Fermium Fermium is a synthetic chemical element; it has symbol Fm and atomic number 100. It is an actinide and the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macros ...

(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 The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, ...

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 Nuclear fallout is residual radioactive material that is created by the reactions producing a nuclear explosion. It is initially present in the radioactive cloud created by the explosion, and "falls out" of the cloud as it is moved by the ...

from

nuclear testing Nuclear weapons tests are experiments carried out to determine the performance of nuclear weapons and the effects of Nuclear explosion, their explosion. Nuclear testing is a sensitive political issue. Governments have often performed tests to si ...

) was ²⁵⁵Fm in 1952. ²⁵⁰Fm was independently synthesized shortly after the discovery of ²⁵⁵Fm. There are 20 known

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

s ranging in

atomic mass Atomic mass ( or ) is the mass of a single atom. The atomic mass mostly comes from the combined mass of the protons and neutrons in the nucleus, with minor contributions from the electrons and nuclear binding energy. The atomic mass of atoms, ...

from ²⁴¹Fm to ²⁶⁰Fm (²⁶⁰Fm is unconfirmed), and 4

nuclear isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states have Half-life, half-lives of ...

s, ^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 Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...

, 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