Actinium (revised)

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Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.


Actinium is the third element in Row 7 of the periodic table, a chart that shows how the chemical elements are related to each other. Some chemists place it in Group 3 (IIIB), with scandium and yttrium. Other chemists call it the first member of the actinides. The actinides are the 14 elements that make up Row 7 of the periodic table. They have atomic numbers from 89 to 103 and are all radioactive. A radioactive atom is unstable and tends to throw off particles and emit energy in order to become stable. Either way of classifying actinium is acceptable to most chemists.

Actinium has chemical properties like those of lanthanum (number 57), the element just above it in the periodic table. Actinium is also similar to radium, the element just before it (number 88) in Row 7.

Naturally occurring actinium is very rare in the Earth's crust. It can be made in the lab by firing neutrons at radium, but it has very few important uses.




Group 3 (IIIB)
Transition metal


Discovery and naming

Four new elements, all radioactive, were discovered between 1898 and 1900. A radioactive element is one that gives off radiation in the form of energy or particles and may change into a different element. The first two of these elementspolonium and radiumwere discovered by Marie Curie (1867-1934) and Pierre Curie (1859-1906). The third, actinium, was discovered in 1899 by a close friend of the Curies, French chemist André Debierne (1874-1949). Debierne suggested the name actinium for the new element. The name comes from the Greek words aktis or aktinos, meaning "beam" or "ray." The fourth element discovered in this series was radon, a gas given off during the radioactive decay of some heavier elements. It was found in 1900 by German chemist Friedrich Ernst Dorn (1848-1916).

Actinium was discovered a second time in 1902. German chemist Friedrich 0. Giesel (1852-1927) had not heard of Debierne's earlier discovery. Giesel suggested the name emanium, from the word emanation, which means "to give off rays." Debierne's name was adopted, however, because he discovered actinium first.

Physical and chemical properties

Only limited information is available about actinium. It is known to be a silver metal with a melting point of 1,050°C (1,920°F) and a boiling point estimated to be about 3,200°C (5,800°F). The element has properties similar to those of lanthanum. Generally speaking, elements in the same column in the periodic table have similar properties. Few compounds of actinium have been produced. Neither the element nor its compounds have any important uses.

Occurrence in nature

Actinium is found in uranium ores. An ore is a mineral mined for the elements it contains. It is produced by the radioactive decay, or breakdown, of uranium and other unstable elements. Actinium can also be artificially produced. When radium is bombarded with neutrons, some of the neutrons become part of the nucleus. This increases the atomic weight and the instability of the radium atom. The unstable radium decays, gives off radiation, and changes to actinium. Actinium metal of 98 percent purityused for research purposescan be made by this process.


About a dozen isotopes of actinium are known. All are radioactive. The two that occur in nature are actinium-227 and actinium-228. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. A radioactive isotope is one that breaks apart and gives off some form of radiation.

The half lives of actinium-227 and actinium-228 are 21.77 years and 6.13 hours, respectively. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. For example, suppose 1.0 gram of actinium-227 is formed by the breakdown of another element. After 21.77 years, only 0.5 gram of actinium-227 would remain. This is known as the half life.


Actinium is rarely, if ever, extracted from natural sources.


There are no practical commercial uses of actinium. Actinium of 98 percent purity is prepared for research studies.


The few compounds of actinium that are known are used solely for research purposes.

Like all radioactive materials, actinium is a health hazard.

Health effects

Like all radioactive materials, actinium is a health hazard. If taken into the body, it tends to be deposited in the bones, where the energy it emits damages or destroys cells. Radiation is known to cause bone cancer and other disorders.


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melting point: 1,050°C
boiling point: 3,200°C
density: 10.07 g/cm3
most common ions: Ac3+

Actinium has thirty-six isotopes , all of which are radioactive and which range in mass number from 209 to 234. The longest-lived isotope has a mass number of 227 and a half-life of 21.8 years. Actinium was discovered in pitchblende in 1899 by French chemist André-Louis Debierne, a member of the Curie laboratory. He named it actinium, using the Greek word aktis, meaning ray. It was discovered independently by Friedrich Giesel in 1902. Actinium in its ground state has an (outer orbital) electronic configuration of 5f 06d 7s 2. Actinium exists in an oxidation state of 3+ in solution and in its compounds. The isotope 227Ac is found in uranium ores in concentrations of approximately 0.15 mg per ton of pitchblende, and at lower concentrations, in thorium ores. Pure actinium forms a silvery-white metal that has a face-centered structure near its melting point.

In 1945 Glenn Seaborg proposed that actinium was the first member of a family of fifteen elements (the "actinides"), characterized by the possession of the 5f orbitals. His proposal was based on the similarity of the chemistry of actinium to that of lanthanum (atomic number 57), which is the first member of the fifteen elements of the trivalent lanthanide family. Actinium is somewhat more basic than lanthanum but, like lanthanum, forms compounds that have strongly ionic bonds. Many actinium compounds are isostructural with the corresponding compounds of lanthanum, due to the similarities in radii and electronic structures among the two set of compounds. Due to the short half-lives of the isotopes of actinium, no significant uses have been developed for these isotopes.

see also Berkelium; Einsteinium; Fermium; Lawrencium; Mendelevium; Neptunium; Nobelium; Plutonium; Protactinium; Rutherfordium; Seaborg, Glenn Theodore; Thorium; Uranium.

Gregory R. Choppin


Choppin, Gregory R.; Liljenzin, Jan-Olov; and Rydberg, Jan (2001). Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn, MA: Butterworth-Heinemann.


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ac·tin·i·um / akˈtinēəm/ • n. the chemical element of atomic number 89, a radioactive metallic element of the actinide series. It is rare in nature, occurring as an impurity in uranium ores. (Symbol: Ac)


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actinium (symbol Ac) Radioactive, metallic element, the first of the actinide series, discovered (1899) by French chemist André Debierne.It is found associated with uranium ores. Ac227, a decay product of U235, emits beta particles during disintegration. Properties:; r.d.10.07; m.p. 1100°C (1900°F); b.p. 3200°C (5800°F); most stable isotope Ac227 (half-life 21.8 yr).

actinium series

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actinium series See DECAY SERIES.

actinium series

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actinium series See decay series.