The actinides are the elements with atomic numbers 89 (actinium) through 103 (Lawrencium). With each unit increase in atomic number, the atom of each succeeding actinium element has one additional 5f electron. Thus, the actinium series of fourteen elements is homologous with the lanthanide series for fourteen elements in which each successive element in the series has one additional 4f electron. Like the lanthanide elements, the actinides have very similar chemical properties. And like the lanthanides, the most common oxidation state in aqueous solution is 3+. In oxide and fluoride compounds, among others, the actinide elements are found in higher oxidation states, from IV to VII as well as III.
The first four actinide elements, actinium (atomic number 89), thorium (90), protactinium (91) and uranium (92) are naturally occurring and have been known for a considerable time. The dates of discovery of these elements as 1789 for uranium, 1829 for thorium, 1899 for actinium and 1913 for protactinium. The next two elements in the actinide series, neptunium (93) and plutonium (94) were discovered as products of nuclear fission (see Fission) experiments. Neptunium (93), the first synthetic element, was discovered in 1940 and plutonium (94) discovered shortly afterward in the same year. Many different isotopes of the remaining elements in the series were produced purposefully using neutron sources of particle accelerators of various types over the following two decades: americium (95) and curium (96) in 1944, berkelium (97) in 1949, californium (98) in 1950, einsteinium (99) and fermium (100) in 1952, mendelevium (101) in 1955, nobelium (102) in 1957 and lawrencium (103) in 1961. The privilege of naming a new element has traditionally been accorded to the first group of scientists that can convince their colleagues that they have successfully produced the element experimentally. As might be surmised from the names of the actinides, credit for their synthesis was given principally the group of scientists led by Glenn Seaborg at the University of California at Berkeley and later the Lawrence Livermore Laboratories.
Two of the actinides, thorium and uranium, are found in relatively high abundance naturally and can often be detected by standard chemical techniques. The other actinides have very low or no known natural abundance and must be analyzed by highly sensitive radiation detection techniques.
Two isotopes of actinides, uranium-235 and plutonium-239, are used extensively in nuclear fission power generators and are also used in nuclear weapons. Tons of plutonium-239 have been produced in fission reactors used for providing electricity. Plutonium-239 poses a great challenge for storage because it is highly toxic due to its intense alpha particle emission and it has a very long half-life of nearly 25,000 years. Because the technology for fabricating nuclear weapons is now so widespread and many nations would be capable of manufacturing these weapons if they have access to uranium-235 or plutonium-239, the quantities of these isotopes that are available also pose a security challenge.
Other isotopes of the actinide elements that have found application are Pu-238 which is used to power heart pacemakers and space instrumentation, americium-241 which is used in smoke detectors, and californium-252 which is used in analytical techniques that involve production of isotopes of other elements by neutron-capture.
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