Thallium
post-transition metal · period 6 · group 13 · p-block
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature.
What the numbers say
At standard temperature and pressure Thallium is a solid, sitting in period 6, group 13 of the p-block.
Its melting point of 577 K (304 °C) is the 79th highest of the 110 elements with a measured value — mid-range among the elements.
At 11.85 g/cm³ it is the 21st densest of 96 elements with a quoted density.
Pauling electronegativity is 1.62. Paired with oxygen (3.44) that is a difference of 1.82, 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 (1, 3), which sets how many distinct compounds and phases it can form.
First identified in 1861.
Crystal structure
Solid Thallium 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 Tl — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 32 · O 18 · P 3
Properties
| Atomic number | 81 |
|---|---|
| Atomic mass | 204.38 u |
| Series | post-transition metal |
| Group | 13 |
| Period | 6 |
| Block | p-block |
| Phase at STP | Solid |
| Density | 11.85 g/cm³ |
| Melting point | 577 K (304 °C) |
| Boiling point | 1746 K (1473 °C) |
| Electronegativity (Pauling) | 1.62 |
| Electron configuration | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ |
| Crystal structure | Simple Hexagonal |
| Oxidation states | 1,3 |
| Discovered | 1861 |
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See Tl in context with all 118 elements.
Working with thallium in a real design?
Ask the materials chat how Tl behaves in your alloy system, what it does to weldability, and which grades to look at — with the source of every number named.