Actinium Element Facts

The chemical element actinium is classed as an actinide metal. It was discovered in 1899 by Andre Debierne.

Actinium is radioactive. Image Ref.(1)
Actinium is radioactive.

Data Zone

Classification: Actinium is an actinide metal
Color: silvery-white
Atomic weight: (227), no stable isotopes
State: solid
Melting point: 1050 oC, 1323 K
Boiling point: 3199 oC, 3471 K
Electrons: 89
Protons: 89
Neutrons in most abundant isotope: 138
Electron shells: 2,8,18,32,18,9,2
Electron configuration: [Rn] 6d1 7s2
Density @ 20oC: 10.07 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 22.54 cm3/mol
Structure: fcc: face-centered cubic
Hardness: mohs
Specific heat capacity 27.2 J mol-1 K-1
Heat of fusion 14.2 kJ mol-1
Heat of atomization 406 kJ mol-1
Heat of vaporization 400 kJ mol-1
1st ionization energy 499 kJ mol-1
2nd ionization energy 1170 kJ mol-1
3rd ionization energy
Electron affinity
Minimum oxidation number 0
Min. common oxidation no. 0
Maximum oxidation number 3
Max. common oxidation no. 3
Electronegativity (Pauling Scale) 1.1
Polarizability volume 32.1 Å3
Reaction with air mild, w/ht, ⇒ Ac2O3
Reaction with 15 M HNO3 mild, ⇒ Ac(NO3)3
Reaction with 6 M HCl mild, ⇒ H2, AcCl3
Reaction with 6 M NaOH
Oxide(s) Ac2O3
Hydride(s) AcH2
Chloride(s) AcCl3
Atomic radius 195 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion) 112 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity
Electrical conductivity
Electrical conductivity
Freezing/Melting point: 1050 oC, 1323 K

Discovery of Actinium

The radioactive element actinium was discovered by Andre Debierne in 1899, in Paris, France.

Debierne worked with Marie Curie and discovered actinium in pitchblende (uranium ore) from which radium and polonium had already been extracted.

He was able to identify the presence of a new element because the radioactive emissions profile of his sample could not be explained by the presence of any known element.

Debierne was unable to isolate pure actinium from the thorium also present in pitchblende.

Appearance and Characteristics

Harmful effects:

highly radioactive


radioactive, glows in the dark

Uses of Actinium

neutron production

Abundance and Isotopes

Abundance earth’s crust: 0.0005 parts per trillion by weight

Abundance solar system: negligible

Cost, pure: $ per 100g

Cost, bulk: $ per 100g

Source: Source: Natural U-235 radioactive decay. A ton of pitcheblende contains about 0.15 milligrams of actinium. Nowadays, actinium is generally produced by the neutron irradiation of 226Ra.

Isotopes: Actinium has 29 isotopes. All are radioactive. 227Ac occurs naturally in uranium ore and, at 21.773 years, has the longest Ac half-life.


1. Photo by ORNL.

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