Definition of Actinium

Babylon English Dictionary
(Ac) radioactive metallic element (Chemistry)
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Actinium Definition from Language, Idioms & Slang Dictionaries & Glossaries
Webster's Revised Unabridged Dictionary (1913)
(n.)
A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.
  
Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter. About
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actinium
\ac*tin"i*um\ (&?;), n. [gr. &?;, &?;, ray.] (chem.) a supposed metal, said by phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.

WordNet 2.0

Noun
1. a radioactive element of the actinide series; found in uranium ores
(synonym) Ac, atomic number 89
(hypernym) chemical element, element
(substance-holonym) uranium ore
Actinium Definition from Science & Technology Dictionaries & Glossaries
Chemistry of the Elements
Name: actinium
Symbol: Ac
Atomic number: 89
Atomic weight: [ 227 ]
Group in periodic table:
Group name:
Period in periodic table: 7 (actinoid)
Block in periodic table: f-block
CAS registry ID: 7440-34-8

Actinium is dangerously radioactive. The chemical behaviour of actinium is similar to that of the rare earths, particularly lanthanum. It is found naturally in uranium ores and actinium is 150 times more radioactive than radium.
Mineralogy Database
Atomic Mass : 227
Atomic Number : 89
Name Origins : From the Greek, aktinos = "ray."
Year Discovered : 1899
Discovery Credits : Discovered by A. Debierne at Paris, France.
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Actinium Definition from Encyclopedia Dictionaries & Glossaries
Wikipedia English - The Free Encyclopedia
Actinium ( ) is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table.

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Actinium Definition from Entertainment & Music Dictionaries & Glossaries
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