= Lutetium
<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.<Periodic Table by Royal Society of Chemistry>[Lutetium: atomic data and uses]{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.
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