Lutetium – facts and properties

Position in the periodic table

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Occurrence and extraction

Lutetium is the last of the fifteen lanthanides and one of the heavy rare earths introduced on the lanthanum page. In the Earth's crust it is more common than cadmium or antimony, but it occurs only as a minute companion in monazite and xenotime. Separating it from ytterbium is particularly laborious, because the two neighbours resemble each other almost down to the last detail. Since only small amounts are needed, lutetium counts among the costly rare-earth metals.

Three claimants shared one year. In 1907 the Frenchman Georges Urbain showed in Paris that the ytterbium described by Marignac almost three decades earlier consisted of two elements. He named the second one lutecium, after Lutetia. At almost the same moment, the Austrian Carl Auer von Welsbach in Vienna and the American Charles James reached the same result. Auer called the element cassiopeium, and in Germany that name stayed in use for a while. In the end Urbain's name prevailed, spelt lutetium.

Properties and reactions

Lutetium is the fifteenth lanthanide. In period 6, ytterbium stands on its left, and in the main body of the table hafnium follows on the right. Because the 4f subshell is already full, the next electron goes into the 5d subshell. The configuration [Xe] 4f¹⁴ 5d¹ 6s² therefore resembles that of scandium and yttrium. This is why people debate whether lutetium belongs below yttrium in the table, and some tables show it there. Both layouts are in use.

With lutetium the lanthanide contraction, described on the lanthanum page, comes to an end. The Lu³⁺ ion is about a sixth smaller than the La³⁺ ion. The atoms sit tightly packed in the metal, with an atomic radius of 173 pm. That leads to the highest values of the row: a density of 9.84 g/cm³, a melting point of 1663 °C and the greatest hardness. The boiling point is 3402 °C and the electronegativity is 1.27.

Chemically, lutetium is a typical trivalent rare-earth metal. The Lu³⁺ ion has a completely filled 4f shell and therefore forms colourless salts, and lutetium oxide, Lu₂O₃, is white. The metal is fairly stable in dry air, tarnishes in moist air and reacts slowly with water.

Uses in everyday life and technology

An important technical use is in PET scanners. They contain scintillator crystals of lutetium yttrium oxyorthosilicate, LYSO for short, doped with a little cerium. In PET a tracer releases positrons, which annihilate with electrons and send two gamma quanta in opposite directions. The dense crystal reliably stops this radiation and turns it into a very brief flash of light that light sensors register. From the places and times of the flashes, a picture of the metabolism in the body is built up.

In cancer medicine the isotope lutetium-177 is used. It is made in a nuclear reactor, has a half-life of about a week and emits beta radiation of short range, at most about two millimetres in tissue, plus some gamma radiation for imaging. In radioligand therapy the isotope is attached to a molecule that binds specifically to tumour cells: in advanced, metastatic prostate cancer to a protein called PSMA, in neuroendocrine tumours to receptors for the hormone somatostatin. The radiation then acts mainly at the tumour. Like any radiotherapy the method has side effects, and it is used only in nuclear medicine under radiation protection.

Natural lutetium is just under three per cent lutetium-176. This isotope is weakly radioactive, and its half-life of about 37 billion years far exceeds the age of the universe. It decays to hafnium-176, and the ratio of the two lets scientists date rocks and meteorites.

Quick question

What is the atomic number of lutetium (Lu)?

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