Lutetium (Lu), with the atomic number 71, is classified as a silvery-white metal. It is a member of the rare earth metals. This dense element is the last and heaviest of the lanthanides, a series often set apart on the periodic table. Lutetium’s unique physical and chemical characteristics make it valuable for highly specialized applications. Its high melting point and tendency to form a trivalent ion confirm its metallic nature.
Lutetium’s Place on the Periodic Table
Lutetium occupies a unique position on the periodic table, located in Period 6 and traditionally categorized as the final element in the Lanthanide series. The Lanthanides and Actinides are often displayed separately below the main body of the table. This group, including Lutetium, is categorized as Rare Earth Elements. They are chemically difficult to separate, which historically made them challenging to isolate.
The placement of the Lanthanide series is a consequence of the filling of the \(4f\) electron subshell. As the atomic number increases from Lanthanum (57) to Lutetium (71), electrons are progressively added to this inner \(4f\) orbital. For Lutetium, the electron configuration is \(\text{[Xe]}4f^{14}5d^16s^2\), meaning the \(4f\) shell is completely filled.
Because the \(4f\) shell is full, Lutetium sometimes exhibits properties aligned with a transition metal. Some chemists classify it as the first element of the \(6\)th-period transition metals and place it in Group 3, directly below Scandium and Yttrium. This debate highlights its role as a bridge between the inner transition metals and the primary transition metal block. Its fundamental chemical behavior, however, is consistently metallic.
Defining Characteristics That Make Lutetium a Metal
Lutetium’s metallic nature is confirmed by several distinct physical and chemical properties. Among all Lanthanide elements, Lutetium is the hardest, densest, and possesses the highest melting point, traits indicative of strong metallic bonding. Its density is approximately \(9.84\) grams per cubic centimeter, and its melting point is high at about \(1663^\circ\text{C}\). The metal also exhibits a characteristic silvery-white metallic luster.
Lutetium is an excellent conductor of both heat and electricity, a hallmark of metallic elements. This high conductivity is due to the free movement of its valence electrons throughout the hexagonal close-packed crystal lattice structure. Its electrical resistivity is approximately \(582\) nanoohm-meters at room temperature, classifying it as a conductor.
Chemically, Lutetium behaves like an active metal, readily participating in reactions by losing electrons. It predominantly forms the \(\text{Lu}^{3+}\) ion, losing the two \(6s\) and the single \(5d\) electron to achieve a stable configuration with a filled \(4f\) orbital. Lutetium tarnishes slowly in moist air, reacting with oxygen to form Lutetium oxide (\(\text{Lu}_2\text{O}_3\)). It also reacts with water and dissolves easily in diluted acids, confirming its tendency toward cation formation.
Practical Uses of Lutetium
Lutetium’s distinctive properties, particularly its stable trivalent ion and unique nuclear characteristics, lead to highly specialized applications. One prominent use involves the radioactive isotope Lutetium-177 (\(\text{Lu}-177\)) in nuclear medicine. This isotope is utilized in targeted radionuclide therapy (TRT) for treating specific cancers, such as neuroendocrine tumors and metastatic prostate cancer. The \(\text{Lu}-177\) is attached to a targeting molecule that binds to cancer cells, delivering a localized dose of cell-destroying beta radiation.
Lutetium is also instrumental in advanced medical imaging, specifically in Positron Emission Tomography (\(\text{PET}\)) scanners. Crystals made from Lutetium Oxyorthosilicate (\(\text{LSO}\)) are a preferred material for the detectors in these scanners. The high density and fast scintillation properties of \(\text{LSO}\) make it highly efficient at detecting the gamma rays produced during a \(\text{PET}\) scan, resulting in clearer images.
Beyond medicine, Lutetium compounds are employed as catalysts in various industrial processes. They are used in the petrochemical industry to facilitate the cracking of hydrocarbons during petroleum refining. Lutetium compounds also find use as catalysts in certain polymerization reactions. In materials science, the element’s high density and hardness make its compounds valuable for creating specialized high-pressure ceramics and unique metal alloys.