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Periodic Table - Extended Periodic Table
Name, Symbol, Number actinium, Ac, 89
Chemical series actinides
Group, Period, Block 3, 7, f
Appearance silvery
Atomic mass (227) g/mol
Electron configuration [Rn] 6d1 7s2
Electrons per shell 2, 8, 18, 32, 18, 9, 2
Physical properties
Phase solid
Density (near r.t.) 10 g·cm−3
Melting point (circa) 1323  K
(1050 ° C, 1922 ° F)
Boiling point 3471 K
(3198 ° C, 5788 ° F)
Heat of fusion 14 kJ·mol−1
Heat of vaporization 400 kJ·mol−1
Heat capacity (25 °C) 27.2 J·mol−1·K−1
Atomic properties
Crystal structure cubic face centered
Oxidation states 3
(neutral oxide)
Electronegativity 1.1 (Pauling scale)
Ionization energies 1st: 499 kJ/mol
2nd: 1170 kJ/mol
Atomic radius 195 pm
Magnetic ordering no data
Thermal conductivity (300 K) 12 W·m−1·K−1
CAS registry number 7440-34-8
Selected isotopes
Main article: Isotopes of actinium
iso NA half-life DM DE ( MeV) DP
225Ac syn 10 days α 5.935 221Fr
226Ac syn 29.37 hours β- 1.117 226Th
ε 0.640 226Ra
α 5.536 222Fr
227Ac 100% 21.773 years β- 0.045 227Th
α 5.042 223Fr

Actinium ( IPA: /akˈtɪniəm/) is a chemical element in the periodic table that has the symbol Ac and atomic number 89.

Notable characteristics

Actinium is a silvery radioactive metallic element. Due to its intense radioactivity, Actinium glows in the dark with a pale blue light. It is found only in traces in uranium ores as 227Ac, an α and β emitter with a half-life of 21.773 years. One ton of uranium ore contains about a tenth of a gram of actinium.


It is about 150 times as radioactive as radium, making it valuable as a neutron source. Otherwise it has no significant industrial applications.

225Ac is used in medicine to produce 213Bi in a reusable generator or can be used alone as an agent for radio-immunotherapy.


Actinium was discovered in 1899 by André-Louis Debierne, a French chemist, who separated it from pitchblende. Friedrich Otto Giesel independently discovered actinium in 1902. The chemical behaviour of actinium is similar to that of the rare earth lanthanum.

The word actinium comes from the Greek aktis, aktinos, meaning beam or ray.


Actinium is found in trace amounts in uranium ore, but more commonly is made in milligram amounts by the neutron irradiation of 226Ra in a nuclear reactor. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300ºC.


Naturally occurring actinium is composed of 1 radioactive isotope; 227Ac. 36 radioisotopes have been characterized with the most stable being 227Ac with a half-life of 21.772 y, 225Ac with a half-life of 10.0 days, and 226Ac with a half-life of 29.37 h. All of the remaining radioactive isotopes have half-lifes that are less than 10 hours and the majority of these have half lifes that are less than 1 minute. The shortest-lived isotope of actinium is 217Ac which decays through alpha decay and electron capture. It has a half-life of 69 ns. Actinium also has 2 meta states.

Purified 227Ac comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.773-year half-life.

The isotopes of actinium range in atomic weight from 206 amu (206Ac) to 236 amu (236Ac).


227Ac is extremely radioactive, and in terms of its potential for radiation induced health effects, 227Ac is even more dangerous than plutonium. Ingesting even small amounts of 227Ac would present a serious health hazard.

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