Actinium gives its name to the actinide series of the periodic table. The series begins with actinium, element 89, and is usually described as running through lawrencium, element 103.
Actinium was discovered in 1899 by André-Louis Debierne in the radioactive residues left after Marie and Pierre Curie extracted radium from pitchblende. Its name comes from the Greek aktis or aktinos, meaning “ray” or “beam,” a reference to its intense radioactivity.
The actinides are associated mainly with the filling of the 5f electron shell and include familiar elements such as thorium, uranium, plutonium and americium. They are often displayed as a detached row beneath the main periodic table, much like the lanthanides.
A common mix-up is confusing actinium with uranium or thorium, which are also actinides. They do not name the series, however: the family name is based on its first member, just as “lanthanides” is derived from lanthanum.