We continue to review some of the most important materials in heat treatment and metallurgy.  

Actinium (chemical symbol: Ac)

Actinium (Fig. 1) is a soft, silvery-white metal that is highly radioactive, giving off a faint blue glow in the dark. When exposed to air, the surface of actinium will immediately react with either oxygen or moisture in air to form a thin crust of white actinium oxide (Fig. 2), which will prevent further oxidation from taking place. Actinium is found only in trace amounts, usually within uranium and thorium ores (only about 0.2 milligrams per ton of ore). Since it is so scarce in nature, actinium is generally produced through neutron irradiation of the radium isotope, 226Ra within a nuclear reactor.

In 1899, French chemist Andre Debierne was the first to discover actinium while working alongside Pierre and Marie Curie with pitchblende (Fig. 3), a uranium-rich mineral. While the Curies had previously discovered both radium and polonium, Debierne continued their research and identified the presence of a new element (based on its radioactive emissions profile) of his sample.

In 1902, German chemist Friedrich Oskar Giesel independently isolated the same element without knowing it was previously discovered a few years prior, but credit for its discovery was given to Debierne. Debierne named actinium after the ancient Greek word “aktis,” meaning “beam.”

Due to its scarcity, actinium’s high cost and radioactivity means it does not have any significant industrial uses. However, it does see some use in the medical industry. Actinium’s high activity level makes it a valuable element for producing neurons that are useful for radiation therapy to target and destroy cancer cells in the human body.

Here are a few important facts about actinium.[2,5]

  • Atomic number: 89
  • Atomic weight: 227
  • Melting point: 1324 K (1051°C or 1924°F)
  • Boiling point: 3471 K (3198°C or 5788°F)
  • Density: 10.07 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Element classification: Metal
  • Period number: 7    
  • Group number: none    
  • Group name: Actinide
  • Electron configuration: [Rn] 6d17s2



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. Royal Society of Chemistry (rsc.org)
  6. Oak Ridge National Laboratory, US Department of Energy (https://www.ornl.gov)