In 1794 Johan Gadolin, a Finnish chemist, isolated yttria from a mineral that had recently been discovered at Ytterby, a village near Stockholm in Sweden. At first Gadolin believed that yttria was the oxide of a single new element, but it was subsequently found to consist of no fewer than ten new elements - yttrium, terbium, erbium, ytterbium, scandium, holmium, thulium, gadolinium, dysprosium and lutetium.
Shortly after Gadolin’s discovery, Martin Heinrich Klaproth and, independently, Jons Jacob Berzelius and W. Hisinger, isolated another new oxide – ceria. This was later shown to contain oxides of cerium, lanthanum, praseodymium, neodymium, samarium and europium.
Thorium, protactinium and uranium are the only naturally occurring actinide elements. In 1789 Klaproth showed that pitchblende, thought previously to be a mixture of zinc, iron and tungsten oxides, also contained the oxide of a new element which he called uranium. 39 years later Berzelius discovered thoria, a new oxide from which he subsequently isolated thorium. Protactinium was not discovered until 1913, when Kasimir Fajans and O. Göhring identified 234Pa as a member of the 238U radioactive decay series. This isotope of protactinium is short-lived (half-life of 6.7 hours), but the more stable 231Pa, identified in 1916 by Otto Hahn, Lise Meitner, F. Soddy and J. A. Cranston has a half-life of 32,760 years.
None of the other actinide elements occur naturally, and must be synthesised by nuclear reactions.
"The f elements", N. Kaltsoyannis and P. Scott
"Inorganic Chemistry", D.F. Shriver and P.W. Atkins