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(ăktĭn`ēəm) [Gr.,=like a ray], radioactive chemical element; symbol Ac; at. no. 89; mass number of most stable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 227; m.p. about 1,050°C;; b.p. 3,200°C;±300°C;; sp. gr. 10.07; valence +3. Actinium is a silver-white metal with a cubic crystalline structure. It is found with uranium minerals in pitchblendepitchblende
, dark, lustrous, heavy mineral, a source of radium and uranium. Largely natural uranium oxides, triuranium octaoxide (U3O8) and uranium dioxide (UO2
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. The pure metal can be prepared by reducing its fluoride with lithium vapor at about 1,200°C;. Actinium-227, the most stable isotope, has a half-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 of 21.77 years. The other seven isotopes of actinium have very short half-lives ranging from 10 days to less than 1 min.

Actinium is in Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. Its chemical properties are similar to those of lanthanumlanthanum
[Gr.,=to lie hidden], metallic chemical element; symbol La; at. no. 57; at. wt. 138.90547; m.p. about 920°C;; b.p. about 3,460°C;; sp. gr. 6.19 at 25°C;; valence +3.
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 and of members of the actinide seriesactinide series,
a series of radioactive metallic elements in Group 3 of the periodic table. Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
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, of which it is usually considered the first member. It reacts with water to form an insoluble hydroxide; with halides to form a trifluoride, trichloride, bromide, or iodide; with oxalic acid to form the oxalate; with oxygen or sulfur to form the sesquioxide or sesquisulfide.

Actinium was first recognized in 1899 by André Debierne in uranium residues from pitchblende after the radium was extracted by Pierre and Marie CurieCurie
, family of French scientists. Pierre Curie, 1859–1906, scientist, and his wife, Marie Sklodowska Curie, 1867–1934, chemist and physicist, b. Warsaw, are known for their work on radioactivity and on radium.
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. It was later found to be identical with an element discovered in 1902 by Fritz Giesel and which he called emanium.



Ac, a radioactive chemical element in the Group III of the Mendeleev periodic table, atomic number 89. No stable isotopes.

Actinium was discovered in 1899 by the French chemist A. Debierne while he was studying uranium ore tailings. The ten known radioactive isotopes of actinium have mass numbers ranging from 221 to 230. The longest-lived actinium isotope, 227Ac (half-life T½=21.8 years), emits beta particles (98.8 percent) and alpha particles (1.2 percent). The isotopes 227Ac and 228Ac (T½=6.13 h; also known as mesothorium II, MsThll) occur naturally in uraniferous and thoriferous ores as members of the naturally occurring radioactive families. The surface layer of the earth’s crust (thickness 1.6 km) contains 11,300 tons of 227Ac, but the actinium content in the earth’s crust is very small compared to that of other elements (6.10-10 percent by mass).

The element actinium is a silvery white metal with a face-centered cubic lattice (Tm=1050° ± 50°C, Tb probably about 3300°C); it glows faintly in the dark because of its high radioactivity. In moist air it becomes coated with a white film of oxide which prevents further oxidation of the metal. Actinium is trivalent in chemical compounds. Almost all actinium salts are white and are colorless in solution. Most of them (except AcP04) are isomorphic with the corresponding lanthanum compounds. Actinium forms the same insoluble compounds as does La (hydroxide, phosphate, oxalate, carbonate, fluosilicate). Actinium hydroxide Ac(OH)3 is more basic than lanthanum hydroxide La(OH)3. Because of the extraordinary similarities between the chemical properties of actinium and lanthanum, isolation of actinium in pure form from its natural hosts (containing La and other rare earths) involves great difficulties, and therefore milligram amounts of actinium (227Ac) are obtained artificially through neutron bombardment of radium 226Ra. Since probability of 227Ac decay with alpha emission is low, and the energy of beta particles emitted by this isotope is very small (46 keV), any emission accompanying the radioactive decay of 227Ac went undetected for a long time; until 1935 it was assumed that radioactive decay of 227Ac occurred without emission. Modern instruments are capable of identifying such soft beta radiation, but it is still quite difficult to make quantitative measurements; therefore, the behavior of 227Ac is usually monitored in terms of the activity of its daughters, in experiments involving trace amounts of the radionuclide. Mixed with beryllium, 227Ac is used in the preparation of neutron sources, in which neutrons are formed by irradiating 9Be nuclei with alpha particles emitted by 227Ac daughters.


Seaborg, G., and J. Katz. Khimiia aktinidnykh elementov. Moscow, 1960. (Translated from English.)
Bagnall, K. Khimiia redkikh radioaktivnykh elementov, poloniiaktinii. Moscow, 1960. (Translated from English.)



A radioactive element, symbol Ac, atomic number 89; its longest-lived isotope is 227Ac with a half-life of 21.7 years; the element is trivalent; chief use is, in equilibrium with its decay products, as a source of alpha rays.


a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no.: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07; melting pt.: 1051°C; boiling pt.: 3200 ± 300°C