Actinium facts for kids

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General properties
Name, symbol, number actinium, Ac, 89
Pronunciation /ækˈtɪniəm/
Element category actinide
Category notes sometimes considered a transition metal
Group, period, block n/a, 7, f
Standard atomic weight (227) g/mol
Electron configuration [Rn] 6d1 7s2
Electrons per shell 2, 8, 18, 32, 18, 9, 2 (Image)
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
Specific heat capacity (25 °C) 27.2 J/(mol·K)
Atomic properties
Oxidation states 3
(neutral oxide)
Electronegativity 1.1 (Pauling scale)
Ionization energies 1st: 499 kJ/mol
2nd: 1170 kJ/mol
Covalent radius 215 pm
Crystal structure face-centered cubic
Magnetic ordering no data
Thermal conductivity (300 K) 12 W/(m·K)
CAS registry number 7440-34-8
Most stable isotopes
Main article: Isotopes of actinium
iso NA half-life DM DE (MeV) DP
225Ac trace 10 d α 5.935 221Fr
226Ac syn 29.37 h β 1.117 226Th
ε 0.640 226Ra
α 5.536 222Fr
227Ac trace 21.772 y β 0.045 227Th
α 5.042 223Fr

Actinium is chemical element 89 on the periodic table. Its symbol is Ac. Actinium's mass is 227 g/mol.

Actinium is a silver radioactive, solid metal. It is so radioactive that it glows in the dark. Even a small amount of actinium is dangerous to people.

Actinium was discovered in 1899 by André-Louis Debierne, a French chemist. In 1899, Debierne described the substance as similar to titanium and (in 1900) as similar to thorium.

Notable characteristics

Uraninite ores have elevated concentrations of actinium.

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. 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. Actinium is used in medicine to produce bismuth 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 Oskar Giesel independently discovered actinium in 1902 and called it "emanium" in 1904. Debierne's name was retained because it had seniority. The chemical behavior of actinium is similar to that of the rare earth lanthanum.

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

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