actinium

Actinium is a chemical element with the symbol Ac and atomic number 89. It is a dense, silvery‑white metal that is highly radioactive and belongs to the actinide series in the periodic table. In nature it occurs only in minute quantities, primarily as a decay product in the uranium‑235 decay chain, making it one of the rarest naturally occurring elements. The name derives from the Greek aktis, meaning ray, in reference to the strong radiation of its compounds.

Naturally occurring actinium is present mainly as the isotope 227Ac, which has a half‑life of about 21.8 years and is the most stable isotope found in nature. Several other radionuclides exist, including 225Ac with a half‑life of about 10 days and 228Ac with a half‑life of about 6 hours. In addition to these natural isotopes, actinium can be produced synthetically in laboratories for research and medical applications.

Chemically, actinium predominantly exhibits the +3 oxidation state and forms compounds such as Ac2O3 and AcCl3, as well as various coordination complexes. It behaves similarly to other actinides, tending to hydrolyze in water and form insoluble compounds. Because of its radioactivity, handling actinium requires specialized facilities and strict safety protocols.

Actinium was discovered in 1899 by French chemist André-Louis Debierne and was named for its radiant emissions. Today, actinium and its isotopes are primarily of interest for scientific research; the isotope 225Ac is studied for targeted alpha therapy in experimental cancer treatments, and actinide isotopes are used as radiation sources in research. Natural occurrence remains extremely limited, reinforcing actinium’s status as a very rare element in the Earth's crust.