Actinium

Actinium is a radioactive metallic chemical element, symbol $\mathrm{Ac}$ and atomic number $89$. Its common chemistry is trivalent, and it is associated with the beginning of the actinide series. Naturally occurring actinium is present in trace quantities in uranium decay chains.^[ref]

Radioactive abundance depends on both production and decay. A daughter isotope can remain rare even when continually replenished by its parent, so chemical separation and radiometric identification answer complementary questions about a sample.