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From New World Encyclopedia
Popular Article: Inner transition metal

Actinide phases.gif
The inner transition metals are two series of elements known as the lanthanides and actinides. They are usually shown below all the other elements in the standard view of the periodic table, but they really belong to periods 6 and 7. The lanthanide series consists of the 14 elements, cerium through lutetium (atomic numbers 58–71) immediately following lanthanum. Likewise, the actinide series consists of the 14 elements, thorium through lawrencium (atomic numbers 90–103), immediately following actinium. These elements were among the last to be discovered and placed in the periodic table. Many of the actinides do not occur naturally but were synthesized through nuclear reactions. Chemically, the elements within each series (especially the lanthanides) are very similar to one another.