Boron
metalloid · period 2 · group 13 · p-block
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder.
What the numbers say
At standard temperature and pressure Boron is a solid, sitting in period 2, group 13 of the p-block.
Its melting point of 2349 K (2076 °C) is the 13th highest of the 110 elements with a measured value — mid-range among the elements.
At 2.46 g/cm³ it is the 73rd densest of 96 elements with a quoted density.
Pauling electronegativity is 2.04. Paired with oxygen (3.44) that is a difference of 1.40, 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 trigonal structure.
It is reported in 3 oxidation states (1, 2, 3), which sets how many distinct compounds and phases it can form.
First identified in 1808.
Crystal structure
Solid Boron adopts a Simple Trigonal arrangement, in the Trigonal (rhombohedral) system — axes a = b = c (rhombohedral setting), angles α = β = γ ≠ 90°. Symmetry controls the anisotropy of stiffness and thermal expansion, and the available slip systems set how far the metal deforms before it fractures.
Crystallographic examples
Boron or one of its compounds is a textbook example for these crystal systems:
- Cubic (isometric) — Cu, Al, Fe-α (BCC), NaCl, diamond, spinel MgAl₂O₄
- Tetragonal — β-Sn, rutile TiO₂, zircon, martensite (BCT)
Check it against computed structures
The structure search queries the Materials Project, OQMD and the Open Materials Database for computed crystal structures containing B — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 3
Properties
| Atomic number | 5 |
|---|---|
| Atomic mass | 10.81 u |
| Series | metalloid |
| Group | 13 |
| Period | 2 |
| Block | p-block |
| Phase at STP | Solid |
| Density | 2.46 g/cm³ |
| Melting point | 2349 K (2076 °C) |
| Boiling point | 4200 K (3927 °C) |
| Electronegativity (Pauling) | 2.04 |
| Electron configuration | [He] 2s² 2p¹ |
| Crystal structure | Simple Trigonal |
| Oxidation states | 1,2,3 |
| Discovered | 1808 |
← Back to the periodic table
See B in context with all 118 elements.
Working with boron in a real design?
Ask the materials chat how B behaves in your alloy system, what it does to weldability, and which grades to look at — with the source of every number named.