actinium
noun
ac·tin·i·um
ak-ˈti-nē-əm 
: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende see Chemical Elements Table
Examples of actinium in a Sentence
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Some other elements in nature with no stable isotope are technetium, polonium, actinium and uranium.
—
Kelling Donald, The Conversation, 13 Oct. 2025
This produces both nobelium and element 89, actinium (Ac), which is found at the opposite end of the same elemental group as nobelium on the periodic table.
—David Szondy
august 27, New Atlas, 27 Aug. 2025
Since 2015 a working group at the International Union of Pure and Applied Chemistry has been refereeing a debate over which elements should go in the third column of the table: lanthanum and actinium (elements 57 and 89) or lutetium and lawrencium (71 and 103).
—Stephanie Pappas, Scientific American, 14 May 2024
Other theorists have predicted that hydrides such as calcium hydride or actinium hydride should superconduct at close to room temperature—and at a pressure considerably less than that needed for CSH.
—Robert F. Service, Science | AAAS, 26 Aug. 2021
The lanthanide and actinide series — those two rows floating by themselves, named after their first members, lanthanum and actinium — are isolated like that as a concession to limited space.
—Bill Andrews, Discover Magazine, 26 June 2019
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Kelling Donald, The Conversation, 13 Oct. 2025
This produces both nobelium and element 89, actinium (Ac), which is found at the opposite end of the same elemental group as nobelium on the periodic table.
—David Szondy
august 27, New Atlas, 27 Aug. 2025
Since 2015 a working group at the International Union of Pure and Applied Chemistry has been refereeing a debate over which elements should go in the third column of the table: lanthanum and actinium (elements 57 and 89) or lutetium and lawrencium (71 and 103).
—Stephanie Pappas, Scientific American, 14 May 2024
Other theorists have predicted that hydrides such as calcium hydride or actinium hydride should superconduct at close to room temperature—and at a pressure considerably less than that needed for CSH.
—Robert F. Service, Science | AAAS, 26 Aug. 2021
The lanthanide and actinide series — those two rows floating by themselves, named after their first members, lanthanum and actinium — are isolated like that as a concession to limited space.
—Bill Andrews, Discover Magazine, 26 June 2019
Cite this Entry
“Actinium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/actinium. Accessed 31 Oct. 2025.
Copy CitationKids Definition
actinium
noun
ac·tin·i·um
ak-ˈtin-ē-əm
: a radioactive metallic element found especially in pitchblende see element
Medical Definition
actinium
noun
ac·tin·i·um
ak-ˈtin-ē-əm
: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende
—symbol Ac
see Chemical Elements Table
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