actinium

noun
ac·​tin·​i·​um | \ak-ˈti-nē-əm \

Definition of actinium 

: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende — see Chemical Elements Table

First Known Use of actinium

1900, in the meaning defined above

History and Etymology for actinium

borrowed from French, from Greek aktīn-, aktís "ray, beam" + -ium -ium; so called from its radioactivity — more at actin-

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The first known use of actinium was in 1900

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actinium

noun
ac·​tin·​i·​um | \ak-ˈtin-ē-əm \

Medical Definition of actinium 

: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende symbol Ac — see Chemical Elements Table

More from Merriam-Webster on actinium

Britannica.com: Encyclopedia article about actinium

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