Isotopes of fermium

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 2 nuclear isomers, 250mFm and 251mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 250mFm with a half-life of 1.8 seconds.

Main isotopes of fermium (100Fm)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
252Fm syn 25.39 h SF
α 248Cf
253Fm syn 3 d ε 253Es
α 249Cf
255Fm syn 20.07 h SF
α 251Cf
257Fm syn 100.5 d α 253Cf

List of isotopes

Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[1][n 1]
spin and
excitation energy
241Fm 100 141 241.07421(32)# 730(60) µs SF(>78%) (various) 5/2#+
α (<14%) 237Cf
242Fm 100 142 242.07343(43)# 0.8(2) ms SF (various) 0+
α (rare) 238Cf
243Fm 100 143 243.07447(23)# 231(9) ms α (91%) 239Cf 7/2−#
SF (9%) (various)
β+ (rare) 243Es
244Fm 100 144 244.07404(22)# 3.12(8) ms SF (99%) (various) 0+
α (1%) 240Cf
245Fm 100 145 245.07535(21)# 4.2(13) s α (95.7%) 241Cf 1/2+#
β+ (4.2%) 245Es
SF (.13%) (various)
246Fm 100 146 246.075350(17) 1.54(4) s α (85%) 242Cf 0+
β+ (10%) 246Es
β+, SF (10%) (various)
SF (4.5%) (various)
247Fm 100 147 247.07695(12)# 31(1) s α (>50%) 243Cf (7/2+)
β+ (<50%) 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+ (85%) 249Es (7/2+)#
α (15%) 245Cf
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−)
α (1.8%) 247Cf
251mFm 200.09(11) keV 21.1(16) µs (5/2+)
252Fm 100 152 252.082467(6) 25.39(4) h α (99.99%) 248Cf 0+
SF (.0023%) (various)
β+β+ (rare) 252Cf
253Fm 100 153 253.085185(4) 3.00(12) d EC (88%) 253Es (1/2)+
α (12%) 249Cf
254Fm 100 154 254.0868544(30) 3.240(2) h α (99.94%) 250Cf 0+
SF (.0592%) (various)
255Fm 100 155 255.089964(5) 20.07(7) h α 251Cf 7/2+
SF (2.4×10−5%) (various)
256Fm 100 156 256.091774(8) 157.6(13) min SF (91.9%) (various) 0+
α (8.1%) 252Cf
257Fm[n 2] 100 157 257.095106(7) 100.5(2) d α (99.79%) 253Cf (9/2+)
SF (.21%) (various)
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 3][n 4] 100 160 260.10281(55)# 4 ms SF (various) 0+
  1. ^ Abbreviations:
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. ^ Heaviest nuclide produced via neutron capture
  3. ^ Discovery of this isotope is unconfirmed
  4. ^ Not directly synthesized, occurs as decay product of 260Md


  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.
  • The isotopes denoted by an isotope number with an "m" are metastable isomers.

Chronology of isotope discovery

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)[2]

260Fm? was not confirmed in 1997.


  1. ^ "Universal Nuclide Chart". nucleonica. (Registration required (help)).
  2. ^ see mendelevium

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, but IUPAC acknowledges its inclusion based on common usage.The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any 5f electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).


Bohrium is a synthetic chemical element with symbol Bh and atomic number 107. It is named after Danish physicist Niels Bohr. As a synthetic element, it can be created in a laboratory but is not found in nature. It is radioactive: its most stable known isotope, 270Bh, has a half-life of approximately 61 seconds, though the unconfirmed 278Bh may have a longer half-life of about 690 seconds.

In the periodic table of the elements, it is a d-block transactinide element. It is a member of the 7th period and belongs to the group 7 elements as the fifth member of the 6d series of transition metals. Chemistry experiments have confirmed that bohrium behaves as the heavier homologue to rhenium in group 7. The chemical properties of bohrium are characterized only partly, but they compare well with the chemistry of the other group 7 elements.


Fermium is a synthetic element with 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 quantities, although pure fermium metal has not yet been prepared. A total of 19 isotopes are known, with 257Fm being the longest-lived with a half-life of 100.5 days.

It was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Enrico Fermi, one of the pioneers of nuclear physics. Its chemistry is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

Georgy Flyorov

Georgy Nikolayevich Flyorov (Russian: Гео́ргий Никола́евич Флёров, IPA: [gʲɪˈorgʲɪj nʲɪkɐˈlajɪvʲɪtɕ ˈflʲɵrəf]; 2 March 1913 – 19 November 1990) was a Soviet nuclear physicist who is known for his discovery of spontaneous fission and his contribution towards the physics of thermal reactions. In addition, he is also known for his letter directed to Joseph Stalin, during the midst of World War II, to start the atomic bomb project in the Soviet Union.

In 2012, element 114 was named flerovium after the research laboratory at the Joint Institute for Nuclear Research bearing his name.


Mendelevium is a synthetic element with chemical symbol Md (formerly Mv) and atomic number 101. A metallic radioactive transuranic element in the actinide series, it is the first element that currently cannot be produced in macroscopic quantities through neutron bombardment of lighter elements. It is the third-to-last actinide and the ninth transuranic element. It can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of sixteen mendelevium isotopes are known, the most stable being 258Md with a half-life of 51 days; nevertheless, the shorter-lived 256Md (half-life 1.17 hours) is most commonly used in chemistry because it can be produced on a larger scale.

Mendelevium was discovered by bombarding einsteinium with alpha particles in 1955, the same method still used to produce it today. It was named after Dmitri Mendeleev, father of the periodic table of the chemical elements. Using available microgram quantities of the isotope einsteinium-253, over a million mendelevium atoms may be produced each hour. The chemistry of mendelevium is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced mendelevium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

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