Thulium
lanthanide · period 6 · group 3 · f-block
Thulium is a chemical element with the symbol Tm and atomic number 69. It is the thirteenth and third-last element in the lanthanide series.
What the numbers say
At standard temperature and pressure Thulium is a solid, sitting in period 6, group 3 of the f-block.
Its melting point of 1818 K (1545 °C) is the 25th highest of the 110 elements with a measured value — mid-range among the elements.
At 9.32 g/cm³ it is the 30th densest of 96 elements with a quoted density.
Pauling electronegativity is 1.25. Paired with oxygen (3.44) that is a difference of 2.19, which puts the bond predominantly ionic — expect a stable oxide ceramic.
In the solid state it takes a simple hexagonal structure — hexagonal packing offers few easy slip systems, so properties are anisotropic and formability is limited.
It is reported in 2 oxidation states (2, 3), which sets how many distinct compounds and phases it can form.
First identified in 1879.
Crystal structure
Solid Thulium adopts a Simple Hexagonal arrangement, in the Hexagonal system — axes a = b ≠ c, angles α = β = 90°, γ = 120°. Symmetry controls the anisotropy of stiffness and thermal expansion, and the available slip systems set how far the metal deforms before it fractures.
Check it against computed structures
The structure search queries the Materials Project, OQMD and the Open Materials Database for computed crystal structures containing Tm — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 31 · O 8 · P 2
Properties
| Atomic number | 69 |
|---|---|
| Atomic mass | 168.934222 u |
| Series | lanthanide |
| Group | 3 |
| Period | 6 |
| Block | f-block |
| Phase at STP | Solid |
| Density | 9.32 g/cm³ |
| Melting point | 1818 K (1545 °C) |
| Boiling point | 2223 K (1950 °C) |
| Electronegativity (Pauling) | 1.25 |
| Electron configuration | [Xe] 4f¹³ 6s² |
| Crystal structure | Simple Hexagonal |
| Oxidation states | 2,3 |
| Discovered | 1879 |
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See Tm in context with all 118 elements.
Working with thulium in a real design?
Ask the materials chat how Tm behaves in your alloy system, what it does to weldability, and which grades to look at — with the source of every number named.