Fermium

Fermium
Atomic number	100
Symbol	Fm
Group	Actinides
Electron configuration	[Rn] 5f12 7s2
Discovery	1952 (Ivy Mike fallout), Ghiorso et al.
Period	7
Main isotopes	257Fm, 255Fm, 253Fm
Phase STP	Solid
Block	f
Oxidation states	+2, +3
Wikidata	Q1896

Fermium (symbol Fm, atomic number 100) is a synthetic, highly radioactive metallic element in the actinide series It lies in period 7 of the periodic table (f-block); it is expected to be a silvery metal and is solid at room temperature In practice, fermium exists only in trace amounts from nuclear reactions – none of its isotopes are stable, so any primordial fermium has long since decayed away The most stable isotope, ^257Fm, has a half-life of about 100.5 days Because of this and its scarcity, fermium has no significant natural abundance and occurs only as a tiny laboratory or fallout product Its chemistry is dominated by the +3 oxidation state (as in other late actinides), with a +2 state also accessible No biological role is known for fermium, and it is extremely radiotoxic Due to these factors, it has no practical uses outside specialized scientific research.

Atomic Structure and Electron Configuration

The fermium atom has 100 protons and, when neutral, 100 electrons. Its ground-state electron configuration is [Rn]5f^12 7s^2 This means it has two 7s electrons (the valence electrons) and a nearly filled 5f subshell. The distribution of electrons among shells is 2, 8, 18, 32, 30, 8, 2 reflecting the fully filled inner shells up to radon plus the actinide electrons. The atomic radius (nonbonded) of fermium is about 2.45 Å consistent with the actinide contraction (a gradual size decrease for heavier actinides). Its covalent (single-bond) radius is about 1.67 Å The first ionization energy is ≈627 kJ/mol indicating the energy required to remove one electron; successive ionization energies are much higher. No reliable electronegativity value is reported for fermium (the Pauling electronegativity is effectively unknown). In general, fermium’s properties continue the trends of the heavy actinides: it is large and electropositive, with its chemistry mainly determined by the 5f and 7s electrons.

Isotopes and Nuclear Properties

All known isotopes of fermium are radioactive. In total about 19 fermium isotopes (mass numbers 241–259) have been characterized The most stable, ^257Fm, decays by α-emission to ^253Cf with a half-life of roughly 100.5 days Other noteworthy isotopes include ^253Fm (t1/2 ≈3 days), ^255Fm (≈20 hours), ^254Fm (≈3.2 hours), and ^256Fm (≈2.6 hours) Shorter-lived isotopes decay in minutes or less (e.g., ^251Fm has t1/2 ≈ 5.3 h). Heavy fermium isotopes beyond mass 257 are essentially instantly unstable: for example, ^258Fm undergoes spontaneous fission with a half-life ≈370 μs In fact, all isotopes heavier than ^257Fm decay almost immediately by fission (e.g. ^259Fm in ~1.5 s, ^260Fm in ~4 ms so neutron-capture processes in reactors cannot build elements beyond Z = 100. Consistently, no fermium isotope decays by β^− to element 101 (mendelevium) – the heaviest actinide accessible by neutron capture is fermium This so-called “fermium gap” means heavier transuranics must be made by charged-particle nuclear reactions rather than slow neutron addition.

Nuclear spins have been measured for some fermium isotopes. For example, ^257Fm has spin 9/2, ^255Fm has spin 7/2, and ^253Fm has spin 1/2 reflecting the alignment of unpaired nucleons. Because all fermium isotopes are short-lived and occur only in minuscule amounts, there is no practical use of fermium in dating or tracing natural processes: its radioisotopes serve only as tracers or markers in laboratory research.

Allotropes and Typical Compounds

No allotropes of fermium are known, since no macroscopic metallic sample has ever been isolated. Chemically, fermium behaves like the other late actinides (and like the heavy lanthanides) in that it forms mainly trivalent compounds. In practice it has been studied only in tracer quantities, so compounds are inferred by analogy. Fermium typically forms +3 salts: for example, in aqueous solution Fm^3+ would be analogous to Yb^3+ or Cf^3+. (Actually, experiments have shown Fm(III) can be reduced to Fm(II) by strong reductants indicating that Fm^2+ is chemically accessible.) Known or expected compounds include fermium(III) oxide (Fm2O3) and dioxide (FmO2), halides like FmF3 and FmCl3, and salts like Fm(NO3)3, all prepared in minute amounts. Trivalent Fm salts are typically soluble only in strongly acidic media; for instance, Fm^3+ would precipitate as Fm(OH)3 or carbonate at high pH. Reductive chemistry yields Fm(II) compounds such as FmCl2 (obtained by reducing FmCl3) Complexation chemistry (with EDTA-like ligands or crown ethers) is presumed similar to that of lanthanides and other actinides, but detailed studies are lacking. In summary, the only known fermium compounds are ionic salts, all characterized spectroscopically or radiochemically on a tracer scale; no covalent or molecular compounds have been isolated in bulk.

