Atomic number 71 · Lanthanide
Lutetium Lu
Lutetium (Lu), element 71, is the last of the lanthanides and the densest, hardest and highest-melting of them. It is expensive and has few bulk uses, but it plays an important role in medicine, from the crystals inside PET scanners to targeted radiation drugs for prostate cancer.
Key properties
| Atomic number | 71 |
|---|---|
| Atomic weight | 174.97 (CIAAW 2024 abridged standard atomic weight) |
| Category | Lanthanide |
| Group | Lanthanides/actinides (no group number) |
| Period | 6 |
| Block | f |
| State (25 °C, 1 atm) | Solid |
| Melting point | 1936 K (1662.8 °C) |
| Boiling point | 3675 K (3401.8 °C) |
| Density | 9.84 g/cm³ |
| Electronegativity (Pauling) | 1.27 |
| Atomic radius (van der Waals) | 221 pm |
| First ionization energy | 5.426 eV |
| Electron affinity | — |
| Oxidation states | +3 |
| Discovered | 1907 |
Electron configuration
- Condensed
- [Xe]6s2 4f14 5d1
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d1 6s2
- Electrons per shell
- 2 · 8 · 18 · 32 · 9 · 2
Orbital diagram
Position in the table
Discovery
Lutetium was discovered almost simultaneously by three chemists in 1907. Georges Urbain in France, Carl Auer von Welsbach in Austria and Charles James in the United States each showed that Marignac's ytterbia was a mixture of two elements. James published too late to enter the priority contest, and Urbain and Auer argued over who had been first.
The International Committee on Atomic Weights sided with Urbain and adopted his name. In German-speaking countries, however, Auer's name "cassiopeium" (symbol Cp) remained in use until the 1950s. Urbain's original spelling was "lutecium"; the modern spelling was adopted in 1949. Pure lutetium metal was first prepared only after separation techniques improved in the mid-twentieth century, and it is still produced in such small amounts that it is one of the most expensive lanthanides.
Origin of the name
The name comes from Lutetia, the Latin name of Paris, Urbain's home city.
Main uses
- Targeted radiotherapy: lutetium-177 attached to molecules that seek out tumours is used to treat neuroendocrine tumours and metastatic prostate cancer. Its beta particles attack the tumour while weak gamma emission lets doctors image where the drug has gone.
- PET detectors: lutetium oxyorthosilicate (LSO) and its yttrium-doped cousin LYSO emit fast, bright flashes when struck by gamma rays and are standard scintillators in PET scanners.
- Catalysis: lutetium compounds have been used or studied as catalysts for cracking and polymerisation.
- Dating: the lutetium-hafnium method helps date rocks and meteorites.
Isotopes
Natural lutetium is a mixture of stable lutetium-175 (about 97.4%) and radioactive lutetium-176 (about 2.6%). Lutetium-176 beta-decays to hafnium-176 with a half-life of roughly 37 billion years, which is the basis of lutetium-hafnium dating. Medical lutetium-177 has a half-life of about 6.6 days.
In everyday life
Because LYSO crystals contain naturally radioactive lutetium-176, PET detectors built from them glow very faintly all the time, and this background signal can even be used for calibration. Lutetium has no known biological role and is considered to have low toxicity. Chemists have long debated whether lanthanum or lutetium belongs beneath yttrium in group 3; arguments based on electron configuration and chemical trends have increasingly favoured lutetium.
Good to know
- Lutetium is the densest, hardest and highest-melting lanthanide.
- Three chemists discovered lutetium independently in the same year.
- Germany called the element cassiopeium until the 1950s.
Same category (Lanthanide)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.