Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where actinium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 188 pm — that is 0.188 nm, so about 2660 million of them side by side would span a millimetre.
Thermal range
Actinium is liquid over a 2148 K window, from 1323 K to 3471 K.
The story
Intensely radioactive and 150 times more so than radium — actinium glows pale blue in the dark.
Actinium was discovered in 1899 by André-Louis Debierne, a French chemist working in Marie Curie's laboratory at the University of Paris. Debierne was investigating the radioactive residues left after extracting radium and polonium from pitchblende ore. He noticed a new radioactive substance that behaved differently from known elements and named it "actinium" from the Greek word "aktinos," meaning ray or beam.
In 1902, Friedrich Oskar Giesel, a German chemist, independently discovered the same element while working with similar radioactive materials. Initially, there was confusion about whether Debierne's actinium and Giesel's "emanium" were the same element. Scientific analysis eventually confirmed they were identical, with priority given to Debierne for his earlier work.
The discovery took place during the golden age of radioactivity research in Marie Curie's legendary laboratory. Debierne was studying the complex mixture of radioactive elements in pitchblende residues, using the newly developed techniques of radioactive decay analysis. The work was painstaking, requiring the processing of tons of uranium ore to obtain minute quantities of radioactive materials.
Early researchers struggled to characterize actinium chemically due to its extreme radioactivity and scarcity. Otto Hahn and Lise Meitner made significant contributions to understanding actinium's properties in the early 1900s. They determined its position in the periodic table and identified its radioactive decay products.
The discovery of actinium was crucial for understanding radioactive decay series and helped establish the foundation of nuclear chemistry. It was the first element of what would later be known as the actinide series, opening up an entirely new area of chemistry and physics that would eventually lead to nuclear energy and nuclear medicine.
Applications
Actinium serves as a crucial element in advanced nuclear medicine and research applications. Its most significant isotope, Actinium-225, is revolutionizing targeted alpha therapy (TAT) for cancer treatment. This isotope emits high-energy alpha particles that can destroy cancer cells with minimal damage to surrounding healthy tissue, making it incredibly valuable for treating metastatic cancers that resist conventional therapies.
Actinium-225 is produced in specialized nuclear reactors and particle accelerators for pharmaceutical applications. The isotope is attached to targeting molecules that seek out specific cancer cells, delivering lethal radiation doses directly to tumors. This precision medicine approach is showing remarkable results in treating neuroendocrine tumors, prostate cancer, and leukemia.
In nuclear research facilities, actinium isotopes serve as sources for studying alpha decay processes and nuclear reactions. Researchers use actinium compounds to investigate the fundamental properties of heavy nuclei and to develop new radiopharmaceuticals. The element's unique decay characteristics make it valuable for understanding nuclear physics and advancing medical applications.
Actinium-227 has historically been used as a neutron source in scientific instruments and research applications. Its long half-life (21.8 years) and predictable decay make it useful for calibrating radiation detection equipment and studying neutron physics. However, due to its radioactivity and associated safety concerns, its use has become more specialized and regulated.
Scientists are exploring actinium's potential in next-generation nuclear technologies, including advanced reactor designs and space exploration applications. Its unique nuclear properties could play a role in developing more efficient nuclear fuel cycles and specialized radiation sources for deep space missions where traditional power sources are impractical.
The primary modern use of actinium is in targeted alpha therapy (TAT) for cancer treatment. Actinium-225 is conjugated with monoclonal antibodies or peptides that specifically target cancer cells, delivering high-energy alpha particles directly to tumors. This approach is particularly effective against blood cancers and metastatic solid tumors.
Pharmaceutical companies are developing actinium-based drugs for treating various cancers. These radiopharmaceuticals combine actinium-225 with targeting molecules to create precision cancer treatments. Clinical trials are ongoing for treatments targeting prostate cancer, neuroendocrine tumors, and acute myeloid leukemia.
Research institutions use actinium isotopes to study nuclear decay processes, develop new medical treatments, and advance our understanding of heavy element chemistry. The element serves as a model system for studying actinide behavior and developing separation techniques for other radioactive elements.
Due to its predictable radioactive decay, actinium compounds serve as reference standards for radiation detection equipment and dosimetry instruments. This ensures accurate measurements in nuclear medicine, research facilities, and radiation protection programs.
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Actinium occurs naturally in trace amounts within uranium ores, particularly in pitchblende and other uranium-bearing minerals. It forms as part of the uranium-235 decay chain, where uranium-235 eventually decays to produce actinium-227. The concentration is extremely low, typically less than 0.2 parts per trillion in uranium ores.
Natural actinium can be found wherever uranium deposits exist, including locations in Canada, Australia, Kazakhstan, Niger, and the United States. The Colorado Plateau, Canadian Shield, and African uranium provinces contain the highest natural concentrations, though extraction from these sources is impractical due to the minute quantities present.
Virtually all actinium used today is artificially produced in nuclear reactors or particle accelerators. Actinium-225 is typically produced by bombarding radium-226 targets with neutrons, or by extracting it from thorium-229 decay. Actinium-227 can be produced by neutron bombardment of radium-226 in nuclear reactors.
Actinium-227 is a member of the uranium-235 decay series (actinium series), while other actinium isotopes are produced artificially. The natural abundance is so low that it was one of the last naturally occurring elements to be discovered, and natural samples are insufficient for practical applications.
Extracting actinium from natural sources is extremely difficult and economically unfeasible due to its scarcity and the complex chemistry required to separate it from other radioactive elements. Modern production methods focus on nuclear synthesis rather than natural extraction.
Handling
Actinium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
WARNING: Actinium and all its isotopes are highly radioactive and pose severe health risks. Alpha radiation from actinium can cause significant cellular damage, particularly to DNA, leading to increased cancer risk and acute radiation syndrome at high exposures.
Actinium must only be handled in specialized radiological facilities with appropriate shielding, ventilation, and containment systems. Personnel require extensive radiation safety training, personal dosimetry monitoring, and must work behind lead or concrete barriers. Remote handling equipment is often necessary.
Inhalation or ingestion of actinium compounds is extremely dangerous. The element can accumulate in bones and liver tissue, causing long-term internal radiation exposure. Even microscopic amounts can pose significant health risks due to the high energy of alpha particles emitted during decay.
Suspected actinium exposure requires immediate medical attention and specialized treatment at facilities equipped for radiation emergencies. Decontamination procedures must be followed, and long-term health monitoring is essential due to the potential for delayed radiation effects.
Quick answers
Actinium (symbol Ac) is element 89 on the periodic table, a actinide in period 7. Intensely radioactive and 150 times more so than radium — actinium glows pale blue in the dark. At room temperature it is a solid, and it is radioactive.
[Rn] 6d¹ 7s², giving 7 occupied shells holding 2, 8, 18, 32, 18, 9, 2 electrons respectively.Actinium melts at 1323 K (1049.9 °C) and boils at 3471 K (3197.9 °C).
Actinium has no stable isotope, so it has no standard atomic weight. The figure quoted, 227, is the mass number of its longest-lived known isotope.
Actinium has a density of 10.07 g/cm³. Water is 1.0 g/cm³, so a block of actinium is about 10.1× heavier.
Actinium has a Pauling electronegativity of 1.1. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.
Actinium was discovered in 1899 by Andre-Louis Debierne. It is named after greek aktis, "ray".
Actinium does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Actinium has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.