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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.

Lawrencium is the synthetic radioactive chemical element $\mathrm{Lr}$, with atomic number $103$. It lies at the end of the actinide series; studying it requires producing small quantities of short-lived isotopes, rather than handling an ordinary macroscopic mineral sample.^[ref]

Its listed neutral-atom ground-state electron configuration is $[\mathrm{Rn}]5f^{14}7s^{2}7p^1$. That assignment is a reminder that a simple extrapolation of light-element shell-filling rules can fail for heavy atoms. A neutral-atom configuration also does not by itself determine every accessible chemical oxidation state.