Actinides ($5f$ series, from $Th$ to $Lr$) are elements with high atomic numbers.

• Characteristic: All actinides are radioactive.
• Nuclei are unstable and undergo spontaneous decay. While early members like Thorium ($Th$) and Uranium ($U$) have long half-lives and occur in nature, the later members are synthetic (transuranium elements) and extremely short-lived.