Fermium

1 · Identity

Symbol, number

Fm, 100

Oxidation states in water

+3 (book entry); never measured in natural water

Note

Identity from the book entry; nothing measured in water.

Sources

Identity

Name and symbol

Fermium, Fm

Atomic number

100 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-72-4

Atomic structure

Atomic mass

257 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹²
[Rn] 5f¹² 7s²

Electrons per shell

2, 8, 18, 32, 30, 8, 2

Valence electrons

14 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,800 K (1,526.85 °C)

Boiling point

not in sources

Density

not in sources

Appearance

not in sources

Thermal conductivity

not in sources

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

face-centered cubic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+3

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

6.5 eV
1st 627 kJ/mol

Electron affinity

not in sources

Atomic radius

not in sources

Ionic radius

not in sources

Reactivity

Studied only in tracer amounts; the metal has never been prepared and no solid compound isolated. Its solution chemistry is that of a late trivalent actinide, Fm3+, with an accessible +2 state reachable with moderately strong reductants such as samarium(II) chloride.

with water

Not known for the metal; in water the Fm3+ ion is the stable species, with an acid dissociation constant of about 1.6 x 10^-4 (pKa 3.8).

with oxygen, air

Not known; no fermium metal or oxide has been prepared.

with acids

In acid solution exists as Fm3+, which forms chloride and nitrate complexes more stable than those of einsteinium or californium and coprecipitates with rare-earth fluorides and hydroxides.

with halogens

Not known; fermium(II) chloride has been detected only as a trace coprecipitate with samarium(II) chloride, never isolated.

Typical compounds

not in sources

Occurrence, production and use

Crustal abundance

Not Applicable

Oceanic abundance

Not Applicable

Occurrence and sources

• synthetic only microgram quantities from neutron bombardment of plutonium

Extraction, production

not in sources

Uses

Due to the small amounts produced and its short half-life, there are currently no uses for fermium outside of basic scientific research.

Owing to the minute amounts of fermium produced and all of its isotopes having relatively short half-lives, there are currently no uses for it outside of basic scientific research that expands knowledge of the rest of the periodic table.

Safety, toxicity

not in sources

Discovery and name

Discovered by

Lawrence Berkeley National Laboratory

Discovered

1953

First isolated

not in sources

Named by

not in sources

Origin of the name

after Enrico Fermi

Fermium does not occur naturally in the Earth’s crust. It was first identified in December 1952 by American scientists from the Argonne National Laboratory near Chicago, Illinois, the Los Alamos National Laboratory in Los Alamos, New Mexico, and The University of California Laboratory in Berkeley, California in the debris of thermonuclear weapons (Fig. IUPAC.100.1). The element was named for Enrico Fermi, who built the first man-made nuclear reactor. 255Fm (with a half-life of 20 h) was the first fermium isotope identified. Fermium is the heaviest element that can be formed by neutron bombardment of lighter elements and is thus the heaviest element that can be synthesized in macroscopic quantities [632], [633].

Fermium is of interest in particle physics research, but it has no commercial applications. 253Fm was one of the decay products used to confirm synthesis of copernicium in a particle accelerator experiment [634].

Fermium is the heaviest synthetic element that can be formed by neutron bombardment of lighter elements, and hence the heaviest element that can be prepared in macroscopic quantities. The chemical properties of fermium have been studied solely using tracer amounts and innovative experimental techniques are required. Fermium metal has not been prepared, however measurements have been made on fermium alloys with rare earth metals and a number of predictions have been made. It was deduced that fermium metal prefers a divalent state but with modest compression can form a trivalent state. Other measurements on mixed fermium alloys and compounds include the magnetic moment, inner-shell binding energies, x-ray energies, sublimation enthalpy, etc.

The chemistry of fermium is typical of the late actinides, with a dominance of the +3 oxidation state but also a tendency toward an accessible +2 oxidation state. In the solid state no pure fermium compounds have been prepared, however Fm(III) has been studied by co-crystallization techniques as a trace component in a rare earth matrix with the same charge. Fermium co-precipitates with rare earth fluorides and hydroxides. In aqueous solution, fermium exists in solution as the Fm3+ ion, which has a hydration number of 16.9 and an acid dissociation constant of 1.6 × 10-4 (pKa = 3.8). Fm3+ forms complexes with a wide variety of organic ligands with hard donor atoms such as oxygen, and these complexes are usually more stable than those of the lighter actinides. It also forms complexes with ligands such as chloride or nitrate and, again, these complexes appear to be more stable than those formed by einsteinium or californium. Bonding in the heavier actinides is mostly ionic in character and the ionic radius of the Fm3+ ion is smaller than the preceding An3+ ions because of the actinide contraction. This is the result of a higher effective nuclear charge of fermium, and thus fermium forms shorter and stronger metal, ligand bonds. In the heavier actinides there is an increasing tendency to form a divalent ion that emerges at einsteinium. Fm3+ can be readily reduced to stable Fm2+ using moderately strong reducing agents such as samarium(II) chloride. In aqueous media, the Fm(III)/Fm(III) redox couple has been investigated via radio-electrochemistry and other techniques. The electrode potentials have been estimated to be similar to that of the ytterbium redox couple. The redox potentials for the various fermium couples have been measured and/or estimated by various workers: Fm3+ → Fm2+ (- 1.15 V); Fm2+ → Fm0 (-2.37 V), all versus the Normal Hydrogen Electrode.