Actinium · element 89
The element that gave the actinides their name glows blue in the dark and is now being aimed at cancer cells
Key facts
Actinium is a soft, silvery metal that glows. Not because it is hot, but because its intense radioactivity ionises the air around it, producing a pale blue light. It is around 150 times more radioactive than radium, which is why very few people have ever handled a visible amount.
It lends its name to the actinides, the fifteen elements from actinium to lawrencium that fill the second long row beneath the main table, and its own name comes from the Greek aktis, a ray. Long a scientific curiosity, actinium has recently become one of the most sought after isotopes in cancer medicine.
Where it is found
Actinium-227 is a member of the uranium-235 decay chain, so it occurs naturally in uranium ores, but only in traces of about a fifth of a milligram per tonne, and its half-life of 21.8 years means it never builds up. Extracting actinium from ore is possible but hopelessly inefficient.
Instead it is manufactured. Actinium-227 is made by irradiating radium-226 with neutrons in a reactor. The medically important actinium-225, with a half-life of ten days, is separated from stocks of thorium-229 held at a few national laboratories, or produced by bombarding thorium or radium targets in particle accelerators. Global supply is tiny and demand from cancer trials far exceeds it.
How it was discovered
Actinium was found in the leftovers of radium. In 1899 André-Louis Debierne, a colleague of the Curies in Paris, reported a new radioactive element in the pitchblende residues after radium had been extracted, and named it actinium. His description was muddled, though: his substance seemed to behave like titanium one year and thorium the next, and it is not certain what he actually had.
In 1902 the German chemist Friedrich Oskar Giesel independently isolated a strongly radioactive substance from the same kind of residue and called it emanium, because it gave off a radioactive gas. Giesel's material was clearly actinium, and many historians consider him the true discoverer, but Debierne's name had priority and was kept. Pure actinium metal was not prepared until the mid twentieth century.
What we use it for
The exciting use is targeted alpha therapy. Actinium-225 decays through a rapid chain of four alpha emissions, delivering a concentrated dose of energy within a few cell widths of wherever it sits. Attached to a molecule that seeks out prostate cancer cells, it has produced striking results in men whose disease had stopped responding to other treatments, with scans showing widespread tumours shrinking away. Trials are also under way for leukaemia and other cancers.
Actinium-227 has smaller roles. Mixed with beryllium it serves as a neutron source, and oceanographers use the natural actinium-227 in seawater as a tracer to study how deep ocean water mixes and rises.
Reading the numbers
The melting point of about 1230 °C and boiling point near 3230 °C are approximate, because measuring a metal this radioactive is difficult. Its density of 10 grams per cubic centimetre puts it close to silver and well above iron, and the atomic mass of 227 is the mass number of the longest lived isotope, as actinium has no stable form.
The electron configuration [Rn] 6d1 7s2 has three electrons outside a radon core, so actinium almost always forms Ac3+ ions. That makes it chemically a near twin of lanthanum, which has the equivalent configuration one row up; the two are hard to separate except by their radioactivity. Ionisation energies of 499 and 1170 kJ/mol and an electronegativity of 1.1 mark it as a reactive metal that tarnishes in air and reacts with water. There is a small irony in its name: the actinides are defined by filling the 5f orbitals, yet actinium itself has no 5f electrons at all. Chemists still debate whether it belongs in group 3 beneath lanthanum or purely heads the actinides.
Safety and the environment
Actinium is extremely radiotoxic. Like radium it is a bone seeker, and once deposited it and its decay products bombard the marrow with alpha particles, so it is handled only in glove boxes and hot cells. In cancer therapy the dose is measured in millionths of a gram and carefully targeted, and the main side effect seen in trials is a dry mouth, because the targeting molecule also collects in salivary glands.
Outside uranium mining and processing there is no environmental actinium to speak of. Its natural traces are in equilibrium with the uranium that produces them, and medical quantities decay away within weeks.
Did you know?
- Actinium glows pale blue in the dark because its radiation ionises the surrounding air.
- The actinide series is named after actinium, yet actinium itself has no electrons in the 5f orbitals that define the series.
- Actinium is roughly 150 times more radioactive than radium.
- Actinium-225 undergoes four alpha decays in quick succession, which is why a single atom attached to a cancer cell delivers such a powerful blow.
- Friedrich Giesel, who independently found actinium in 1902 and called it emanium, was a chemist at a quinine factory and the first person to produce radium commercially.
- Oceanographers use the trace of natural actinium-227 in seawater to measure how deep ocean water mixes.
More in the actinides
The nearest neighbours of actinium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
What is actinium used for?
Its most important use is the isotope actinium-225 in targeted alpha therapy, an experimental cancer treatment that attaches the isotope to molecules that seek out tumour cells. Actinium-227 is also used in neutron sources and as an ocean tracer. There are no everyday consumer uses.
Is actinium the same as an actinide?
Actinium is one element, number 89. The actinides are the fifteen elements from actinium to lawrencium, numbers 89 to 103, which sit in the second row beneath the main table and are named after actinium. All of them are radioactive.
Why does actinium glow?
Its intense radioactivity knocks electrons off nitrogen and oxygen molecules in the surrounding air. When those molecules recapture electrons they emit light, mostly in the blue and ultraviolet, producing a faint blue glow visible in the dark.
Who discovered actinium?
André-Louis Debierne in Paris announced it in 1899 and gave it its name, but his description of the substance was inconsistent. Friedrich Oskar Giesel independently isolated a clearly identified sample in 1902 and called it emanium. Both are often credited, with Debierne's name retained.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Actinium reference article. Spotted an error? Tell us.