Actinium

Actinium is a chemical element with the symbol Ac and atomic number 89. It is a radioactive metal, meaning its atomic nucleus undergoes decay, releasing energy and particles. It occupies the beginning of the actinide series on the periodic table, a group named after this element.

Actinium occurs naturally in very small amounts. It is found in uranium ore in trace quantities, produced by the radioactive decay of heavier actinides. For this reason, it is usually produced artificially by bombarding radium with neutrons inside a nuclear reactor.

Physical and chemical properties

Actinium is a silvery-white metal with a lustrous surface. When exposed to air, its surface oxidizes to form an oxide coating that resists further corrosion. It emits blue light in the dark, caused by the radiation it releases ionizing the surrounding air. Its chemistry closely resembles that of lanthanum, and it commonly exhibits an oxidation state of +3.

Isotopes

All isotopes of actinium are radioactive. The most common are Ac-227, which is part of the decay chain of uranium-235, and Ac-225. Half-life—the time required for half of a sample to decay—varies significantly: Ac-227 has a half-life of several decades, while Ac-225 has a half-life of days.

Applications

Actinium use is limited to research and medicine. Ac-225 is being tested in cancer treatment through targeted alpha-particle therapy, in which the element is attached to a molecule that directs it to cancer cells, allowing the alpha radiation to destroy the targeted cells. Ac-227 is sometimes used to generate a neutron source when combined with beryllium.

History

Actinium was discovered in the early twentieth century during studies of radioactivity in uranium ore. Its name comes from the Greek word meaning 'ray' or 'radiation'. Handling actinium requires specialized equipment because the radiation it emits poses health risks if ingested or inhaled.