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Actinium(III) oxide is a chemical compound containing the rare radioactive element actinium. It has the formula Ac2O3. It is similar to its corresponding lanthanum compound, lanthanum(III) oxide, and contains actinium in the oxidation state +3.[1][2] Actinium oxide is not to be confused with Ac2O (acetic anhydride), where Ac is an abbreviation for acetyl instead of the symbol of the element actinium.

Actinium(III) oxide
Actinium sesquioxide.jpg
Unit cell, ball and stick model of actinium(III) oxide
IUPAC name
Actinium(III) oxide
Systematic IUPAC name
Actinium(3+) oxide
Other names
Actinium sesquioxide
3D model (JSmol)
ECHA InfoCard 100.031.275
EC Number 234-417-6
Molar mass 502.00 g·mol−1
Appearance white
Trigonal, hP5
P-3m1, No. 164
Related compounds
Other cations
Scandium(III) oxide
Yttrium(III) oxide
Lanthanum(III) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references


  • Ac2O3 + 6HF → 2AcF3 + 3H2O
  • Ac2O3 + 6HCl → 2AcCl3 + 3H2O
  • 4Ac(NO3)3 → 2Ac2O3 + 12NO2 + 3O2
  • 4Ac + 3O2 → 2Ac2O3
  • Ac2O3 + 2AlBr3 → 2AcBr3 + Al2O3
  • 2Ac(OH)3 → Ac2O3 + 3H2O
  • Ac2(C2O4)3 → Ac2O3 + 3CO2 + 3CO
  • Ac2O3 + H2S → Ac2S3 + 3H22


  1. ^ Actinium, Great Soviet Encyclopedia (in Russian)
  2. ^ Sherman, Fried; Hagemann, French; Zachariasen, W. H. (1950). "The Preparation and Identification of Some Pure Actinium Compounds". Journal of the American Chemical Society. 72 (2): 771–775. doi:10.1021/ja01158a034.