Physical Properties

Solid fermium metal (if produced) would be a heavy, dense metal. It is solid at standard conditions The melting point is estimated at about 1527 °C (1800 K) Its boiling point has not been measured (expect it to exceed 3000 °C, but data are lacking). The crystal structure of fermium metal has not been observed, but by analogy to nearby actinides it is plausibly face-centered cubic. No reliable density measurement exists; it is expected to be very high (on the order of 15–20 g/cm³ or more) due to the large atomic weight. Because only microgram samples have ever been made, most bulk properties (hardness, conductivity, thermal expansion) are unknown. We expect fermium metal to conduct heat and electricity like other metals, and to be paramagnetic (from its unpaired 5f electrons). Spectroscopically, atomic lines for fermium are essentially uncharacterized; atomic absorption or emission lines are only known for a few transitions in isolated ions. In practical terms, known physical data come from theory or by analogy. For example, tables list a nonbonded atomic radius of 2.45 Å and a covalent radius of 1.67 Å The first ionization energy (627 kJ/mol) ties well with predictions for a heavy actinide metal

Chemical Reactivity and Trends

Fermium’s chemical behavior follows the trends of the late actinides. In aqueous solution, Fm^3+ is a small, highly charged cation (similar to the trivalent lanthanides) and thus acts as a strong Lewis acid. It will hydrolyze or precipitate at high pH (for example, Fm^3+ + 3 OH^– → Fm(OH)3↓). The redox chemistry of Fm is dominated by the Fm^3+⇌Fm^2+ couple. The standard potential for Fm^3+ + e^– → Fm^2+ is about –1.15 V (vs. SHE) comparable to ytterbium’s (meaning Fm^3+ can be fairly readily reduced to Fm^2+). Indeed, experiments confirm that potent reductants (e.g. Sm^2+) can convert Fm^3+ to Fm^2+ The Fm^2+/Fm^0 potential is roughly –2.37 V implying that if pure fermium metal could be isolated it would be a very strong reducing agent (analogous to Eu or Yb metal). In acid or complexing media, Fm(III) forms stable salts and complexes just like its lanthanide analogs: one expects, for instance, Fm(NO3)3 to be soluble and Fm2(CO3)3 to precipitate in carbonate solution. Fm^2+ salts (like FmCl2) would be more soluble or more reducing.

Practically no detailed reactivity data exist beyond these analogies. One can infer that metallic Fm would rapidly corrode (tarnish) in air to form oxides or hydroxides (similar to europium or americium metal). Its aqueous salts show the usual trends: Fm^3+ is roughly as hard to oxidize or reduce as its neighbors, and its complex-formation constants resemble those of other UF block elements. In summary, fermium’s chemistry is qualitatively like a combination of heavy lanthanide and actinide behavior: generally trivalent, mildly reducible to M(II), and forming coordination complexes and insoluble hydroxides in a typical lanthanide/actinide manner

Occurrence and Production

Fermium has essentially no natural occurrence today. All of its isotopes decay within months, so any primordial fermium (from Earth’s formation) disappeared long ago The only place transuranic elements like fermium were once present naturally is the Oklo natural reactor (~2 billion years ago), but fermium itself is no longer found in nature. In the cosmos, fermium would only be briefly made in extreme processes (supernovae or neutron-star mergers), if at all, and would decay quickly.

All known fermium is artificial, produced on Earth. The main method is multiple neutron capture on heavy actinides in high-flux reactors For instance, curium and californium targets in the Oak Ridge HFIR reactor are bombarded with neutrons over long times. In such an irradiation, atoms absorb neutrons stepwise, creating ^254Cm, ^255Cm, etc., which eventually decay to fermium isotopes. The Oak Ridge HFIR (85 MW) is the primary current source of fermium Even so, the yields are extremely small: after irradiating tens of grams of curium, one gets only picograms (10^–12 g) of ^257Fm Lesser isotopes (^254Fm, ^255Fm) can be made similarly, but they have shorter half-lives.

Thermonuclear explosions also produce fermium: the enormous neutron flux can create fermium isotopes from uranium. For example, in the 1969 U.S. “Hutch” thermonuclear test, about 250 micrograms of ^257Fm were eventually recovered from debris However, collecting and isolating that quantity is extremely difficult. (No practical extraction from bombs exists beyond a few isolated measurements.) After production, fermium must be chemically separated from other actinides and fission products. This is done by specialized radiochemistry—typically ion-exchange chromatography using highly selective resins with α-hydroxyisobutyrate eluents—so that Fm^3+ can be isolated in tracer amounts No terrestrial ores or natural deposits of fermium exist; it is obtained only on the “laboratory scale” during reactor operations or from filtered explosion debris.

