Actinium Facts - Element 89 or Ac

Actinium Properties, Uses, and Sources

Actinium is a radioactive actinide element.
Actinium is a radioactive actinide element. Science Picture Co / Getty Images

Actinium is the radioactive element that has atomic number 89 and element symbol Ac. It was the first non-primordial radioactive element to be isolated, although other radioactive elements had been observed before actinium. This element possesses several unusual and interesting characteristics. Here are the properties, uses, and sources of Ac.

Actinium Facts

  • Actinium is a soft, silver-colored metal that glows pale blue in the dark because the radioactivity ionizes air. Actinium reacts with moisture and oxygen to form a white coating of actinium oxide that protects the underlying metal from further oxidation. The shear modulus of element 89 is estimated to be similar to that of lead.
  • Andre Debierne claimed discovery of an element he named actinium, working from a sample of pitchblende supplied by Marie and Pierre Curie. Debierne was unable to isolate the new element (which modern analysis reveals might not have been element 89, but rather protactinium). Friedrich Oskar Giesel independently discovered actinium in 1902, calling it "emamium". Giesel went on to become the first person to isolate a pure sample of the element. Debierne's name was retained because his discovery had seniority. The name comes from the Ancient Greek word aktinos, which means ray or beam.
  • The actinide series of elements, a group of metals between actinium and lawrencium possessing similar properties, takes its name from actinium. Actinium is considered the first transition metal in period 7 (although sometimes lawrencium is assigned that position).
  • Although the element gives its name to the actinide group, most of the chemical properties of actinium are similar to those of lanthanum and other lanthanides.
  • The most common oxidation state of actinium is +3. Actinium compounds have similar properties to lanthanum compounds.
  • Natural actinium is a mix of two isotopes: Ac-227 and Ac-228. Ac-227 is the most abundant isotope. It is primarily a beta emitter, but 1.3% of decays yield alpha particles. Thirty-six isotopes have been characterized. The most stable is Ac-227, which has a half-life of 21.772 years. Actinium also has two meta states.
  • Actinium occurs naturally in trace amounts in uranium and thorium ores. Because it's difficult to isolate the element from ore, the most common way to produce actinium is by neutron irradiation of Ra-226. Milligram samples may be prepared in this manner within nuclear reactors.
  • To date, there has been minimum industrial use of actinium because it is rare and expensive. The isotope actinium-227 might have use in radioisotope thermoelectric generators. Ac-227 pressed with beryllium is a good neutron source and may be used as a neutron probe for well logging, radiochemistry, radiography, and tomography. Actinium-225 is used for radiation cancer treatment. Ac-227 may also be used to model water mixing in the ocean.
  • There is no known biological function for actinium. It is both radioactive and toxic. It is considered slightly less toxic than the radioactive element plutonium and americium. When rats were injected with actinium trichloride, about half of the actinium was deposited in the liver and one-third into the bones. Because of the health risk it presents, actinium and its compounds should only be handled with a glove box.

Actinium Properties

Element Name: Actinium

Element Symbol: Ac

Atomic Number: 89

Atomic Weight: (227)

First Isolated By (Discoverer): Friedrich Oskar Giesel (1902)

Named By: André-Louis Debierne (1899)

Element Group: group 3, d block, actinide, transition metal

Element Period: period 7

Electron Configuration: [Rn] 6d1 7s2

Electrons per Shell: 2, 8, 18, 32, 18, 9, 2

Phase: solid

Melting Point: 1500 K (1227 °C, 2240 °F) 

Boiling Point:3500 K (3200 °C, 5800 °F) extrapolated value

Density: 10 g/cm3 near room temperature

Heat of Fusion: 14 kJ/mol

Heat of Vaporization: 400 kJ/mol

Molar Heat Capacity: 27.2 J/(mol·K)

Oxidation States3, 2

Electronegativity: 1.1 (Pauling scale)

Ionization Energy: 1st: 499 kJ/mol, 2nd: 1170 kJ/mol, 3rd: 1900 kJ/mol

Covalent Radius: 215 picometers

Crystal Structure: face-centered cubic (FCC)


  • Debierne, André-Louis (1899). "Sur un nouvelle matière radio-active." Comptes Rendus (in French). 129: 593–595.
  • Emsley, John (2011). Nature's building blocks: An A-Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-960563-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
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Helmenstine, Anne Marie, Ph.D. "Actinium Facts - Element 89 or Ac." ThoughtCo, Aug. 26, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 26). Actinium Facts - Element 89 or Ac. Retrieved from Helmenstine, Anne Marie, Ph.D. "Actinium Facts - Element 89 or Ac." ThoughtCo. (accessed June 10, 2023).