Definition of actinium in English:



mass noun
  • The chemical element of atomic number 89, a radioactive metallic element of the actinide series. It is rare in nature, occurring as an impurity in uranium ores.

    • ‘The first four actinide elements, actinium, thorium, protactinium and uranium are naturally occurring and have been known for a considerable time.’
    • ‘Actinides - the radioactive chemical elements that span from actinium to lawrencium on the periodic table - have generated a great deal of interest in recent years.’
    • ‘A proposal was patented for a ceramic block pessary using radioactive elements such as thorium, actinium and even radium.’
    • ‘Francium was discovered in 1939 by the French physicist Marguerite Perey while she was analyzing the products formed during the radioactive decay of actinium.’
    • ‘Along the way, these disintegration series produce radioactive isotopes of protactinium, thorium, actinium, radium, francium, radon, astatine, polonium, bismuth, lead, thallium and mercury.’


Early 20th century: from Greek aktis, aktin- ‘ray’ + -ium.