Atomic number 89 · Actinide
Actinium Ac
Actinium (Ac), element 89, is the first member of the actinide series (elements 89 to 103) and gives the series its name. It is a silvery radioactive metal so intensely radioactive that it glows pale blue in the dark. Actinium-225 has recently become one of the most sought-after isotopes for cancer therapy.
Key properties
| Atomic number | 89 |
|---|---|
| Atomic weight | [227] (No stable isotope; mass number of a representative isotope in brackets) |
| Category | Actinide |
| Group | Lanthanides/actinides (no group number) |
| Period | 7 |
| Block | f |
| State (25 °C, 1 atm) | Solid |
| Melting point | 1324 K (1050.8 °C) |
| Boiling point | 3471 K (3197.8 °C) |
| Density | 10.07 g/cm³ |
| Electronegativity (Pauling) | 1.1 |
| Atomic radius (van der Waals) | 260 pm |
| First ionization energy | 5.17 eV |
| Electron affinity | — |
| Oxidation states | +3 |
| Discovered | 1899 |
Electron configuration
- Condensed
- [Rn]7s2 6d1
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 9 · 2
Orbital diagram
Position in the table
Discovery
In 1899 the French chemist André-Louis Debierne found a new radioactive substance in the pitchblende residues left after the Curies had extracted radium, and called it actinium. In 1902 Friedrich Oskar Giesel in Germany independently isolated a similar substance, which he named "emanium".
Later analysis suggested that the properties Debierne first reported did not match actinium well, and some historians of science argue that Giesel deserves the credit for the discovery. Debierne's earlier name was nevertheless adopted and is still used. In the 1950s, milligram amounts of actinium made by irradiating radium-226 with neutrons in reactors finally allowed its chemistry to be studied properly.
Origin of the name
The name comes from the Greek aktis (genitive aktinos), meaning "ray" or "beam", reflecting its radioactivity. Under the actinide concept proposed by Glenn Seaborg in the 1940s, the elements from 89 to 103 are named after it.
Main uses
- Targeted alpha therapy: actinium-225 attached to molecules that seek out prostate cancer (PSMA ligands) and other tumours has shown striking results in advanced cancers that no longer respond to standard treatment. With a half-life of about ten days, each atom releases four alpha particles through its decay chain, delivering a powerful dose to cancer cells. Supply is very limited, so accelerator-based production is being developed alongside extraction from thorium-229.
- Neutron sources: actinium-227 mixed with beryllium has been used as a neutron source, for example in oil-well logging.
- Heat sources: actinium-227 has been studied as a possible heat source for radioisotope power.
Isotopes
All actinium isotopes are radioactive. The main natural isotope, actinium-227, has a half-life of about 21.8 years and occurs in the uranium-235 decay chain. Medical actinium-225 has a half-life of about ten days, and actinium-228 in the thorium series about six hours.
In everyday life
Uranium ore contains well under a milligram of actinium per tonne, so no one encounters it in daily life. Chemically it closely resembles lanthanum, forming a 3+ ion and behaving much like lanthanum compounds. Because of its strong radioactivity it is handled only in shielded specialist facilities.
Good to know
- The whole actinide series is named after actinium.
- Actinium glows blue in the dark from its own radiation.
- Each actinium-225 atom emits four alpha particles as it works its way down its decay chain.
Same category (Actinide)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.