= Lanthanide
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.
<Periodic Table by Royal Society of Chemistry>[Lanthanum through lutetium, atomic numbers57–71; element properties]{ref}
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