Lanthanides
Thulium
The rarest stable rare earth, used in portable X-ray sources.
Atomic #69Mass168.93Blockf-blockPeriod6Group—
Tm69 · 168.93
3D Atom Explorer
Inside the Thulium atom
Switch between Bohr and Quantum Cloud modes to compare a simple teaching model with a more realistic probability-based view, and follow the guided tour to explore the Thulium atom step by step.
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Electron configuration
[Xe] 4f13 6s2
A neutral Thulium atom has 69 electrons (equal to its proton count). Choosing a different isotope above changes only the neutron count.
Shell distribution
Shell 12 e⁻Shell 28 e⁻Shell 318 e⁻Shell 431 e⁻Shell 58 e⁻Shell 62 e⁻
Electrons fill inner shells before outer ones; the outermost (valence) shell drives the element's chemistry.
Properties
Physical & atomic properties
- State (room temp)
- Solid
- Melting point
- 1818 K (1545 °C)
- Boiling point
- 2223 K (1950 °C)
- Density
- 9.321 g/cm³
- Electronegativity
- 1.25 Pauling
- Atomic radius
- 175 pm
- 1st ionization energy
- 597 kJ/mol
- Category
- Lanthanides
History
Discovery & naming
- Discovered
- 1879
- Discovered by
- Per Teodor Cleve
- Origin of name
- Thule, a mythical northern land.
Notable uses
Portable X-ray sources and lasers.
Cosmic origin
Where Thulium comes from
Neutron star mergers
An r-process element, produced where free neutrons are briefly, spectacularly abundant.
Simplified origin map — many elements form through more than one astrophysical pathway.
Summary
- Atomic number
- 69
- Atomic mass
- 168.93
- Category
- Lanthanides
- Group · Period
- — · 6
- Block
- f-block
- Shells
- 2 · 8 · 18 · 31 · 8 · 2