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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.

Lutetium is the chemical element with atomic number $71$ and chemical symbol $\mathrm{Lu}$. It is a dense rare-earth element whose ordinary chemistry is dominated by the $+3$ oxidation state. The neutral atom has a filled $4f$ shell with outer $5d6s^2$ occupancy.^[ref]

Its close chemical similarity to neighbouring rare earths makes separation difficult. This illustrates why identifying a new element and obtaining a pure metallic specimen are different experimental achievements. Element identity is fixed by nuclear charge, whereas similar outer-electron chemistry controls the practical separation.