The actinides are the elements with atomic numbers 89 (actinium) through 103 (Lawrencium). With each unit increase in atomic number, the atom of each succeeding actinium element has one additional 5f electron. Thus, the actinium series of fourteen elements is homologous with the lanthanide series for fourteen elements in which each successive element in the series has one additional 4f electron. Like the lanthanide elements, the actinides have very similar chemical properties. And like the lanthanides, the most common oxidation state in aqueous solution is 3+. In oxide and fluoride compounds, among others, the actinide elements are found in higher oxidation states, from IV to VII as well as III.

The first four actinide elements, actinium (atomic number 89), thorium (90), protactinium (91) and uranium (92) are naturally occurring and have been known for a considerable time. The dates of discovery of these elements as 1789 for uranium, 1829 for thorium, 1899 for actinium and 1913 for protactinium. The...