Element Actinium, Ac, Actinide or Actinoid

Actinium was discovered in 1899 in pitchblende residues by Andre-Louis Debierne, Curie's collaborator. The word actinium comes from the Greek aktis, aktinos, meaning radiant, analogous to the radium (ray). In 1901 Fritz Giesel, investigating the fraction, separated from pitchblende, noted the presence of a new radioactive substance, containing rare-earth elements, which were called emanium. This name also emphasized the radioactive properties (from Lat. emanare meaning emanation). In 1904 it was identified as actinium, after which the latter name was confirmed. Actinum is a member of actinide (or actinoid) group of chemical elements.

10 actinium isotopes with mass numbers from 221 to 230 are known. The most long-lived of them is ²²⁷Ac (half-life T_1/2 = 21.8 years) which emits β-particles (98.8%) and α-particles (1.2%). Isotopes ²²⁷Ac and ²²⁸Ac (also called mesothorium, MsThll T_1/2 = 6.13 hours) occur in uranium ores as members of natural radioactive series). Earth's crust surface layer 1.6 km deep contains 11 300 metric tons of Actinum-227. However, comparing with other elements actinium abundance is extremely small, 6x10^-10% by mass.