Actinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902. Actinium gave the name to the actinoid series, a group of 15 similar elements between actinium and lawrencium in the periodic table.
Actinium is a silvery, radioactive, metallic element. Due to its intense radioactivity, actinium glows in the dark with a pale blue light. The chemical behaviour of actinium is similar to that of the rare earth element lanthanum. Actinium shows similar chemical behaviour to lanthanum. Due to this similarity the separation of actinium from lanthanum and the other rare earth elements, which are also present in uranium ores was difficult. Solvent extraction and ion exchange chromatography was used for the separation.