Actinium (Ac)


  • Symbol: Ac
  • Atomic Number: 89
  • Atomic Weight: 227
  • Element Classification: Actinide
  • Discovered By: Friedrich Oskar Giesel
  • Discovery Date: Independently discovered by Friedrich Oskar Giesel in 1902 and by André-Louis Debierne in 1899, but Debierne is often credited with the discovery.
  • Name Origin: From the Greek ‘aktinos’, meaning ray or beam, referring to its radioactivity
  • Density(g/cc): 10.07
  • Melting Point: 1050°C
  • Boiling Point: 3200°C (estimated)
  • Appearance: Silvery-white metallic; it glows in the dark due to its intense radioactivity
  • Atomic Radius(pm): 195


Actinium was independently discovered by André-Louis Debierne, a French chemist, in 1899 and by Friedrich Oskar Giesel, a German chemist, in 1902. Both scientists discovered actinium as a result of their work with pitchblende, a uranium-rich mineral. Debierne initially described it as similar to titanium and (later) to thorium, while Giesel described it as a substance with properties similar to lanthanum. Due to the priority of publication and the detailed characterization, Debierne is often credited with the discovery. The name “actinium” comes from the Greek ‘aktinos’, meaning a ray, reflecting its radioactive nature.

Relation to Other Elements

Actinium is the first element in the actinide series, a group of 15 metallic elements from actinium to lawrencium in the periodic table. These elements are characterized by their filling of the 5f electron orbital. Actinium exhibits typical metallic properties and has similar chemical behaviors to the rare earth metals, particularly lanthanum. It has a +3 oxidation state in most of its compounds. Being radioactive, actinium’s most stable isotope, actinium-227, has a half-life of 21.77 years and decays to produce radium-223 and alpha particles.

Natural Occurrence

Actinium is found naturally in trace amounts in uranium and thorium ores, where it arises from the radioactive decay of those elements. Due to its scarcity and radioactivity, actinium does not have significant natural deposits, and its presence in the Earth’s crust is extremely low. It is usually produced synthetically in nuclear reactors or particle accelerators.


Given its radioactivity and rarity, actinium’s practical applications are limited:

  • Medical Research: Actinium-225 is used in targeted alpha therapy (TAT), an experimental cancer treatment that uses alpha particles emitted by radioactive isotopes to kill cancer cells. Actinium-225 is especially researched for its potential in treating certain types of cancer, such as leukemia and prostate cancer, due to its high energy and short range in biological tissues.
  • Scientific Research: Actinium is used in research related to its chemical properties, behavior as an actinide, and potential uses in new types of nuclear reactors and medical applications.

The discovery of actinium marked the beginning of the actinide series and contributed to the understanding of radioactive elements and their properties. While its uses are primarily confined to scientific research and experimental cancer treatments, ongoing studies continue to explore its potential benefits in medicine and other fields.



Radium (Ra)

Thorium (Th)