actinium (Ac)

actinium atom

Electron configuration of actinium. Credit: Wikipedia.

Actinium is a silvery-white, radioactive, metallic element and first member of the actinide series; it is also a transition element in group IIIB of the periodic table. Actinium was discovered by Andre Debierne in 1899 and is found associated with ores of uranium; however, it is difficult to separate and is best prepared by bombardment of radium with neutrons. It is separated from other species by ion exchange or solvent extraction. Its most stable isotope, 227Ac, with a half-life of 21.8 years, is a decay product of uranium-235. Actinium glows blue and is oxidized in air.


Very few actinium compounds have been isolated because of the element's intense radioactivity. In its compounds actinium is tripositive, forming very similar compounds to lanthanum. Actinium salts are colorless.


atomic number 89
relative atomic mass 227
relative density 10.07
melting point 1,100°C (1,900°F)
boiling point 3,200°C (5,800°F)