naturally on Earth: it has been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; thus, it i ...

, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no

stable isotope Stable nuclides are Isotope, isotopes of a chemical element whose Nucleon, nucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. The Atomic nucleus, nuclei of such isotopes are no ...

s. The first

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

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 Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of 100.5 days, and the longest-lived isomer is ^247mFm with a half-life of 5.1 seconds.

List of isotopes

, - , rowspan=3, ²⁴¹Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 141 , rowspan=3, 241.07431(32)# , rowspan=3, 730(60) μs , SF? , (various) , rowspan=3, 5/2+# , - , α (<14%) , ²³⁷Cf , - , β⁺ (<12%) , ²⁴¹Es , - , rowspan=2, ²⁴²Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 142 , rowspan=2, 242.07343(43)# , rowspan=2, 0.8(2) ms , SF , (various) , rowspan=2, 0+ , - , α? , ²³⁸Cf , - , rowspan=3, ²⁴³Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 143 , rowspan=3, 243.07441(14)# , rowspan=3, 231(9) ms , α (91%) , ²³⁹Cf , rowspan=3, (7/2−) , - , SF (9%) , (various) , - , β⁺? , ²⁴³Es , - , rowspan=3, ²⁴⁴Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 144 , rowspan=3, 244.07404(22)# , rowspan=3, 3.12(8) ms , SF (>97%) , (various) , rowspan=3, 0+ , - , β⁺ (<2%) , ²⁴⁴Es , - , α (<1%) , ²⁴⁰Cf , - , rowspan=3, ²⁴⁵Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 145 , rowspan=3, 245.07535(21)# , rowspan=3, 4.2(13) s , α (88.5%) , ²⁴¹Cf , rowspan=3, (1/2+) , - , β⁺ (11.5%) , ²⁴⁵Es , - , SF (<0.3%) , (various) , - , rowspan=3, ²⁴⁶Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 146 , rowspan=3, 246.075350(17) , rowspan=3, 1.54(4) s , α (93.2%) , ²⁴²Cf , rowspan=3, 0+ , - , SF (6.8%) , (various) , - , EC (<1.3%) , ²⁴⁶Es , - , rowspan=2, ²⁴⁷Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 147 , rowspan=2, 247.07694(19)# , rowspan=2, 31(1) s , α (~64%) , ²⁴³Cf , rowspan=2, (7/2+) , - , β⁺? (~36%) , ²⁴⁷Es , -id=Fermium-247m , rowspan=3 style="text-indent:1em" , ^247mFm , rowspan=3 colspan="3" style="text-indent:2em" , 49(8) keV , rowspan=3, 5.1(2) s , α (88%) , ²⁴³Cf , rowspan=3, (1/2+) , - , IT? , ²⁴⁷Fm , - , β⁺? , ²⁴⁷Es , - , rowspan=3, ²⁴⁸Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 148 , rowspan=3, 248.0771855(91) , rowspan=3, 34.5(12) s , α (99.9%) , ²⁴⁴Cf , rowspan=3, 0+ , - , SF (0.1%) , (various) , - , β⁺? , ²⁴⁸Es , -id=Fermium-248m , rowspan=3 style="text-indent:1em" , ^248mFm , rowspan=3 colspan="3" style="text-indent:2em" , 1200(100)# keV , rowspan=3, 10.1(6) ms , IT? , ²⁴⁸Fm , rowspan=3, 6+# , - , α? , ²⁴⁴Cf , - , β⁺? , ²⁴⁸Es , - , rowspan=2, ²⁴⁹Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 149 , rowspan=2, 249.0789260(67) , rowspan=2, 1.6(1) min , β⁺? (67%) , ²⁴⁹Es , rowspan=2, 7/2+ , - , α (33%) , ²⁴⁵Cf , - , rowspan=3, ²⁵⁰Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 150 , rowspan=3, 250.0795198(85) , rowspan=3, 31.0(11) min , α (99.99%) , ²⁴⁶Cf , rowspan=3, 0+ , - , SF (6.9×10⁻³%) , (various) , - , EC? , ²⁵⁰Es , -id=Fermium-250m , rowspan=4 style="text-indent:1em" , ^250mFm , rowspan=4 colspan="3" style="text-indent:2em" , 1199.2(10) keV , rowspan=4, 1.92(5) s , IT , ²⁵⁰Fm , rowspan=4, (8−) , - , α? , ²⁴⁶Cf , - , β⁺? , ²⁵⁰Es , - , SF? , (various) , - , rowspan=2, ²⁵¹Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 151 , rowspan=2, 251.081545(15) , rowspan=2, 5.30(8) h , β⁺ (98.20%) , ²⁵¹Es , rowspan=2, 9/2− , - , α (1.80%) , ²⁴⁷Cf , -id=Fermium-251m , style="text-indent:1em" , ^251mFm , colspan="3" style="text-indent:2em" , 200.0(1) keV , 21.8(8) μs , IT , ²⁵¹Fm , 5/2+ , - , rowspan=2, ²⁵²Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 152 , rowspan=2, 252.0824660(56) , rowspan=2, 25.39(4) h , α (99.99%) , ²⁴⁸Cf , rowspan=2, 0+ , - , SF (.0023%) , (various) , - , rowspan=2, ²⁵³Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 153 , rowspan=2, 253.0851809(16) , rowspan=2, 3.00(12) d , EC (88%) , ²⁵³Es , rowspan=2, 1/2+ , - , α (12%) , ²⁴⁹Cf , -id=Fermium-253m , style="text-indent:1em" , ^253mFm , colspan="3" style="text-indent:2em" , 351(6) keV , 0.56(6) μs , IT , ²⁵³Fm , 11/2-# , - , rowspan=2, ²⁵⁴Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 154 , rowspan=2, 254.0868524(20) , rowspan=2, 3.240(2) h , α (99.94%) , ²⁵⁰Cf , rowspan=2, 0+ , - , SF (0.0592%) , (various) , - , rowspan=2, ²⁵⁵Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 155 , rowspan=2, 255.0899635(42) , rowspan=2, 20.07(7) h , α , ²⁵¹Cf , rowspan=2, 7/2+ , - , SF (2.4×10⁻⁵%) , (various) , - , rowspan=2, ²⁵⁶Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 156 , rowspan=2, 256.0917717(32) , rowspan=2, 157.1(13) min , SF (91.9%) , (various) , rowspan=2, 0+ , - , α (8.1%) , ²⁵²Cf , - , rowspan=2, ²⁵⁷FmHeaviest nuclide produced via neutron capture , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 157 , rowspan=2, 257.0951054(47) , rowspan=2, 100.5(2) d , α (99.79%) , ²⁵³Cf , rowspan=2, 9/2+ , - , SF (.210%) , (various) , - , rowspan=2, ²⁵⁸Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 158 , rowspan=2, 258.09708(22)# , rowspan=2, 370(14) μs , SF , (various) , rowspan=2, 0+ , - , α? , ²⁵⁴Cf , -id=Fermium-259 , ²⁵⁹Fm , style="text-align:right" , 100 , style="text-align:right" , 159 , 259.10060(30)# , 1.5(2) s , SF , (various) , 3/2+# , -id=Fermium-260 , ²⁶⁰FmDiscovery of this isotope is unconfirmedNot directly synthesized, occurs as decay product of ²⁶⁰Md , style="text-align:right" , 100 , style="text-align:right" , 160 , 260.10281(47)# , 1# min , SF , (various) , 0+

Chronology of isotope discovery

²⁶⁰Fm was not confirmed in 1997.

References

* Isotope masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** ** ** {{Navbox element isotopes Fermium Fermium