Selenium
polyatomic nonmetal · period 4 · group 16 (chalcogens) · p-block
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic.
What the numbers say
At standard temperature and pressure Selenium is a solid, sitting in period 4, group 16 of the p-block.
Its melting point of 494 K (221 °C) is the 84th highest of the 110 elements with a measured value — mid-range among the elements.
At 4.819 g/cm³ it is the 65th densest of 96 elements with a quoted density.
Pauling electronegativity is 2.55. Paired with oxygen (3.44) that is a difference of 0.89, which puts the bond polar covalent — oxides here tend to be semiconducting or refractory rather than salt-like.
In the solid state it takes a simple monoclinic structure.
It is reported in 5 oxidation states (-2, 1, 2, 4, 6), which sets how many distinct compounds and phases it can form.
First identified in 1817.
Crystal structure
Solid Selenium adopts a Simple Monoclinic arrangement, in the Monoclinic system — axes a ≠ b ≠ c, angles α = γ = 90°, β ≠ 90°. 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 Se — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 6
Properties
| Atomic number | 34 |
|---|---|
| Atomic mass | 78.9718 u |
| Series | polyatomic nonmetal |
| Group | 16 |
| Period | 4 |
| Block | p-block |
| Phase at STP | Solid |
| Density | 4.819 g/cm³ |
| Melting point | 494 K (221 °C) |
| Boiling point | 958 K (685 °C) |
| Electronegativity (Pauling) | 2.55 |
| Electron configuration | [Ar] 3d¹⁰ 4s² 4p⁴ |
| Crystal structure | Simple Monoclinic |
| Oxidation states | -2,1,2,4,6 |
| Discovered | 1817 |
← Back to the periodic table
See Se in context with all 118 elements.
Working with selenium in a real design?
Ask the materials chat how Se behaves in your alloy system, what it does to weldability, and which grades to look at — with the source of every number named.