Periodic table / Actinoids

100Fermium

Fermium is an actinoid in period 7, in the actinoid series. It has only ever been made a few atoms at a time, so nobody has seen it as a solid, liquid or gas.

Shells: 2, 8, 18, 32, 30, 8, 2. A simple picture: electrons do not move in circles.

Symbol: Fm
Atomic number: 100 protons in the nucleus
Standard atomic weight: None. Fermium has no stable isotopes, so the mass of a sample depends on which isotopes it holds.
Electron configuration: [Rn] 5f¹² 7s²
Electrons in each shell: 2, 8, 18, 32, 30, 8, 2
Group, period, block: –, 7, f
Family: Actinoids
State at 25 °C: not known
Electronegativity: 1.3 (Pauling scale)
First ionisation energy: 627.2 kJ/mol (6.5 eV) (estimated by NIST)

Where these numbers come from

Atomic weight: IUPAC CIAAW, abridged standard atomic weights 2024. Electron configuration and ionisation energy: NIST Atomic Spectra Database. Family, state and electronegativity: PubChem. See Sources. Melting points, boiling points and densities are left out for now: published values differ between sources, and we will add them once each has been checked.