ACTINIUM, Ac (Latin "Actinium" * EN: actinium; DE: Aktinium; FR: actinium; ES: actinio; RU: актиний) is the radioactive chemical element of the III group of the periodic system by Mendeleev, the atomic number is 89, the most long living isotope is 227Ac. It has been discovered by the french chemist André-Louis Debierne in 1899 in the wastes of the processing of the uranium ores. The Clarke number of the actinium in the Earth crust is 6x10^-10% (by the mass). The isotopes of the Ac and 228Ac are the members of the radioactive series of the actinium and thorium.

Actinium is the metal of the silvery-white colour. It crystallizes in the cubic crystal system, the parameter "a" of the unit cell is 53.11 nanometres (5.311 A), the atomic radius is 18.8 nanometres (1.88 A); the temperature of the melting point is the 1040 ± 50 degrees Celsius, the temperature of the boiling point is around the 3590 degrees Celsius. It is easily oxidized in the air and is dissolved in the hydrochloric and nitric acids. The oxidation state in the chemical compounds is +3. As the highest homologue of the lanthanum, actinium in the pure form is produced only artificially during the irradiation of the 226Ra with neutrons. The isomorphism of the actinium and lanthanum is so great, that the same types of their compounds have the same properties, but the major properties of the actinium are expressed more strongly [the ionic radius of the Ac3+ is 11.1 nanometres (1.11 A), of the La3+ is 10.6 nanometres (1.06 A)]. The Ac2O3 oxide is produced by the baking of the actinium hydroxide or actinium oxalate at 1100 degrees Celsius. There has been mastered the production of the halides, oxyhalides and sulfides of the actinium. Almost all salts of the actinium are of the white colour and colourless in the solutions. The 227Ac in the mixture with Be is used for the manufacturing of the neutron sources, in which the neutrons are formed during the irradiation of the 9Be by the alpha particles, which are emitted by the progeny products of the 227Ac with the 227 atomic weight.