Boron
[He] 2s² 2p¹ · 2 · 3
A metalloid on the border between metals and nonmetals: boron itself is a dark, very hard, brittle substance, while its compounds give heat-proof glass and industry's best neutron absorber.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 5
- Atomic mass
- 10.81 а.е.м.
- Electron configuration
- [He] 2s² 2p¹
- Electrons per shell
- 2 · 3
- Block
- p-block
- Group
- Boron group
- Period
- 2
- Electronegativity (Pauling)
- 2.04
- Electronegativity (Allen)
- 2.05
- Atomic radius
- 87 pm
- Covalent radius
- 84 pm
- Van der Waals radius
- 192 pm
- Ionisation energy 1
- 800.6 kJ/mol
- Ionisation energy 2
- 2 427.1 kJ/mol
- Ionisation energy 3
- 3 659.7 kJ/mol
- Ionisation energy 4
- 25 026 kJ/mol
- Ionisation energy 5
- 32 827 kJ/mol
- Electron affinity
- 26.7 kJ/mol
- Common oxidation states
- +3
- Oxidation states
- -5, -1, +1, +2, +3
Physical properties
- State at 25 °C
- solid
- Density
- 2.34 g/cm³
- Melting point
- 2 349 K · 2 075.9 °C
- Boiling point
- 4 200 K · 3 926.9 °C
- Speed of sound
- 16 200 m/s
Thermal properties
- Heat of fusion
- 50.2 kJ/mol
- Heat of vaporisation
- 508 kJ/mol
- Specific heat capacity
- 1.026 J/(g·K)
- Thermal conductivity
- 27.4 W/(m·K)
Mechanical properties
- Young's modulus
- 460 GPa
- Shear modulus
- no data
- Bulk modulus
- 185 GPa
- Poisson ratio
- no data
- Mohs hardness
- 9.3
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- 1·10¹² nΩ·m
- Magnetic ordering
- diamagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- rhombohedral
- Lattice constants
- a = 506 pm · c = 1 257 pm
Abundance
- In the crust
- 10 mg/kg
- In the ocean
- 4.44 mg/L
- In the universe
- 0.001 mg/kg
- In the human body
- 0.7 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 10B | 19.9 % | stable | — |
| 11B | 80.1 % | stable | — |
Boron-10 (19.9 %) absorbs thermal neutrons 250 000 times more readily than boron-11, and all of nuclear boron chemistry rests on that.
Discovery
- Year of discovery
- 1808
- Discovered by
- Gay-Lussac, Thénard and Davy
- Where
- France and England
- Origin of the name
- from Arabic buraq (borax), the mineral it was isolated from
History
Gay-Lussac and Thénard in France and Davy in England independently isolated boron in 1808 by reducing boric acid. Purity above 99 % had to wait for the twentieth century — boron is stubbornly hard to refine.
Where it occurs
10 mg/kg in the crust, but four hundred times more concentrated in seawater. The commercial deposits are borate salts from dried lakes in Turkey and California.
How it is produced
Borax and colemanite give boric acid, then the oxide, which magnesium reduces. Very pure boron comes from decomposing boranes on a hot filament.
Role in living things
Boron is an essential micronutrient for plants; for humans the case is unproven, though traces sit in bone.
Safety
Boron itself is of low toxicity, but boric acid and borates damage the reproductive system on repeated ingestion, and the EU lists them as substances of very high concern.
Uses
- Borosilicate glass: low thermal expansion, hence laboratory and oven-proof ware
- Control rods and dissolved boric acid in nuclear reactors — boron-10 devours neutrons
- Boron carbide, the third-hardest known substance after diamond and boron nitride, used in armour plate
- A micronutrient fertiliser: without boron plants cannot build cell walls or pollen
- Doping silicon to make a p-type semiconductor
Curiosities
- Boron's isotope ratio varies noticeably in nature, so its atomic mass is quoted as an interval rather than a number.
- Boron nitride exists in a form isostructural with diamond and nearly as hard.
- Boron burns with a vivid green flame.
- Boric acid is the oldest cockroach remedy: it destroys their waxy coating.