Lutetium

Lutetium is a silvery white metal with the symbol Lu and atomic number 71. It resists corrosion in dry air but tarnishes in moist air. As the final member of the lanthanide series, it is traditionally grouped with the rare earth elements, though it can also be considered the first transition metal of the sixth period.

The element was discovered independently in 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, and American chemist Charles James. All three found lutetium as an impurity in ytterbium. A priority dispute erupted between Urbain and Welsbach, each accusing the other of basing results on the other’s published work. Urbain published first, so he won the naming rights, choosing “lutecium” after the Latin name for Paris, Lutetia. The spelling was changed to lutetium in 1949. Priority was officially granted to Urbain in 1909, and his names became official, though German scientists used Welsbach’s proposed name “cassiopeium” (later “cassiopium”) into the 1950s.

Lutetium is not especially abundant, though it is more common in Earth’s crust than silver. It has few specific uses. The radioisotope lutetium-176, which makes up 2.6% of natural lutetium and has a half-life of about 38 billion years, is used to date minerals and meteorites. Lutetium typically occurs with yttrium and is occasionally used in metal alloys and as a catalyst. The compound 177Lu-DOTA-TATE is employed in radionuclide therapy for neuroendocrine tumors. Lutetium has the highest Brinell hardness of any lanthanide, ranging from 890 to 1300 MPa.

A lutetium atom has 71 electrons arranged as [Xe] 4f¹⁴ 5d¹ 6s². It almost always appears in the +3 oxidation state, having lost its two 6s electrons and the single 5d electron. Because of lanthanide contraction, the lutetium atom is the smallest among the lanthanides, giving it the highest density, melting point, and hardness in the series. Its 4f orbitals are highly stabilized, so only the 5d and 6s orbitals participate in chemical reactions and bonding. This leads some to classify it as a d-block element and a transition metal like scandium and yttrium, rather than a lanthanide.

Lutetium compounds nearly always feature the +3 oxidation state. Aqueous solutions of most lutetium salts are colorless and form white crystalline solids when dried; the iodide is a brown exception. Soluble salts like nitrate, sulfate, and acetate form hydrates upon crystallization. The oxide, hydroxide, fluoride, carbonate, phosphate, and oxalate are insoluble in water. Lutetium metal is slightly unstable in air at standard conditions but burns readily at 150 °C to form lutetium oxide, which absorbs water and carbon dioxide and can be used to remove those vapors from closed atmospheres. The metal reacts slowly with cold water and quickly with hot water, producing lutetium hydroxide. It reacts with the four lightest halogens to form trihalides; except for the fluoride, these are water-soluble. Lutetium dissolves easily in weak acids and dilute sulfuric acid, yielding colorless lutetium ions coordinated by seven to nine water molecules, averaging [Lu(H₂O)₈.₂]³⁺.

Naturally occurring lutetium consists of two isotopes: lutetium-175 (stable) and lutetium-176 (radioactive). Lutetium-176 decays via beta decay with a half-life of 3.70×10¹⁰ years and makes up about 2.6% of natural lutetium, making the element monoisotopic. Forty synthetic radioisotopes have been characterized, with mass numbers from 149 to 188. The most stable synthetic isotopes are lutetium-174 (half-life 3.31 years) and lutetium-173 (half-life 1.37 years). All other radioactive isotopes have half-lives under 9 days, most under half an hour. Isotopes lighter than lutetium-175 decay mainly by electron capture (producing ytterbium isotopes), with some alpha and positron emission; heavier isotopes decay primarily by beta decay, producing hafnium isotopes. Experiments at the Facility for Rare Isotope Beams have reported lutetium-190 from fragments of platinum-198 colliding with a carbon target. The element also has 43 known nuclear isomers; the most stable are lutetium-177m³ (half-life 160.4 days) and lutetium-174m (half-life 142 days), which outlast the ground states of all lutetium isotopes except 173 through 176.

Three scientists discovered lutetium: Urbain, Welsbach, and James. They found it as an impurity in ytterbia, which Swiss chemist Jean Charles Galissard de Marignac had believed to be pure ytterbium. Urbain published first, then Welsbach; James was about to publish when he learned of Urbain’s work and withdrew his claim. Though he stayed out of the priority dispute, James worked on a much larger scale and at the time possessed the largest supply of lutetium. Urbain and Welsbach proposed different names. Urbain chose neoytterbium for ytterbium.