Applications and Technology

Fermium has no commercial or practical uses beyond scientific research Its applications are limited to fundamental studies of nuclear physics and actinide chemistry, where minute amounts are needed. For example, modern experiments have used fermium isotopes to probe nuclear structure. In 2024, an international team generated picogram samples of ^255Fm and ^257Fm (from Oak Ridge and Grenoble reactors) and performed laser spectroscopy to measure nuclear charge radii, testing shell-model predictions for very heavy nuclei Such measurements may help define where the periodic table ends.

Fermium can also be used as a tracer in chemistry: for instance, preparing a known amount of Fm to study how it behaves in complexation or separation processes. It was once used as a target to discover heavier elements: the Berkeley group produced mendelevium (Z=101) by bombarding ^253Es, and subsequent discoveries of nobelium/lawrencium similarly involved trans-targets in this region (though nuclei beyond Fm typically require accelerator beams). Nevertheless, no routine technological or medicinal applications exist. There are no alloys, catalysts, or devices involving fermium. In short, it is strictly a laboratory curiosity: as one source summarizes, “fermium has no uses outside research”

Biology, Environment, and Safety

Because fermium is entirely artificial and highly radioactive, it has no role in biology or the environment. It is not an essential element for any organism If encountered, its hazard comes purely from radioactivity. All fermium isotopes emit energetic α-particles (and some spontaneous fission fragments). Internally, α-emitters are extremely damaging to tissue; externally, they are stopped by even a sheet of paper or skin. In practical terms, any fermium released (e.g. in nuclear fallout) would pose only a short-term local hazard and then disappear by decay.

In laboratory handling, fermium is treated with extreme caution. Its radiation levels are comparable to those of other transuranics (like plutonium or americium). There are no established exposure limits specific to fermium, but by analogy any exposure would be regulated under standard radiological safety limits. Handling is done in dedicated hot cells or glove boxes with shielding; strict controls prevent ingestion or inhalation. The radiotoxicity of fermium (per unit mass) is very high, so even microgram amounts must be strictly contained. In summary: fermium is biologically inert (no uptake by living systems aside from contamination) but highly radiotoxic, requiring full heavy-metal and alpha-emitter precautions

History and Etymology

Fermium was the first element created in a hydrogen-bomb explosion. It was discovered in 1952 from the debris of the “Ivy Mike” test (the first U.S. thermonuclear bomb) on 1 November 1952 Analyzing the fallout, Albert Ghiorso’s team at the University of California, Berkeley, identified a short-lived new isotope (later recognized as ^255Fm) The result was initially classified for security reasons and kept secret until 1955. Independently, in 1954 a group at the Nobel Institute in Stockholm bombarded uranium with oxygen ions and produced fermium-250 Ultimately, the Berkeley team’s claim was accepted and they chose the name fermium (Fm) in honor of Italian-American physicist Enrico Fermi (1901–1954) The suffix “-ium” reflects its metallic character. Thus, fermium was named for a pioneer of nuclear science.

Key milestones: early work also produced elements 99 (einsteinium) as neighbors in the test debris, and by 1954 the Berkeley group produced Fm by reactor bombardment of plutonium Tracer studies then measured its properties. However, attempts to go beyond Fm by neutron capture quickly ran into the short half-lives (“fermium gap”), so heavier actinides had to be made by particle accelerators. No folklore or prior usage existed (it was entirely man-made); its “place in history” is as the heaviest element first observed in a nuclear explosion and the last actinide identified by simple neutron capture methods

Data Table

Property	Value / Comment	Source(s)
Name	Fermium	–
Symbol	Fm	–
Atomic number (Z)	100
Atomic weight	[257] (≈257 u, from longest-lived isotope)
Element category	Actinide (transuranic)
Period	7
Block	f
Electron configuration	[Rn] 5f^12 7s^2
Electron shells	2, 8, 18, 32, 30, 8, 2
Valence electrons	2 (the 7s electrons)	–
Common oxidation states	+3 (dominant) +2 also possible
Phase at STP	Solid (radioactive metal)
Crystal structure	(Not observed, predicted metal lattice, likely fcc)	–
Atomic (nonbonded) radius	~2.45 Å
Covalent radius	~1.67 Å
First ionization energy	627 kJ/mol (estimated)
Electronegativity (Pauling)	unknown/estimated ~1.3	– (no experimental data)
Melting point	1527 °C (1800 K)
Boiling point	Unknown (not measured)
Density	Not measured (expected very high)	–
Longest-lived isotope	^257Fm (t½ = 100.5 days)
Decay modes	α-decay (to ^253Cf), spontaneous fission (heavy isotopes)	–
Named after	Enrico Fermi (physicist, 1901–1954)
Discovery	1952 (Ivy Mike thermonuclear test, Berkeley)
CAS registry number	7440-72-4	–