Actinides (Encyclopedia of Science)
The actinides (or actinoids) are the chemical elements with atomic numbers between 90 and 109 inclusively. (An atomic number indicates the number of protons in an atom.) Actinides occur between Groups 3 and 4 in Period 7 of the periodic table. All elements in this family are radioactive (that is, they spontaneously release subatomic particles or energy as their nuclei decay). Five actinides have been found in nature: thorium, protoactinium, uranium, neptunium, and plutonium. The other actinides have been produced artificially in nuclear reactors or particle accelerators (atom-smashers).
For many years, the list of chemical elements known to scientists ended with number 92, uranium. Scientists were uncertain as to whether elements heavier than uranium would ever be found. Then, in 1940, a remarkable discovery was made while University of California physicists Edwin McMillan (1907991) and Philip Abelson (1913) were studying nuclear fission. (Nuclear fission is the splitting of an atomic nucleus, a process that releases large amounts of energy. Atomic bombs and nuclear power plants operate on nuclear fission.) During their research, the duo found evidence for the existence of a new element with atomic number 94, two greater than that of uranium.
This new element was the first transuranium (heaver than uranium) element ever discovered....
(The entire section is 1097 words.)
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