Actinium
Atomic Data
| Atomic Number | 89 |
| Symbol | Ac |
| Atomic Weight | 227 u |
| Density (STP) | 10.07 g/cm³ |
| Melting Point | 1049.85 °C (1323 K) |
| Boiling Point | 3197.85 °C (3471 K) |
| Electronegativity | 1.1 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2 |
| Oxidation States | +3 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-34-8 |
Electron Configuration
[Rn] 6d1 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| P | 6 | 6s | 2 | 80 |
| P | 6 | 6p | 6 | 86 |
| P | 6 | 6d | 1 | 87 |
| Q | 7 | 7s | 2 | 89 |
| Total | 89 | 89 | ||
Isotopes of Actinium
Actinium is monoisotopic: ²²⁷Ac is its only naturally occurring stable isotope, accounting for 100% of all natural Actinium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Actinium-227 | ²²⁷Ac | 89 | 138 | trace | Stable |
Abundance & Occurrence
Actinium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactivity: Actinium-227 (t½ = 21.8 years, beta/gamma) and its decay chain emit significant radiation; actinium is a source material for radium-223 used in targeted alpha therapy and requires handling in shielded, radiologically controlled environments.
- Internal dose hazard: Actinium behaves chemically like lanthanum and accumulates in bone if ingested or inhaled; internal contamination must be prevented with full containment and negative-pressure systems.
- Handling requirements: All actinium work requires a radioactive materials licence, purpose-built shielded glove boxes or hot cells, and dosimetry; contamination monitoring is essential given actinium's complex decay chain.
- Environmental persistence: Ac-227's 21.8-year half-life means contaminated materials remain hazardous for over a century; disposal must follow national nuclear regulatory requirements for long-lived alpha-emitting waste.
Actinium in the Real World
Real-World Uses
- Targeted alpha therapy (clinical): Actinium-225 (t½ = 9.9 d) decays through a chain that releases four alpha particles; Ac-225-PSMA-617 and Ac-225-DOTATATE are in clinical trials and approved use for treating metastatic prostate cancer and neuroendocrine tumours resistant to other therapies.
- Generator for Bi-213: Ac-225 decays to bismuth-213, which is itself a clinically used alpha emitter for targeted cancer therapy; Ac-225/Bi-213 generator systems provide a portable alpha source for hospital radiopharmacy.
- Research neutron sources: Actinium-beryllium (Ac-Be) sources were used in early nuclear physics research; the study of actinium chemistry in solution and the solid state informs understanding of actinide behaviour relevant to nuclear waste management.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
Has actinium ever been used for anything?
Actinium-225 is used as a parent isotope for bismuth-213 in targeted alpha therapy for cancer. Ac-225 itself is being investigated as a therapeutic radioisotope: its four alpha-emitting decay products in the decay chain can all damage tumour DNA. Actinium-228 (from thorium decay) has historical use in luminous paint. As a neutron source (actinium-beryllium), it has been used in research.
Why is actinium so rare?
Natural actinium consists of actinium-227, which has a half-life of 21.8 years. It occurs only as a decay product of uranium-235, and only in minuscule traces: about 0.2 parts per trillion in uranium ores. There is estimated to be less than 1 gram of naturally occurring actinium in all of Earth's crust. Usable quantities are produced by neutron irradiation of radium-226 in nuclear reactors.
How was actinium discovered?
Actinium was discovered in 1899 by French chemist André-Louis Debierne, a colleague of Marie Curie, while working with uranium ore residues after the Curies had extracted radium and polonium. He initially called it 'emanium'. German chemist Friedrich Oskar Giesel independently discovered it in 1902 and called it 'emanium'. The name actinium was retained, from the Greek 'aktis', meaning ray, for its radioactivity.
What is actinium-225 targeted alpha therapy?
Actinium-225 (half-life 9.9 days) decays through a chain of four alpha-emitting daughter isotopes, ultimately reaching stable bismuth-209. When Ac-225 is attached to a molecule that seeks out cancer cells (such as a PSMA ligand for prostate cancer), each actinium atom delivers a cascade of four alpha particles within the tumour. Each alpha particle has a range of only 40–90 micrometres in tissue, concentrating the lethal dose in the tumour while minimising damage to surrounding normal tissue.