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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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
Those protons smashing and staying put creates the right kind of actinium through a process called spallation.
—Sarah Scoles, Scientific American, 13 Feb. 2024
For years, scientists have produced this species of actinium by waiting for thorium (element 90, with one extra proton than actinium) to decay.
—Sarah Scoles, Scientific American, 13 Feb. 2024
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
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
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Stephanie Pappas, Scientific American, 14 May 2024
Those protons smashing and staying put creates the right kind of actinium through a process called spallation.
—Sarah Scoles, Scientific American, 13 Feb. 2024
For years, scientists have produced this species of actinium by waiting for thorium (element 90, with one extra proton than actinium) to decay.
—Sarah Scoles, Scientific American, 13 Feb. 2024
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
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
Cite this Entry
“Actinium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/actinium. Accessed 8 Jul. 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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