A chemical element, Lu, atomic number 71, atomic weight 174.97, a very rare metal and the heaviest member of the rare-earth group. The naturally occurring element is made up of the stable isotope ¹⁷⁵Lu, 97.41%, and the long-life β-emitter ¹⁷⁶Lu with a half-life of 2.1 × 10¹⁰ years.