Two of the actinides found in nature have isotopes with such a long half - life that they have not completely decayed since the Earth was formed. One isotope of thorium, Th-232, has a half - life of 14 billion years and has an abundance of 12 parts per million in the Earth's crust. It is a principal constituent of some minerals, notably thorite and monazite (a mixed rare-earth and thorium phosphate). Three isotopes of uranium are found in nature. Their isotopic abundances and half-lives are U-234, 0.006%, 230,000 years; U-235, 0.72%, 696 million years; and U-238, 99.27%, 4.51 billion years. Some laboratory produced isotopes of uranium also have long halflives. The overall abundance of uranium in the Earth's crust is about 4 parts per million, and it is concentrated in many minerals, principally pitchblende, autunite, torbernite , and carnotite.

Deposits of uranium minerals large enough to be profitably mined are found principally in parts of Africa and in Canada, the Soviet Union, and the southwestern United States. Actinium and protactinium, as well as some isotopes of thorium and uranium, are found in nature as decay products of Th-232, U-235, or U - 238. All of the heavier actinide elements, the transuranium elements, as well as some isotopes of the lighter actinides, have been synthesized since 1940. Small amounts of neptunium-239 and plutonium-239 have been found in uranium ores; they are produced by the absorption of neutrons generated by the spontaneous fission of U238. Discovery of Actinide Series. Many chemists at first considered actinium to be chemically similar to lanthanum, thorium to hafnium, protactinium to tantalum, and uranium to tungsten. Neils Bohr suggested in 1923, however, that actinium might begin a series of elements similar to the series of rare earth elements and filling the 5f subshell, parallel to the lanthanide series, which fills the 4f subshell.

In 1944 Glenn Seaborg and co-workers hypothesized that the elements following uranium would indeed parallel the lanthanides. These predictions were correct, and many of the actinides are in fact chemically similar to the lanthanides-- for example, the chemistry of curium is much like that of gadolinium. All of the actinide elements are shiny, hard metals that tarnish in air and are so electropositive that they are difficult to reduce from their compounds. Oxides have been prepared for all the elements through einsteinium the elements thorium through berkelium have stable dioxides. Thorium, uranium, and plutonium dioxides are used in nuclear reactors. Because U-235 and Pu-239 undergo nuclear fission when they absorb neutrons, they are used in nuclear reactors and in nuclear weapons. The actinide elements are among the most prevalent of radioactive waste products from nuclear reactors. Many compounds of the actinide elements have been studied,often using only a few micrograms because of the elements' scarcity and intense radioactivity.