Fermium (Fm)

Fermium is the heaviest element that can be made by bombarding a target with neutrons in a reactor. Past it the chain stops: fermium-258 fissions spontaneously within about 370 microseconds of forming, so neutrons cannot build anything heavier and every element from mendelevium up has to be made by smashing nuclei together in an accelerator instead. That barrier is known as the fermium gap, and it is the reason element 101 onward were discovered one atom at a time.

Properties

Atomic number

100

Symbol

Fm

Mass number of most stable isotope

[257]

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f¹²

Electrons per shell

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

Oxidation states

+3, +2

Discovered

1952 by Albert Ghiorso and colleagues at Berkeley

Electron configuration

Full

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

Noble gas shorthand

[Rn] 7s² 5f¹²

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 30 · P: 8 · Q: 2

What it is used for

• Research only; no application outside the laboratory

Hazards

• Intensely radioactive; has only ever existed in quantities of at most a few million atoms

Melting point, boiling point, density and electronegativity are omitted. Published melting points disagree sharply — 1,800 K in some tables, 1,125 K in others — because both are extrapolations: fermium has never been prepared in a weighable quantity. The mass given is the mass number of fermium-257, the longest-lived at 100.5 days.

Several physical properties are not listed for fermium because no bulk sample has ever existed. Only a handful of atoms have been produced, often with half-lives measured in seconds, so published melting points and densities for elements in this range are calculations rather than measurements. They are omitted rather than presented as observed values.

Neighbours in the table