Fermium – facts and properties
- Symbol
- Fm
- Atomic number
- 100
- Atomic mass
- 257 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f¹² 7s²
- Electronegativity
- 1.3
- Valence electrons
- 14
- State at standard conditions
- solid
- Melting point
- 1527 °C
- Discovered
- 1952
Fermium is a radioactive element that can only be made artificially, with atomic number 100. It was identified in 1952, together with einsteinium, in the fallout of "Ivy Mike" and named after the physicist Enrico Fermi. Fermium is the heaviest element that can still be produced by neutron capture. Nobody has ever isolated a weighable amount, and the element is known only from traces.
Position in the periodic table
Occurrence and extraction
Fermium does not occur in nature. It was discovered in the same samples as einsteinium, in the fallout of the Ivy Mike test in November 1952. In filter papers and coral material the team around Albert Ghiorso in Berkeley found the isotope fermium-255, whose half-life is about twenty hours. It had formed from uranium through a long chain of neutron captures and beta decays. The discovery stayed secret at first and was published in 1955.
The name honours Enrico Fermi, who put the first nuclear reactor into operation in Chicago in 1942 and died in 1954. Fermi had studied early on what happens when atomic nuclei are bombarded with neutrons, so the name suits the way the element forms.
Today fermium is produced in high-flux reactors such as the one at Oak Ridge. Curium and californium are irradiated with neutrons for a long time, and through many neutron captures and beta decays einsteinium and finally fermium result. The amounts are minute. The longest-lived isotope, fermium-257, has a half-life of a little over a hundred days.
Properties and reactions
Fermium is in period 7 and is the twelfth actinoid, in the actinoid row between einsteinium on its left and mendelevium on its right. Erbium sits above it in the lanthanoid row. The configuration of the atom's electrons is [Rn] 5f¹² 7s². The electronegativity of 1.3 and the melting point of 1527 °C are estimates, since no sample of the metal has ever been measured. Its atomic mass is given as 257 u, the mass number of the longest-lived isotope, fermium-257.
Its chemistry is known almost entirely from experiments with traces, in which behaviour in an ion-exchange column is followed. In solution plus three is the rule. Divalent fermium can also be produced with suitable reducing agents. That fits the trend that the divalent state becomes ever more stable towards the end of the series.
Many fermium isotopes decay by spontaneous fission, in which the nucleus splits in two without any outside trigger. Fermium-256 does so predominantly, whereas fermium-257 decays mainly by alpha radiation. Specialists work with it in shielded facilities. Everyday health risks do not arise, because the element exists only as traces in specialist laboratories.
Research and synthesis
Fermium finds no use outside research. Scientifically, fermium matters because a limit becomes visible with it, the fermium gap. Fermium-258 undergoes spontaneous fission with a half-life of only about 370 microseconds. There is no time left to capture another neutron, and no beta decay to the next element occurs. The chain of neutron capture therefore ends at fermium. Heavier elements can only be made in accelerators, by bringing atomic nuclei to fuse.
Research uses fermium to understand the structure of the heaviest nuclei. In 2024 at GSI in Darmstadt, laser light was used to measure single fermium atoms. From the spectrum the researchers worked out how the size of the nucleus changes from isotope to isotope. Such data test computer models of nuclei that are hard to study directly.
Fermium is also used to test methods that will later be needed for even heavier and shorter-lived elements.
Quick question
What is the atomic number of fermium (Fm)?
FAQ
Who is fermium named after?
It is named after the physicist Enrico Fermi, who started the first nuclear reactor. He died in 1954, shortly before the discovery was published and the name became known.
What is the fermium gap?
The isotope fermium-258 undergoes spontaneous fission after a fraction of a millisecond. So the series of elements that neutron capture can build ends at fermium, and heavier elements must be made another way.
Does fermium have a practical use?
It is used only in basic research. With tiny amounts one studies the chemistry and the nuclear properties of the heaviest actinoids.