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Actinium (Ac)

Actinium is a chemical element of the periodic table with chemical symbol Ac and atomic number 89 with an atomic weight of 227 u and is classed as actinide and is part of group 3 (scandium group). Actinium is solid at room temperature.

Actinium in the periodic table

Atomic number89
Group3 (Scandium group)
Color Silver
Number of protons89 p+
Number of neutrons138 n0
Number of electrons89 e-
From Wikipedia, the free encyclopediaActinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.

Physical properties

Phase at STPSolid
Density10 g/cm3
Atomic weight227 u

Thermal properties

Melting point1500 K
1226.85 °C
2240.33 °F
Boiling point3500 K
3226.85 °C
5840.33 °F
Heat of vaporization400 kJ/mol

Atomic properties

Electronegativity (Pauling Scale)1.1
Electron affinity33.77 kJ/mol
Oxidation states+3
(a strongly basic oxide)
Ionization energies
  1. 499 kJ/mol
  2. 1170 kJ/mol
  3. 1900 kJ/mol
  4. 4700 kJ/mol

Electron configuration for actinium

Electron configuration
Shorthand configuration
[Rn] 6d1 7s2
Electron configuration
Full configuration
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2
Electron configuration chart
Electrons per shell2, 8, 18, 32, 18, 9, 2
Valence electrons 2
Valency electrons 3
Bohr model
ActiniumElectron shell for Actinium, created by Injosoft ABAc
Figure: Shell diagram of Actinium (Ac) atom.
Orbital Diagram

The history of Actinium

DiscoveryFriedrich Oskar Giesel (1902)
First isolationFriedrich Oskar Giesel (1903)
Named byAndré-Louis Debierne (1899)
Discovery of actinium
André-Louis Debierne, a French chemist, announced the discovery of a new element in 1899. He separated it from pitchblende residues left by Marie and Pierre Curie after they had extracted radium. In 1899, Debierne described the substance as similar to titanium and (in 1900) as similar to thorium. Friedrich Oskar Giesel found in 1902 a substance similar to lanthanum and called it "emanium" in 1904. After a comparison of the substances' half-lives determined by Debierne, Harriet Brooks in 1904, and Otto Hahn and Otto Sackur in 1905, Debierne's chosen name for the new element was retained because it had seniority, despite the contradicting chemical properties he claimed for the element at different times. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be isolated. The name actinium originates from the Ancient Greek aktis, aktinos (ακτίς, ακτίνος), meaning beam or ray. Its symbol Ac is also used in abbreviations of other compounds that have nothing to do with actinium, such as acetyl, acetate and sometimes acetaldehyde.


List of unique identifiers for Actinium in various chemical registry databases
CAS Number7440-34-8
ChemSpider ID22404
EC number-
PubChem CID Number23965