'actinium' definitions:

Definition of 'actinium'

From: WordNet
noun
A radioactive element of the actinide series; found in uranium ores [syn: actinium, Ac, atomic number 89]

Definition of 'actinium'

From: Elements
  • Symbol: Ac
  • Atomic number: 89
  • Atomic weight: (227)
  • Silvery radioactive metallic element, belongs to group 3 of the periodic table. The most stable isotope, Ac-227, has a half-life of 217 years. Ac-228 (half-life of 6.13 hours) also occurs in nature. There are 22 other artificial isotopes, all radioactive and having very short half-lives. Chemistry similar to lanthanum. Used as a source of alpha particles. Discovered by A. Debierne in 1899.