Skip to Main Content

About Actinium

Actinium Bohr

Actinium, a silver-white metallic element, is the namesake of the actinides group in the periodic table, a series of elements known for their radioactivity. Due to its radioactive nature and relative scarcity on Earth, actinium has not historically enjoyed many commercial or industrial uses or applications; however, that may be changing with ongoing scientific research into both medicine and spacecraft power systems.

The element was discovered in 1899 by French chemist André-Louis Debierne, who made his discovery by isolating residue left by Marie and Pierre Curie in their extraction of radium from uranium ore. In fact, actinium is so scarce, only 0.2 mg of the element can be extracted from one ton of uranium ore. Because of this scarcity, the element is usually obtained by irradiating a radium isotope (226Ra) with neutrons in a nuclear reactor. The resulting actinium then becomes a neutron source of its own, and can be used for targeted radiation therapy in cancer treatments. It is currently becoming a preferred element in medical research for this purpose due to its high radioactivity – roughly 150 times that of radium. This high radioactivity is also attractive to spacecraft designers, who may pursue actinium as the active element in future radioisotope thermoelectric generators.

Actinium rapidly oxidizes in the presence of oxygen and moisture, and it is in this state where the vast majority of its chemical compounds occur. The oxide of 227Ac pressed with beryllium, often referred to as AcBe, is also an efficient neutron source with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs. AcBe-based neutron probes are used to measure water presence and density in soil, in neutron radiography, and in other radiochemical testing applications. Though these compounds are readily achievable in the laboratory, they are typically used solely for research purposes with few commercial applications.

The only naturally occurring isotope of actinium is 227Ac. Thirty-six radioisotopes of actinium have been identified, all with half-lives ranging from 69 ns at the shortest (for 217Ac) to 21.77 years at the longest (227 Ac). Due to its convenient half-life attributes, the presence of 227Ac in oceanic waters is utilized as an estimate to model and calculate vertical mix rates. Over time, actinium decays into thorium and francium, with beta decay dominating over alpha decay by a factor of roughly 71:1.

Actinium Properties

Actinium Bohr Model Actinium is an F-Block, Period 7 element. The number of electrons in each of Actinium's shells is 2,8,18,32,18,9,2 and its electron configuration is [Rn] 6d1 7s2. Actinium The actinium atom has a radius of 195 pm. In its elemental form, CAS 7440-34-8, actinium has a silvery-white appearance. Actinium is a radioactive metal that rapidly reacts rapidly with oxygen when exposed to air, forming a white coating of actinium oxide which prevents further oxidation. Actinium is found naturally in uranium ores. It rarely occurs as a free element in the earth’s crust. It is more frequently produced in the lab, largely for use as a radiation source. Actinium was first noted as a new element by a French chemist, André-Louis Debierne, in 1899. It was discovered again as an independent element in 1902 by German chemist Friedrich Oskar Giesel.

Actinium information, including properties, research, applications and other useful facts are discussed below. Scientific facts such as the atomic structure, ionization energy, abundance on Earth, conductivity and thermal properties are included.

Symbol: Ac
Atomic Number: 89
Atomic Weight: 227
Element Category: Actinide
Group, Period, Block: n/a, 7, f
Color: silvery white
Other Names: Attinio
Melting Point: 1050 °C, 1922°F, 1323.15 K
Boiling Point: 3200 °C, 5792 °F, 3473.15 K
Density: 10060 kg·m3
Liquid Density @ Melting Point: N/A
Density @ 20°C: 10.07 g/cm3
Density of Solid: 10070 kg·m3
Specific Heat: N/A
Superconductivity Temperature: N/A
Triple Point: N/A
Critical Point: N/A
Heat of Fusion (kJ·mol-1): 14
Heat of Vaporization (kJ·mol-1): 400
Heat of Atomization (kJ·mol-1): 406
Thermal Conductivity: 12 W·m-1·K-1
Thermal Expansion: N/A
Electrical Resistivity: N/A
Tensile Strength: N/A
Molar Heat Capacity: N/A
Young's Modulus: N/A
Shear Modulus: N/A
Bulk Modulus: N/A
Poisson Ratio: N/A
Mohs Hardness: N/A
Vickers Hardness: N/A
Brinell Hardness: N/A
Speed of Sound: N/A
Pauling Electronegativity: 1.1
Sanderson Electronegativity: N/A
Allred Rochow Electronegativity: 1
Mulliken-Jaffe Electronegativity: N/A
Allen Electronegativity: N/A
Pauling Electropositivity: 2.9
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 89
Protons: 89
Neutrons: 139
Electron Configuration: [Rn] 6d1 7s2
Atomic Radius: 215 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: N/A
Covalent Radius (Å): 2.01
Van der Waals Radius: N/A
Oxidation States: 3 (neutral oxide)
Phase: Solid
Crystal Structure: Cubic
Magnetic Ordering: N/A
Electron Affinity (kJ·mol-1) 33.77
1st Ionization Energy: 499 kJ·mol-1
2nd Ionization Energy: 1170 kJ·mol-1
3rd Ionization Energy: N/A
CAS Number: 7440-34-8
EC Number: N/A
MDL Number: N/A
Beilstein Number: N/A
SMILES Identifier: [Ac]
InChI Identifier: InChI=1S/Ac
PubChem CID: 23965
ChemSpider ID: 22404
Earth - Total: N/A
Mercury - Total: N/A
Venus - Total: N/A
Earth - Seawater (Oceans), ppb by weight: N/A
Earth - Seawater (Oceans), ppb by atoms: N/A
Earth -  Crust (Crustal Rocks), ppb by weight: N/A
Earth -  Crust (Crustal Rocks), ppb by atoms: N/A
Sun - Total, ppb by weight: N/A
Sun - Total, ppb by atoms: N/A
Stream, ppb by weight: N/A
Stream, ppb by atoms: N/A
Meterorite (Carbonaceous), ppb by weight: N/A
Meterorite (Carbonaceous), ppb by atoms: N/A
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: N/A
Universe, ppb by atom: N/A
Discovered By: André-Louis Debierne
Discovery Date: 1899
First Isolation: Carl Gustaf Mosander (1839)

