A lanthanide is an element in the series from lanthanum, atomic number 57, through lutetium, atomic number 71. The progression is associated with filling the 4f shell and the systematic size change called the lanthanide contraction.
Many lanthanides favour trivalent ions with similar coordination chemistry. Their similarity makes separation demanding, while differences in ionic size and accessible oxidation states permit selectivity.
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Periodic Table by Royal Society of Chemistry. Lanthanum through lutetium, atomic numbers57–71; element properties
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