#### 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.