Actinium

Actinium is a rare, silver-white radioactive metal that glows faintly blue in the dark: one of the most visually striking effects of radioactivity. It gives its name to the actinide series of elements (89-103). Actinium-225 is a promising medical isotope for targeted cancer radiotherapy.

• Atomic Number8989 protons, 89 electrons
• Atomic Mass227.02775 u89× heavier than hydrogen
• State at Room TempSolidSolid
• Density10.07 g/cm³
• Melting / Boiling1050.8°C / 3197.8°C
• Discovered1899

What is Actinium?

Actinium is the first actinide element with 89 protons. Like the lanthanides below it, the actinides share very similar chemistry, they all prefer +3 oxidation states and occur together in radioactive minerals. Actinium-227 has a half-life of 21.8 years. No stable isotopes exist.

Named from the Greek aktinos meaning beam or ray, because of its intense radioactivity. Discovered by André-Louis Debierne in Paris in 1899, shortly after the Curies discovered polonium and radium. Friedrich Giesel also identified it independently in 1902.

Fact Actinium-225 is currently the most promising isotope for targeted alpha therapy (TAT) in cancer treatment. When bound to a molecule that homes in on cancer cells, actinium-225 delivers a chain of four alpha particles directly to tumour cells, killing them while sparing surrounding healthy tissue. Clinical trials show remarkable results in leukaemia and prostate cancer.

Where you find Actinium

On Earth

Actinium occurs in uranium ores as a decay product. It is extremely rare, only approx. 0.2 mg per tonne of uranium ore.

• Uranium ores. Actinium-227 is produced by the decay of uranium-235 in uranium ores worldwide.

How we use Actinium

• Medical radiotherapy.. Actinium-225 produces a chain of alpha-particle emitting daughters. Attached to tumour-targeting molecules, it delivers lethal radiation doses selectively to cancer cells, particularly in clinical trials for leukaemia and prostate cancer.
• Neutron sources.. Actinium-beryllium mixtures emit neutrons and are used as portable neutron sources in research.

Did you know? Actinium-227 is so radioactive that 1 gram of it emits approx. 37 billion radioactive disintegrations per second, almost exactly the same as 1 gram of radium-226, which is why the unit of radioactivity, the curie, was originally defined as the activity of 1 gram of radium.

How it was discovered

Discovered in 1899 by André-Louis Debierne while working with the Curies on uranium ore residues. He found a new radioactive element inseparable from thorium by ordinary chemistry. Friedrich Giesel independently discovered it in 1902, calling it emanium.

Deeper dive: actinium properties and applications

Actinium heads the actinide series, 15 elements from actinium (89) to lawrencium (103) in which the 5f electron subshell fills progressively. Unlike the lanthanides, where all elements are chemically very similar, the actinides show more variety because the 5f, 6d and 7s orbitals are closer in energy, allowing heavier actinides like uranium and plutonium to show a wider range of oxidation states.

Moving to 90 protons on the periodic table brings us to Thorium.