Actinium Isotopes

Actinium has no stable isotopes.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
206Ac 206.01450(8) 25(7) ms Unknown (3+) N/A 1555.03 -
207Ac 207.01195(6) 31(8) ms [27(+11-6) ms] a to 203Fr 9/2-# N/A 1563.11 -
208Ac 208.01155(6) 97(16) ms [95(+24-16) ms] a to 204Fr; ß+ to 208Ra (3+) N/A 1571.19 -
209Ac 209.00949(5) 92(11) ms a to 205Fr; ß+ to 209Ra (9/2-) N/A 1588.59 -
210Ac 210.00944(6) 350(40) ms a to 206Fr; ß+ to 210Ra 7+# N/A 1596.66 -
211Ac 211.00773(8) 213(25) ms a to 207Fr; ß+ to 211Ra 9/2-# N/A 1604.74 -
212Ac 212.00781(7) 920(50) ms a to 208Fr; ß+ to 212Ra 6+# N/A 1612.82 -
213Ac 213.00661(6) 731(17) ms a to 209Fr; ß+ to 213Ra (9/2-)# N/A 1620.9 -
214Ac 214.006902(24) 8.2(2) s a to 210Fr; ß+ to 214Ra (5+)# N/A 1628.98 -
215Ac 215.006454(23) 0.17(1) s a to 211Fr; ß+ to 215Ra 9/2- N/A 1637.06 -
216Ac 216.008720(29) 0.440(16) ms a to 212Fr; ß+ to 216Ra (1-) N/A 1645.14 -
217Ac 217.009347(14) 69(4) ns a to 213Fr; ß+ to 217Ra 9/2- N/A 1653.22 -
218Ac 218.01164(5) 1.08(9) µs a to 214Fr (1-)# N/A 1651.98 -
219Ac 219.01242(5) 11.8(15) µs a to 215Fr; ß+ to 219Ra 9/2- N/A 1660.06 -
220Ac 220.014763(16) 26.36(19) ms a to 216Fr; ß+ to 220Ra (3-) N/A 1668.14 -
221Ac 221.01559(5) 52(2) ms a to 217Fr 9/2-# N/A 1676.21 -
222Ac 222.017844(6) 5.0(5) s a to 218Fr; ß+ to 222Ra 1- N/A 1684.29 -
223Ac 223.019137(8) 2.10(5) min a to 219Fr; EC to 223Ra (5/2-) N/A 1692.37 -
224Ac 224.021723(4) 2.78(17) h ß- to 224Th; a to 220Fr; EC to 224Ra 0- N/A 1691.13 -
225Ac 225.023230(5) 10.0(1) d a to 221Fr (3/2-) N/A 1699.21 -
226Ac 226.026098(4) 29.37(12) h ß- to 226Th; a to 222Fr; EC to 226Ra (1)(-#) N/A 1707.29 -
227Ac 227.0277521(26) 21.772(3) y ß- to 227Th; a to 224Fr 3/2- 1.1 1715.37 -
228Ac 228.0310211(27) 6.15(2) h ß- to 228Th 3+ N/A 1714.13 -
229Ac 229.03302(4) 62.7(5) min ß- to 229Th (3/2+) N/A 1722.21 -
230Ac 230.03629(32) 122(3) s ß- to 230Th (1+) N/A 1730.29 -
231Ac 231.03856(11) 7.5(1) min ß- to 231Th (1/2+) N/A 1738.37 -
232Ac 232.04203(11) 119(5) s ß- to 232Th (1+) N/A 1737.13 -
233Ac 233.04455(32)# 145(10) s ß- to 233Th (1/2+) N/A 1745.21 -
234Ac 234.04842(43)# 44(7) s ß- to 234Th N/A N/A 1753.29 -
235Ac 235.05123(38)# 40# s ß- to 235Th 1/2+# N/A 1752.05 -
236Ac 236.05530(54)# 2# min ß- to 236Th N/A N/A 1760.13 -