Thulium (Tm)
Lanthanide
Thulium is the rarest stable rare-earth element — scarcer than gold's neighbours in price for years. It powers portable X-ray devices and adds efficiency to some surgical lasers.
Position on the periodic table
Atomic properties
| Atomic number | 69 |
|---|---|
| Atomic mass | 168.934 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13 |
| Noble gas shorthand | [Xe] 4f13 6s2 |
| Electrons per shell | 2, 8, 18, 31, 8, 2 |
| Valence electrons (outer shell) | 2 |
| Common oxidation states | +3 |
| Electronegativity (Pauling) | 1.25 |
| Covalent radius (approx.) | 190 pm |
| First ionization energy | 596.7 kJ/mol |
| Electron affinity | 99 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 9.32 g/cm³ |
| Melting point | 1544.8 °C (1818 K) |
| Boiling point | 1949.8 °C (2223 K) |
| Appearance | silvery gray |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of thulium
Thulium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13, usually shortened to [Xe] 4f13 6s2. Its electrons occupy 6 shells (2, 8, 18, 31, 8, 2), placing it in period 6 of the f-block. Explore it interactively in the electron configuration calculator.
Uses of thulium
- Portable X-ray sources
- Fibre lasers for surgery and materials processing
- Euro banknote fluorescence (blue)
Biological role: None known.
Occurrence: The least abundant stable rare earth; from xenotime and clays.
Common compounds of thulium
Tm2O3
Thulium(III) oxide
History and discovery
Discovered: 1879 — Per Teodor Cleve. Name origin: From Thule, the ancient name for the far North.
Per Teodor Cleve discovered thulium in 1879 while combing through erbium earth impurities.
Safety notes
Low toxicity; standard precautions.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Thulium
1. What is the chemical symbol of Thulium?
2. What is the atomic number of Thulium?
3. Which category does Thulium belong to?
4. What is the state of Thulium at room temperature?
5. Which period of the periodic table is Thulium in?
Thulium FAQs
What is the atomic number of thulium?
Thulium's atomic number is 69 — every thulium atom has 69 protons in its nucleus.
What is the symbol for thulium?
Tm. From Thule, the ancient name for the far North.
Is thulium a metal, nonmetal, or metalloid?
Thulium is classified as a lanthanide.
What state is thulium at room temperature?
At about 25 °C, thulium is a solid. It melts at 1544.8 °C.
What is thulium used for?
Key uses include: portable x-ray sources; fibre lasers for surgery and materials processing; euro banknote fluorescence (blue).
Related elements
Keep working with this element
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