Fermium Fermium is a synthetic element with the 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 macroscopic qua ...

(₁₀₀Fm) is a

synthetic element A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; ...

, 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, is ...

cannot be given. Like all artificial elements, it has no

stable isotope The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s. The first

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

to be discovered (in

fallout Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...

from

nuclear testing Nuclear weapons tests are experiments carried out to determine nuclear weapons' effectiveness, yield, and explosive capability. Testing nuclear weapons offers practical information about how the weapons function, how detonations are affected by ...

) 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 nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s ranging in

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nbs ...

from ²⁴¹Fm to ²⁶⁰Fm (²⁶⁰Fm is unconfirmed), and 2

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, higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited ...

s, ^250mFm and ^251mFm. The longest-lived isotope is ²⁵⁷Fm with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 100.5 days, and the longest-lived isomer is ^250mFm with a half-life of 1.92 seconds.

List of isotopes

, - , rowspan=2, ²⁴¹Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 241.07421(32)# , rowspan=2, 730(60) μs , SF(>78%) , (various) , rowspan=2, 5/2#+ , - , α (<14%) , ²³⁷Cf , - , 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 (rare) , ²³⁸Cf , - , rowspan=3, ²⁴³Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 143 , rowspan=3, 243.07447(23)# , rowspan=3, 231(9) ms , α (91%) , ²³⁹Cf , rowspan=3, 7/2−# , - , SF (9%) , (various) , - , β⁺ (rare) , ²⁴³Es , - , rowspan=2, ²⁴⁴Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 144 , rowspan=2, 244.07404(22)# , rowspan=2, 3.12(8) ms , SF (99%) , (various) , rowspan=2, 0+ , - , 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 (95.7%) , ²⁴¹Cf , rowspan=3, 1/2+# , - , β⁺ (4.2%) , ²⁴⁵Es , - , SF (.13%) , (various) , - , rowspan=4, ²⁴⁶Fm , rowspan=4 style="text-align:right" , 100 , rowspan=4 style="text-align:right" , 146 , rowspan=4, 246.075350(17) , rowspan=4, 1.54(4) s , α (85%) , ²⁴²Cf , rowspan=4, 0+ , - , β⁺ (10%) , ²⁴⁶Es , - , β⁺, SF (10%) , (various) , - , SF (4.5%) , (various) , - , rowspan=2, ²⁴⁷Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 147 , rowspan=2, 247.07695(12)# , rowspan=2, 31(1) s , α (>50%) , ²⁴³Cf , rowspan=2, (7/2+) , - , β⁺ (<50%) , ²⁴⁷Es , - , rowspan=3, ²⁴⁸Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 148 , rowspan=3, 248.077186(9) , rowspan=3, 35.1(8) s , α (93%) , ²⁴⁴Cf , rowspan=3, 0+ , - , β⁺ (7%) , ²⁴⁸Es , - , SF (.10%) , (various) , - , rowspan=2, ²⁴⁹Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 149 , rowspan=2, 249.078928(7) , rowspan=2, 1.6(1) min , β⁺ (85%) , ²⁴⁹Es , rowspan=2, (7/2+)# , - , α (15%) , ²⁴⁵Cf , - , rowspan=3, ²⁵⁰Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 150 , rowspan=3, 250.079521(9) , rowspan=3, 30.4(15) min , α (90%) , ²⁴⁶Cf , rowspan=3, 0+ , - , EC (10%) , ²⁵⁰Es , - , SF (6.9×10⁻³%) , (various) , - , style="text-indent:1em" , ^250mFm , colspan="3" style="text-indent:2em" , 1199.2(10) keV , 1.92(5) s , IT , ²⁵⁰Fm , (8−) , - , rowspan=2, ²⁵¹Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 151 , rowspan=2, 251.081540(16) , rowspan=2, 5.30(8) h , β⁺ (98.2%) , ²⁵¹Es , rowspan=2, (9/2−) , - , α (1.8%) , ²⁴⁷Cf , - , style="text-indent:1em" , ^251mFm , colspan="3" style="text-indent:2em" , 200.09(11) keV , 21.1(16) μs , , , (5/2+) , - , rowspan=3, ²⁵²Fm , rowspan=3 style="text-align:right" , 100 , rowspan=3 style="text-align:right" , 152 , rowspan=3, 252.082467(6) , rowspan=3, 25.39(4) h , α (99.99%) , ²⁴⁸Cf , rowspan=3, 0+ , - , SF (.0023%) , (various) , - , β⁺β⁺ (rare) , ²⁵²Cf , - , rowspan=2, ²⁵³Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 153 , rowspan=2, 253.085185(4) , rowspan=2, 3.00(12) d , EC (88%) , ²⁵³Es , rowspan=2, (1/2)+ , - , α (12%) , ²⁴⁹Cf , - , rowspan=2, ²⁵⁴Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 154 , rowspan=2, 254.0868544(30) , rowspan=2, 3.240(2) h , α (99.94%) , ²⁵⁰Cf , rowspan=2, 0+ , - , SF (.0592%) , (various) , - , rowspan=2, ²⁵⁵Fm , rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 155 , rowspan=2, 255.089964(5) , 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, 256.091774(8) , rowspan=2, 157.6(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, ...

, rowspan=2 style="text-align:right" , 100 , rowspan=2 style="text-align:right" , 157 , rowspan=2, 257.095106(7) , rowspan=2, 100.5(2) d , α (99.79%) , ²⁵³Cf , rowspan=2, (9/2+) , - , SF (.21%) , (various) , - , ²⁵⁸Fm , style="text-align:right" , 100 , style="text-align:right" , 158 , 258.09708(22)# , 370(14) μs , SF , (various) , 0+ , - , ²⁵⁹Fm , style="text-align:right" , 100 , style="text-align:right" , 159 , 259.1006(3)# , 1.5(3) s , SF , (various) , 3/2+# , - , 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(55)# , 4 ms , SF , (various) , 0+

Chronology of isotope discovery

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. ** ** ** ** ** {{Navbox element isotopes Fermium