Actinium
Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.
Identity
- Atomic mass
- [227] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +3
- Phase (25 °C)
- Solid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1
- Short configuration
- [Rn] 6d1 7s2
- Electrons per shell
- 2, 8, 18, 32, 18, 9, 2
- Electronegativity (Pauling)
- 1.10
- Electron affinity
- 33.8 kJ/mol
- 1st ionization energy
- 499 kJ/mol
- Ionization energies
- 499 · 1170 kJ/mol
Physical properties
- Melting point
- 1226.8 °C (1500 K)
- Boiling point
- 3226.8 °C (3500 K)
- Density
- 10 g/cm³
- Molar heat capacity
- 27.2 J/(mol·K)
Chemical behavior of actinium
Actinium has three outer electrons ([Rn] 6d¹ 7s²) and almost always acts as +3, with very low electronegativity (1.10). It is the first actinide and the one most similar to a lanthanide: highly electropositive, soft, and radioactive. In the dark it glows blue through ionization of the air. All of its chemistry is studied in shielded cells because of its intense alpha radiation.
Reactivity
4 Ac + 3 O₂ → 2 Ac₂O₃
In air it rapidly oxidizes to the white oxide, which also glows through radioluminescence. The heat of decay keeps the sample warm.
2 Ac + 6 H₂O → 2 Ac(OH)₃ + 3 H₂
It reacts with water, releasing hydrogen and forming a gelatinous hydroxide. The reaction is carried out in glove boxes with shielding.
2 Ac + 6 HCl → 2 AcCl₃ + 3 H₂
It dissolves in acids to give colorless actinium(III) salts. The chloride is the route for separating Ac-225 from radium or thorium blanks.
Ac₂O₃ + 6 HF → 2 AcF₃ + 3 H₂O
With hydrofluoric acid it gives an insoluble fluoride, useful for precipitating and purifying actinium against its radioactive daughters.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Actinium oxide | Ac₂O₃ | Medical Ac-225 source |
| Actinium-225 chloride | ²²⁵AcCl₃ | Targeted alpha therapy |
| Actinium fluoride | AcF₃ | Radiochemical purification |
| Actinium oxalate | Ac₂(C₂O₄)₃ | Separation intermediate |
| Actinium hydride | AcH₂ | Actinide studies |
Electron shell distribution
View in the atom builderIsotopes
No actinium isotope is stable. Actinium-227 (²²⁷Ac, 21.8 years) dominates the uranium-235 decay chain and is the most abundant in minerals. Actinium-225 (²²⁵Ac, 10 days) emits four alpha particles in a cascade and is the star of targeted therapy. Actinium-228 (²²⁸Ac, 6 hours) marks the thorium chain.
Occurrence and production
It occurs in trace amounts in uranium minerals such as pitchblende, on the order of 0.15 mg per tonne: all natural actinium comes from the decay of uranium-235. Medical Ac-225 is produced by bombarding radium-226 with neutrons or thorium-232 with protons, and then separating it by chromatography in specialized laboratories.
History and origin of the name
André-Louis Debierne separated it in 1899 from pitchblende residues and named it actinium, from the Greek aktis (ray): the element of rays. Giesel obtained purer samples in 1902. For decades it remained a curiosity until Ac-225, a bismuth-213 generator, brought it into oncology hospitals.
Applications and relevance of actinium
Uses of the element
As a source of Ac-225 it feeds bismuth-213 generators for alpha therapy: the parent actinium is fixed on the column and the daughter bismuth is eluted to label antibodies. It also serves as a neutron source alloyed with beryllium.
Uses of its compounds
Actinium-225 chloride bound to PSMA or DOTATATE ligands irradiates prostate and neuroendocrine tumors with short-range alpha particles. The fluoride and oxalate purify the isotope in hot cells.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: it accumulates in bone, liver, and kidney. Therapeutic Ac-225 is administered only in hospitals, with doses calculated to the becquerel and monitoring of radioactive daughters.
Safety
It emits alpha particles and generates emitting daughters that require shielding, glove boxes, ventilation, and continuous monitors. It is handled in micrograms under strict protocols; it does not exist in consumer products.
Environment
There are no routine spills: all inventory is traceable and waste decays within weeks or is contained. Producing facilities manage effluents and filters as radioactive waste under regulatory control.
Frequently asked questions about Actinium
What is actinium-225 used for in medicine?
Actinium-225 is used in targeted alpha therapy, a type of radiation therapy that attaches the isotope to a molecule capable of recognizing tumor cells. Once at the tumor, it emits alpha particles with a very short range and high energy, able to damage the DNA of cancer cells with limited effects on nearby tissues. It is being researched especially in advanced prostate cancer and neuroendocrine tumors.
Where does actinium come from?
Actinium occurs in tiny amounts in uranium minerals as part of the natural decay chains. To obtain actinium-225 for medical purposes, it is produced artificially by neutron bombardment of radium-226 or from thorium irradiated in reactors. It is then separated and purified through radiochemical processes. Its scarcity and radioactivity make it an expensive, highly specialized material.
What is the difference between actinium-225 and actinium-227?
Actinium-225 has a half-life of about 10 days and emits alpha particles, so it is studied for targeted radiation therapy against cancer. Actinium-227 has a much longer half-life, around 21.8 years, and is part of the natural uranium-235 decay chain. Therefore, isotope 225 is better suited to short-duration treatments, while 227 is mainly relevant in research and in natural radioactive chains.
What is the relationship between actinium, thorium, and uranium?
Actinium is part of the decay chains of uranium and thorium. In nature it forms when uranium or thorium isotopes decay and produce actinium atoms, which in turn transform into other radioactive elements. This relationship explains why it appears in uranium minerals and why it can be produced artificially from irradiated radium or thorium.
Is actinium dangerous?
Yes. Actinium is radioactive and toxic: its isotopes emit radiation that can damage tissues and increase the risk of cancer with significant exposure. For this reason, it is not found in household products and its use is restricted to authorized laboratories and medical centers. In therapy, actinium-225 is administered encapsulated in radiotracers, with calculated doses, safety controls, and clinical follow-up of the patient.
Why does actinium glow blue in the dark?
Actinium and its decay products emit ionizing radiation that can excite molecules in the surrounding air. As those molecules return to their ground state, they release energy as visible light, perceived as a faint bluish glow. This is not a property of an everyday object but a phenomenon observed in intensely radioactive samples under controlled conditions. The light does not come from the metal itself but from the interaction of its radiation with the environment.
Why is actinium so difficult to handle?
The difficulty comes from its intense radioactivity and its rapid decay into other radioactive elements. Samples generate heat, radiation, and daughter products that change over time, so they require shielding, containment, ventilation, and continuous monitoring. In addition, actinium is handled in tiny amounts under strict protocols to avoid contamination, inhalation, or exposure of personnel.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.