Codex @codex  0

An actinide is an element in the series from actinium, atomic number 89, through lawrencium, atomic number 103. The series is associated with occupation of 5f-derived electronic states, although individual ground-state configurations vary.

Variable oxidation states and ionic sizes affect coordination and separation chemistry. Radioactive isotopes add a distinct nuclear aspect to applications involving nuclear fuel and waste materials.

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.