Atomic number 69 · Lanthanide
Thulium Tm
Thulium (Tm), element 69, is a soft, silvery-grey lanthanide. Apart from radioactive promethium it is the least abundant of the lanthanides, and it is used in niche roles such as portable X-ray sources and surgical lasers.
Key properties
| Atomic number | 69 |
|---|---|
| Atomic weight | 168.93 (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 | 1818 K (1544.8 °C) |
| Boiling point | 2223 K (1949.8 °C) |
| Density | 9.32 g/cm³ |
| Electronegativity (Pauling) | 1.25 |
| Atomic radius (van der Waals) | 227 pm |
| First ionization energy | 6.184 eV |
| Electron affinity | — |
| Oxidation states | +3 |
| Discovered | 1879 |
Electron configuration
- Condensed
- [Xe]6s2 4f13
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f13 5s2 5p6 6s2
- Electrons per shell
- 2 · 8 · 18 · 31 · 8 · 2
Orbital diagram
Position in the table
Discovery
In 1879 Per Teodor Cleve at Uppsala was chasing the impurities left in erbia when he separated two new oxides: a brown one he named holmia and a green one he named thulia.
Getting thulium pure was notoriously hard. In 1911 Charles James at the University of New Hampshire is said to have carried out around fifteen thousand fractional crystallisations of thulium bromate to obtain a fairly pure sample, a story often told to show how laborious rare-earth chemistry used to be. Truly pure thulium metal became readily available only after ion-exchange methods spread in the 1950s.
Origin of the name
The name comes from Thule, the legendary land that ancient Greek and Roman geographers placed at the far north of the known world. Cleve used it as a poetic name for Scandinavia.
Main uses
- Portable X-ray sources: thulium-170, made by irradiating thulium-169 with neutrons, emits relatively low-energy X-rays and powers portable radiography devices that need no electricity.
- Thulium lasers: thulium-doped fibre lasers emit around 2 micrometres and are increasingly used as an alternative to holmium lasers for stone treatment and prostate surgery.
- Phosphors: trivalent thulium emits blue light and appears in some phosphors and anti-counterfeiting inks.
- Materials research: it is used in studies of superconductors and other advanced materials.
Isotopes
Natural thulium consists only of stable thulium-169. The most useful radioactive isotope is thulium-170, with a half-life of about 128 days; thulium-171, with a half-life of about 1.9 years, is also known.
In everyday life
Thulium is too rare and expensive to turn up in daily life, although it may be inside a surgical laser or a non-destructive testing device. It has no known biological role and is regarded as having low toxicity. Despite being called the rarest stable lanthanide, its crustal abundance is estimated to be greater than that of silver and comparable to iodine; its high price comes more from the cost of separating it from its neighbours than from its scarcity.
Good to know
- Thulium is named after Thule, the mythical land at the northern edge of the world.
- An early chemist reportedly needed about fifteen thousand crystallisations to purify thulium.
- Thulium-170 sources power X-ray devices that work without mains electricity.
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.