Periodic table / Actinoids
89Actinium
Actinium is an actinoid in period 7, in the actinoid series. At 25 °C it is a solid.
- Symbol
- Ac
- Atomic number
- 89 protons in the nucleus
- Standard atomic weight
- None. Actinium has no stable isotopes, so the mass of a sample depends on which isotopes it holds.
- Electron configuration
- [Rn] 6d1 7s2
- Electrons in each shell
- 2, 8, 18, 32, 18, 9, 2
- Group, period, block
- –, 7, f
- Family
- Actinoids
- State at 25 °C
- solid
- Electronegativity
- 1.1 (Pauling scale)
- First ionisation energy
- 519.1 kJ/mol (5.380235 eV)
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.