- 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. (Symbol: Ac)More example sentences
- Along the way, these disintegration series produce radioactive isotopes of protactinium, thorium, actinium, radium, francium, radon, astatine, polonium, bismuth, lead, thallium and mercury.
- 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.
- The first four actinide elements, actinium, thorium, protactinium and uranium are naturally occurring and have been known for a considerable time.
early 20th century: from Greek aktis, aktin- 'ray' + -ium.
More definitions of actiniumDefinition of actinium in:
- The US English dictionary