Periodic Table


July 8, 2012, 4:39 pm
Content Cover Image

Some ores such as this North Carolina uranite have high concentrations of actinium. Source: Creative Commons

Previous Element: Radium

Next Element: Thorium


Physical Properties
Color silvery-white
Phase at Room Temp. solid
Density (g/cm3) 10.7g/cm3
Hardness (Mohs) ---
Melting Point (K) 1323.2
Boiling Point (K) 2743
Heat of Fusion (kJ/mol) 14.2
Heat of Vaporization (kJ/mol) 293
Heat of Atomization (kJ/mol) 385
Thermal Conductivity (J/m sec K) ---
Electrical Conductivity (1/mohm cm) ---
Source Synthetic (U-235 decay)
Atomic Properties
Electron Configuration [Rn]7s26d1
Number of Isotopes 35 (0 natural)
Electron Affinity (kJ/mol) ---
First Ionization Energy (kJ/mol) 499
Second Ionization Energy (kJ/mol) 1170
Third Ionization Energy (kJ/mol) ---
Electronegativity 1.1
Polarizability (Å3) 32.1
Atomic Weight 227.03
Atomic Volume (cm3/mol) ---
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) ---
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) 126
Common Oxidation Numbers +3
Other Oxid. Numbers ---
In Earth's Crust (mg/kg) 5.5x10-10
In Earth's Ocean (mg/L) ---
In Human Body (%) ---
Regulatory / Health
CAS Number 7440-34-8
OSHA Permissible Exposure Limit No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit No limits
Mineral Information Institute
Jefferson Accelerator Laboratory

Actinium is a radioactive, metallic element designated by the symbol Ac.  It has a soft texture and is silvery-white in appearance.

Actinium gets its name from the Greek word "aktis" or "aktinos" which means ray or beam.

Actinium is a rare earth element within the Actinides series, which includes all elements from actinium (atomic number 89) to lawrencium (atomic number 103).

caption Actinium. Photo Credit: Prof James Marshall (U. North Texas, USA), Source: National Research Council Canada

Actinium was independently discovered by French chemist André-Louis Debierne in 1899 and subsequently by Friedrich Otto Giesel in 1902. Debierne, who was not only a friend of  Marie Curie and Pierre Curie, but also worked with them, discovered actinium while working with uranium oxide or uraninite, which is a major uranium ore.


Trace amounts of actinium are naturally found in very low concentrations in uranium ores as 227-Ac, its most stable and abundant isotope.  It can also be synthetically produced by neutron irradiation of radon in a nuclear reactor.


27 radioisotopes have been identified. The three with the longest half-lives are:

  • 227-Ac with a half life of 21.773 years, emitting both alpha and beta rays.
  • 225-Ac with a half life of 10 days, and
  • 226-Ac with a half life of 29.37 hours. 

The remaining isotopes have much shorter half-lives ranging from less than 10 hours to less than a minute.

Electron shell configuration

2s2   2p6          
3s2   3p6   3d10      
4s2   4p6   4d10   4f14  
5s2   5p6   5d10      
6s2   6p6   6d1      


Currently actinium has it has no commercial or industrial applications. However, because it is extremely radioactive (150 times more radioactive than radium), it is used to produce neutrons.

Further reading

Actinium - Element Properties and Periodic Table Information



information), U. (2012). Actinium. Retrieved from


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