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General properties
Name, symbol, number actinium, Ac, 89
Pronunciation /ækˈtɪniəm/
Element category actinide
Category notes sometimes considered a transition metal
Group, period, block n/a7, f
Standard atomic weight (227) g·mol−1
Electron configuration [Rn] 6d1 7s2
Electrons per shell 2, 8, 18, 32, 18, 9, 2 (Image)
Physical properties
Phase solid
Density (near r.t.) 10 g·cm−3
Melting point (circa) 1323 K, 1050 °C, 1922 °F
Boiling point 3471 K, 3198 °C, 5788 °F
Heat of fusion 14 kJ·mol−1
Heat of vaporization 400 kJ·mol−1
Specific heat capacity (25 °C) 27.2 J·mol−1·K−1
Atomic properties
Oxidation states 3
(neutral oxide)
Electronegativity 1.1 (Pauling scale)
Ionization energies 1st: 499 kJ·mol−1
2nd: 1170 kJ·mol−1
Covalent radius 215 pm
Crystal structure face-centered cubic
Magnetic ordering no data
Thermal conductivity (300 K) 12 W·m−1·K−1
CAS registry number 7440-34-8
Most stable isotopes
Main article: Isotopes of actinium
iso NA half-life DM DE (MeV) DP
225Ac trace 10 d α 5.935 221Fr
226Ac syn 29.37 h β 1.117 226Th
ε 0.640 226Ra
α 5.536 222Fr
227Ac trace 21.772 y β 0.045 227Th
α 5.042 223Fr

Actinium is chemical element 89 on the periodic table. Its symbol is Ac. Actinium's mass is 227 g/mol.

Actinium is a silver radioactive, solid metal. It is so radioactive that it glows in the dark. Even a small amount of actinium is dangerous to people.

Actinium was discovered in 1899 by André-Louis Debierne, a French chemist. In 1899, Debierne described the substance as similar to titanium[1] and (in 1900) as similar to thorium.[2]


  1. Debierne, André-Louis (1899). "Sur un nouvelle matière radio-active" (in French). Comptes rendus 129: 593–595.
  2. Debierne, André-Louis (1900–1901). "Sur un nouvelle matière radio-actif – l'actinium" (in French). Comptes rendus 130: 906–908.