Actinium is a soft, silvery-white radioactive metallic element, the namesake of the actinide series.
Core Principles & Analysis
Andre-Louis Debierne identified actinium in pitchblende residues in 1899, naming it from the Greek 'aktis' (ray); Friedrich Giesel independently found it in 1902 and called it emanium.
Actinium-227, the longest-lived isotope at 21.8 years, is so intensely radioactive that concentrated samples glow pale blue as their radiation ionizes the surrounding air.
Actinium-225, with a 10-day half-life, is a leading isotope for targeted alpha therapy against cancer, and its scarce worldwide supply is a major bottleneck for clinical trials.