Actinium is a silver-white metallic element that glows blue in the dark. It resembles the rare earths in chemical behavior and valence. Its atomic number is 89 and its atomic weight is 227.

It is a member of the actinide series, which follows lanthanum and includes the 15 elements from thorium to lawrencium inclusive. It is one of the first non-primordial radioactive elements to be isolated.

In 1899, Andre-Louis Debierne separated actinium from a substance called pitchblende. Friedrich Oskar Giesel later independently discovered actinium in 1902.

225 Ac is a very stable isotope that has been studied for its potential as a cancer therapy agent. It emits alpha particles and is used in medical research, and is also a source of the high-energy radionuclides produced in nuclear reactors for gamma radiation therapy.

The chemistry of actinium is very similar to that of lanthanum, which is a nearly ideal surrogate for this element in analytical procedures and preparative chromatography. However, because of its low microgram level and the lack of availability of the long-lived isotopes, much of the chemistry of actinium remains unknown relative to other elements in the periodic table.

Actinides can be dissolved in strong ammonium carbonate solutions or precipitated as hydrous oxides. UO22+ is readily hydroxodicated in ammonia, NpO2+ is precipitated with hydrogen sulfide unless a complexing agent is present. All trivalent actinides are soluble in strong ammonium carbonate solutions, and Uranium and Thorium can be separated from other members of the ammonium hydroxide group (Fe, Ti, Al and other rare earth metals). The most common actinide oxidation states are +3, and +6; all are stable to reducing conditions, six and seven coordinate structures are typical.