= 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.<Periodic Table by Royal Society of Chemistry>[Actinium: atomic data, natural abundance and history]{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.
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