On the periodic table, the first member of the actinide series is actinium.
Actinium has atomic number 89 and the symbol Ac. It begins the 15-element actinide series, which continues through lawrencium at atomic number 103. Actinium is a radioactive, silvery metal and is found naturally only in tiny amounts because its isotopes are unstable.
The element was discovered in 1899 by French chemist André-Louis Debierne while he was studying pitchblende residues. Its name comes from the Greek word aktis, meaning ray or beam, referring to its intense radioactivity. Actinium-227 is one of its important naturally occurring isotopes.
Actinium is often confused with lanthanum, which begins the f-block row commonly placed above the actinides. The two names reflect the historical layout of the table, but actinium belongs to the radioactive actinides, while lanthanum is a much more stable lanthanide-related element.