Formula for radioactive carbon dating
Of the actinides, primordial thorium and uranium occur naturally in substantial quantities.
The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.
Small samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with mass number greater than 257 could be detected, despite predictions that such isotopes would have relatively long half-lives of α-decay.
This inobservation was attributed to spontaneous fission owing to the large speed of the products and to other decay channels, such as neutron emission and nuclear fission.
In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence.
They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties.
At present, there are two major methods of producing isotopes of transplutonium elements: (1) irradiation of the lighter elements with either neutrons or (2) accelerated charged particles.
The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.
By reduction of thorium tetrachloride with potassium, he isolated the metal and named it thorium after the Norse god of thunder and lightning Thor.
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.
By 1944 an observation that curium failed to exhibit oxidation states above 4 (whereas its supposed 6th period homolog, platinum, can reach oxidation state of 6) prompted Glenn Seaborg to formulate a so-called "actinide hypothesis".
Studies of known actinides and discoveries of further transuranic elements provided more data in support of this point of view, but the phrase "actinide hypothesis" (the implication being that a "hypothesis" is something that has not been decisively proven) remained in active use by scientists through the late 1950s.