Actinoids

The fourteen elements immediately following actinium -- running from thorium (Th, atomic number 90) through to lawrencium (Lr, atomic number 103) -- are collectively termed the actinoids, directly analogous in periodic-table role to the lanthanoids already studied in sections 4.5.1 through 4.5.4, but now involving the 5f rather than the 4f sub-shell. Unlike the lanthanoids, however, EVERY SINGLE actinoid is radioactive, and most of them additionally have very short half-lives, making their bulk chemistry considerably harder to study experimentally than lanthanoid chemistry.

Of the fourteen actinoids, only thorium (Th) and uranium (U) occur in nature in genuinely significant quantities, and trace amounts of plutonium (Pu) are additionally found naturally within uranium ores (formed there by neutron-capture nuclear reactions occurring within the ore body itself over geological time). Neptunium (Np) and every actinoid heavier than neptunium are, by contrast, produced entirely synthetically, through deliberate artificial nuclear transformation of naturally occurring elements (typically by neutron bombardment in a nuclear reactor, or by charged-particle bombardment in a particle accelerator) -- none of these heavier actinoids has ever been found to occur naturally on Earth in any meaningful quantity. …