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 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 numb ...
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) 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&nb ...
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 higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ...
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 at ...
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,
257Fm
[Heaviest 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+#
, -
,
260Fm
[Discovery of this isotope is unconfirmed][Not 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
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 ...