Fermium (Fm)
Element 100 · Actinide · atomic mass [257] u · f-block, period 7
Electron shells: 2, 8, 18, 32, 30, 8, 2
Fermium was also discovered in hydrogen-bomb debris and has never been produced in weighable amounts. It marks the practical limit of what neutron capture in a reactor can build.
Fermium is the heaviest element that can be made by neutron bombardment — past it you need particle accelerators.
- Atomic number
- 100
- Atomic mass
- [257] u
- Electron configuration
- [Rn] 5f¹² 7s²
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +3
- Electronegativity
- 1.3 (Pauling)
- Melting point
- 1,527 °C
- Boiling point
- —
- Density
- Not measured
- State at 20 °C
- Solid
- Appearance
- Unknown; presumed metallic
- Discovered
- Albert Ghiorso and colleagues, 1952
100 protons, and 100 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Fermium is used for
- Scientific research only
Atomic structure of Fermium
A neutral fermium atom has 100 protons and 100 electrons. Its most common isotope carries about 157 neutrons, which is where the mass number 257 comes from. Change the neutron count and you have a different isotope of fermium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 30, 8, 2. In orbital notation that is [Rn] 5f¹² 7s². As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Fermium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Fermium sits in the periodic table
The second f-block row, elements 89-103. All are radioactive, and everything past uranium is essentially man-made.
- One proton lighter: Einsteinium.
- One proton heavier: Mendelevium.
Discovery and naming
Fermium was discovered by Albert Ghiorso and colleagues in 1952. Its name comes from Enrico Fermi, which is also why its symbol (Fm) does not match its English name.
Every isotope of fermium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Fermium?
Fermium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 257 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Fermium have?
Fermium has 100 protons, because the atomic number is the proton count and that is what makes an atom Fermium at all. A neutral Fermium atom has the same number of electrons, 100. The neutron count varies by isotope; the most common one has about 157, since 257 − 100 = 157.
What is the electron configuration of Fermium?
The electron configuration of Fermium is [Rn] 5f¹² 7s². Its electrons are arranged 2, 8, 18, 32, 30, 8, 2 across 7 shells.
Is Fermium a metal?
Yes. Fermium is classified as a actinide, on the metallic side of the table.
What group and period is Fermium in?
Fermium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Fermium?
Fermium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Fermium?
Fermium melts at 1,527 °C. That puts it in the solids at everyday temperatures.
What is Fermium used for?
Scientific research only. Fermium was also discovered in hydrogen-bomb debris and has never been produced in weighable amounts.
Who discovered Fermium, and where does its name come from?
Fermium (Fm) was discovered by Albert Ghiorso and colleagues in 1952. The name comes from Enrico Fermi.
Is Fermium radioactive?
Yes. Every isotope of Fermium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.