actinium

[ak-tin-ee-uhm]
noun Chemistry.
a radioactive silver-white metallic element that glows blue in the dark, resembling the rare earths in chemical behavior and valence. Symbol: Ac; atomic number: 89; atomic weight: 227.

Origin:
1900–05; actin- + -ium

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World English Dictionary
actinium (ækˈtɪnɪəm)
 
n
a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07; melting pt: 1051°C; boiling pt: 3200 ± 300°C
 
[C19: New Latin, from actino- + -ium]

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Medical Dictionary

actinium ac·tin·i·um (āk-tĭn'ē-əm)
n.
Symbol Ac
A radioactive element found in uranium ores. Its longest lived isotope is Ac 227 with a half-life of 21.6years. Atomic number 89; melting point 1,050°C; boiling point (estimated) 3,200°C; specific gravity (calculated) 10.07; valence 3.

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Science Dictionary
actinium   (āk-tĭn'ē-əm)  Pronunciation Key 
Symbol Ac
A silvery-white, highly radioactive metallic element of the actinide series that is found in uranium ores. It is about 150 times more radioactive than radium and is used as a source of alpha rays and neutrons. Its most stable isotope has a half-life of about 22 years. Atomic number 89; melting point 1,050°C (1,922°F); boiling point (estimated) 3,200°C (5,792°F); specific gravity (calculated) 10.07; valence 3. See Periodic Table.
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Encyclopedia Britannica
Encyclopedia

actinium

(Ac), radioactive chemical element, in Group IIIb of the periodic table, atomic number 89. Actinium was discovered (1899) by Andre-Louis Debierne in pitchblende residues left after Pierre and Marie Curie had extracted radium and was also discovered (1902) independently by Friedrich Otto Giesel. A ton of pitchblende ore contains about 0.15 mg of actinium. The rare, silvery-white metal is highly radioactive, glowing blue in the dark

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Good ventilation should be provided where radium, thorium, or actinium is stored to prevent build-up of the element.
Francium, the heaviest known member of the alkali metals series, occurs as a result of an alpha disintegration of actinium.
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