ElementsWhat is thulium?
Thulium is a soft, silvery metal and one of the least common lanthanides.
What is thulium used for?
Portable X-ray sources and some surgical lasers.
Where the name comes from
From Thule, an ancient name for a far northern land.
Key facts
- Symbol
- Tm
- Atomic number
- 69
- Atomic mass
- 168.93422 u
- Family
- Lanthanide
- Period
- 6
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 1,545 °C (1,818 K, 2,813 °F)
- Boiling point
- 1,950 °C (2,223 K, 3,542 °F)
- Density
- 9.32 g/cm³
- Electrons per shell
- 2, 8, 18, 31, 8, 2
- Electron configuration
- [Xe]6s2 4f13
- Common oxidation states
- +3
- Electronegativity (Pauling)
- 1.25
- First ionisation energy
- 6.184 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 227 pm
- Discovered
- 1879
Place in the periodic table
Thulium is element number 69. It sits in period 6 (row 6) of the periodic table, in the separate row of lanthanides drawn below the main table, and belongs to the f-block. Just before it comes erbium (68), and just after it comes ytterbium (70).
Family: Lanthanides.
Physical properties
At room temperature thulium is a solid. It melts at 1,545 °C and boils at 1,950 °C, so it stays liquid over a range of 405 degrees. Its density is 9.32 grams per cubic centimetre, so a one-litre block would weigh about 9.32 kilograms. By density it ranks 29th among the 96 elements with a measured value.
Inside the atom
An atom of thulium has 69 protons in its nucleus and, when neutral, 69 electrons around it. The electrons are arranged in 6 shells: 2, 8, 18, 31, 8, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Xe]6s2 4f13.
Its electronegativity on the Pauling scale is 1.25. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 6.184 electronvolts (about 597 kilojoules per mole). One mole of thulium weighs 168.93422 grams, so a single gram contains about 3.56 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains thulium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.