ACTINIDE SERIES, the group of elements, all with similar properties, beginning with actinium (atomic number 89) and ending with lawrencium (atomic number 103). The first four members of the series—actinium, thorium, protactinium, and uranium—occur naturally. The trans-uranium elements (atomic number 93 to 103) are not found in nature but are produced artificially. All elements of the actinide series are radioactive; they decay by emitting alpha particles. In some isotopes of these elements, however, electron capture, spontaneous fission, or emission of beta particles is the predominant mode of decay.

Two isotopes of uranium, $\displaystyle {{U}^{{233}}}$ and $\displaystyle {{U}^{{235}}}$, and au isotope of plutonium, $\displaystyle P{{u}^{{239}}}$, undergo fission when they capture slow neutrons. These isotopes are fuels for nuclear reactors. The much more abundant $\displaystyle T{{h}^{{232}}}$ and $\displaystyle {{U}^{{238}}}$ are not used directly as nuclear fuels, but $\displaystyle T{{h}^{{232}}}$ can be converted to $\displaystyle {{U}^{{233}}}$, and $\displaystyle {{U}^{{238}}}$ can be converted to Pu239. All elements in the actinide series, except uranium and thorium, are so radioactive that special handling precautions are required to avoid health hazards.

Danish physicist Niels Bohr was the first to propose that the transuranium elements might resemble the lanthanide series of elements. He suggested that electrons fill into the inner 5f subshell of the transuranium elements in a fashion similar to the filling of the inner 4f subshell in elements of the lanthanide series. In 1944 the American chemist Glenn T. Seaborg suggested that actinium was the first member of a series that included the transuranium elements. This series is now called the actinide series.

All elements of the lanthanide series and all transplutonium elements of the actinide series form a +3 ion when in an aqueous solution. For this reason, the transplutonium elements closely resemble the lanthanide elements in their chemistry. However, other members of the actinide series form ions of a different charge than + 3. Thorium forms a +4 ion, and protactinium, uranium, neptunium, and plutonium exhibit a variety of ionic charges. This variation in the behavior of these early members of the actinide series is attributable to the relatively, small difîerence in the energy levels of the electrons in the 5f and 6d subshells of these elements.

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