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Fm

Fermium

Fermium (Fm) is an actinide with atomic number 100. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 257. Its electron configuration is [Rn] 5f12 7s2.

Fermium key data

Symbol Fm
Atomic number 100
Mass number (longest-lived isotope) 257
Protons 100
Neutrons (longest-lived isotope, fermium-257) 157
Electrons (neutral atom) 100
Category Actinide
Period 7
Group f-block, no group
Block f-block
Noble gas notation [Rn] 5f12 7s2
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f12 6s2 6p6 7s2
Electrons per shell 2, 8, 18, 32, 30, 8, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Properties of fermium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Predicted melting point 1800 K (1526.9 °C), 28th of 103 by melting point
Predicted state at room temperature Solid
Predicted electronegativity 1.3 (Pauling), joint 61st of 95 by electronegativity
Predicted first ionisation energy 6.5 eV (627 kJ/mol), 61st of 102 by first ionisation energy
Common oxidation states +3
Discovered 1952

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Fermium electron configuration

The full electron configuration of fermium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f12 6s2 6p6 7s2. With the noble-gas core collapsed it is [Rn] 5f12 7s2, and the outermost subshell is 7s2.

Bohr model of fermium (Fm): a nucleus and seven shells holding 2, 8, 18, 32, 30, 8 and 2 electrons from the inside out.
Bohr model of fermium (Fm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of fermium (Fm), [Rn] 5f12 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing two unpaired electrons, in 5f.
Orbital diagram of fermium (Fm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one fermium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 257 / (6.02214076 × 10²³)
  2. = 4.2676 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 257
  2. = 2.3432 × 10²¹ atoms

Fermium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of fermium-257, so treat these as nominal.

Fermium on the periodic table

Fermium is in period 7, outside the numbered groups, in the f-block.

The rest of period 7

Work with fermium