Ac → Lr
The lighter actinides occur naturally to varying degrees, while many heavier members are produced mainly in reactors or laboratories. Their nuclei are unstable and transform through radioactive decay.
Uranium is the best-known member because some isotopes can sustain nuclear fission chain reactions. Thorium has also been studied as a possible nuclear fuel. Plutonium has important nuclear applications and requires strict controls.
Their 5f electrons give actinides a complex chemistry with several oxidation states and unusual bonding. This series is one of the clearest places where chemistry and nuclear physics meet.