Lutetium

Lutetium closes the lanthanide series with the configuration [Xe]4f14 5d1 6s2. Its 4f shell is full, so its chemistry is almost entirely that of the Lu3+ ion. Across the series, each added proton pulls the poorly shielding 4f electrons inward, a trend called the lanthanide contraction, and lutetium sits at its end: Lu3+ is the smallest lanthanide ion. As a result, lutetium is the densest and hardest lanthanide metal and has the highest melting point of the series, 1663 °C.

For a long time lutetium had few uses because it is scarce and expensive to separate. That changed with nuclear medicine. Lutetium-177 emits short-range beta particles that destroy cells within a few millimetres, plus gamma rays that allow imaging. Attached to molecules that seek out specific tumours, it is the basis of approved therapies for neuroendocrine and prostate cancers. Lutetium silicate crystals are also excellent scintillators and form the detectors in many PET scanners.

Discovery

In 1907 three chemists independently showed that Marignac's ytterbia was a mixture of two oxides: Georges Urbain in Paris, Carl Auer von Welsbach in Austria and Charles James in the United States. Urbain published first and named the new element lutecium after Lutetia, the Roman name of Paris; the spelling became lutetium in 1949. Auer von Welsbach called it cassiopeium, a name German-speaking chemists used until the 1950s.