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Actinium facts for kids

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General properties
Name, symbol, number actinium, Ac, 89
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 in actinide group. 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.


Actinium is a soft, silvery-white, radioactive metal.

Because its strong radioactivity, actinium glows in the dark with a pale blue light.

Actinium reacts quickly with oxygen and moisture in air forming a white coating (actinium oxide) that stops the actinium from oxidizing.


Actinium is only found in very tiny amounts in uranium and thorium ores.


Actinium is now being studied for use in cancer treatments. It can be used in radiation therapy.

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