Compared to the lanthanides, which (except for promethium) are found in nature in appreciable quantities, most actinides are rare.
The majority of them do not even occur in nature, and of those that do, only thorium and uranium do so in more than trace quantities.
The advantage of the second method is that elements heavier than plutonium, as well as neutron-deficient isotopes, can be obtained, which are not formed during neutron irradiation.
In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six underground nuclear explosions.
All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium.
The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any f-electrons, and curium and lawrencium have the same number as the preceding element.
While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.
Actinium was discovered in 1899 by André-Louis Debierne, an assistant of Marie Curie, in the pitchblende waste left after removal of radium and polonium.
The actinide series derives its name from the first element in the series, actinium.
The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.
Like the lanthanides, the actinides form a family of elements with similar properties.
Within the actinides, there are two overlapping groups: transuranium elements, which follow uranium in the periodic table—and transplutonium elements, which follow plutonium.