Actinides
Actinides (Z = 89 to 103) make up the second f-block. All radioactive, dominated by uranium and plutonium, they are at the heart of nuclear energy and heavy astrochemistry.
Actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawrencium. Only Th and U exist naturally in significant quantities (~4 ppm and 2.7 ppm in the crust); all the others are either ultra-trace or entirely synthetic.
Their chemistry is dominated by states +3 to +6 (especially +4 and +6 for U and Pu), with a richness comparable to the transition metals. The isotopes U-235 and Pu-239 are fissile under slow neutrons — the basis of civilian and military nuclear energy.
Beyond uranium, the synthetic actinides are produced in tiny quantities in reactors or accelerators, for fundamental research and a few specialised applications (dating, smoke detectors with Am-241).