From New World Encyclopedia

89 radiumactiniumthorium


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/cm³
Melting point (circa) 1323 K
(1050 °C, 1922 °F)
Boiling point 3471 K
(3198 °C, 5788 °F)
Heat of fusion 14 kJ/mol
Heat of vaporization 400 kJ/mol
Heat capacity (25 °C) 27.2 J/(mol·K)
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·K)
CAS registry number 7440-34-8
Notable 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 (chemical symbol Ac, atomic number 89) is a silvery, radioactive, metallic element. Due to its intense radioactivity, it glows in the dark with a pale blue light. It is about 150 times as radioactive as radium and is therefore a valuable source of neutrons. Its isotope 225Ac is used in nuclear medicine.

Occurrence and production

Actinium is found in trace amounts in uranium ore, but it is more commonly 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 1300 ºC.

Etymology and history

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

Actinium was discovered in 1899 by André-Louis Debierne, a French chemist, who separated it from pitchblende. Friedrich Oskar Giesel independently discovered actinium in 1902.

Notable characteristics

In the periodic table, actinium is situated in period seven and group three. It lies just below the rare earth lanthanum, and its chemical behavior is similar to that of lanthanum.

Actinium is often considered as the first of the actinides, which are a group of "inner transition metals." From the standpoint of their electronic structure, the actinides are characterized by the gradual filling of the 5f subshell. Actinium, however, has no electrons in its 5f subshell, and it best fits with the elements of group three.

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.


Naturally occurring actinium is composed of one 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 ten days, and 226Ac with a half-life of 29.37 h. All of the remaining radioactive isotopes have half-lives that are less than ten hours and the majority of these have half lives that are less than one 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 two 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).


Actinium 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.


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.

See also

ISBN links support NWE through referral fees

  • Actinium Los Alamos National Laboratory. Retrieved September 12, 2007.
  • Chang, Raymond. Chemistry (9th ed.) New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry, (4th ed.), New York: Wiley, 1980. ISBN 0-471-02775-8
  • Greenwood, N.N. and A. Earnshaw. Chemistry of the Elements, (2nd Edition). Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654 Online version available at Knovel Retrieved September 12, 2007.

External links

All links retrieved June 15, 2023.


New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.