Boron
Boron is a metalloid chemical element with symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in both the Solar system and the Earth's crust. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals.
Identity
- Atomic mass
- [10.806, 10.821] u
- Category
- Metalloids
- Group
- 13
- Period
- 2
- Block
- p
- Oxidation States
- −3, +3
- Phase (25 °C)
- Solid
- Appearance
- black-brown
Electronic Structure
- Electron configuration
- 1s2 2s2 2p1
- Short configuration
- [He] 2s2 2p1
- Electrons per shell
- 2, 3
- Electronegativity (Pauling)
- 2.04
- Electron affinity
- 27.0 kJ/mol
- 1st ionization energy
- 801 kJ/mol
- Ionization energies
- 801 · 2427 · 3660 · 25026 · 32827 kJ/mol
Physical properties
- Melting point
- 2075.8 °C (2349 K)
- Boiling point
- 3926.8 °C (4200 K)
- Density
- 2.08 g/cm³
- Molar heat capacity
- 11.1 J/(mol·K)
Chemical behavior of boron
Boron has three valence electrons ([He] 2s² 2p¹) and a high electronegativity for its position (2.04), so it almost always acts as +3 forming essentially covalent bonds. It is an electron-deficient metalloid: it does not achieve the octet by itself and compensates with multicentre bonds, as in boranes. Its chemistry is situated between that of aluminium (more metallic) and carbon, with acidic oxides and halides that are Lewis acids.
Reactivity
4 B + 3 O₂ → 2 B₂O₃
When heated to redness it burns in oxygen forming boron oxide, a vitreous acidic solid that dissolves in water giving boric acid. At room temperature it is slightly reactive due to its hardness and passivation.
2 B + 3 F₂ → 2 BF₃
With fluorine it reacts directly giving boron trifluoride, a gas that is a classic Lewis acid: accepts electron pairs and catalyses polymerisations and alkylations.
2 B + 6 NaOH → 2 Na₃BO₃ + 3 H₂
Hot, concentrated bases attack it forming soluble borates with hydrogen release. This reactivity confirms its non-metallic character, far from aluminium.
2 B + 3 Mg → Mg₃B₂
With very electropositive metals it forms borides, where it formally acts as −3. Magnesium diboride (MgB₂) is a superconductor below 39 K.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Boric acid | H₃BO₃ | Gentle antiseptic, eye drops, flame retardant and fluxes |
| Borax | Na₂B₄O₇·10H₂O | Detergents, borosilicate glasses and ceramic glazes |
| Boron trifluoride | BF₃ | Lewis acid catalyst in petrochemicals and polymerisation |
| Boron carbide | B₄C | One of the hardest materials; armor and abrasives |
| Hexagonal boron nitride | BN | White 'graphite' lubricant; thermal and electrical insulating ceramics |
Electron shell distribution
View in the atom builderIsotopes
Natural boron combines two stable isotopes: boron-11 (¹¹B), with six neutrons and 80.1%, and boron-10 (¹⁰B), with five neutrons and 19.9%. Boron-10 absorbs neutrons with great efficiency and is used in control rods for reactors, armour and neutron capture therapy against cancer (BNCT).
Occurrence and production
It is a scarce element (about 10 ppm of crust) that never appears free. Its ores are borates: borax and kernite of Turkish and Californian deserts, and colemanite and ulexite. It is obtained by reducing the oxide with magnesium or by electrolysis of molten salts; ultrapure boron for electronics is prepared by diborane decomposition.
History and origin of the name
Borates were used since antiquity in glazes and metallurgy, but the element was isolated in 1808 almost simultaneously: Joseph Louis Gay-Lussac and Thenard in France, and Humphry Davy in England, reducing boric acid. Berzelius baptised boron, from carbonised borax, and pure crystalline boron was not obtained until 1909, by Weintraub.
Applications and relevance of boron
Uses of the element
Amorphous boron is used in pyrotechnics, metallurgical deoxidant and igniters; crystalline boron is a Semiconductor dopant; and its fibres are in lightweight composite materials for aeronautics and sports goods. Boron carbide armours vehicles and protects reactors.
Uses of its compounds
Borax and boric acid feed borosilicate glass (Pyrex), detergents, glazes and flame retardants; hexagonal nitride lubricates and insulates in electronics; borosilicates form fibreglass.
Health, safety and environment
Biological role
It is not essential for humans, although plants need it in traces for cell walls. Borates are toxic in high doses: they irritate skin and mucous membranes and damage kidney and liver.
Safety
Boron dust is flammable and boranes (such as diborane) are toxic and spontaneously flammable gases. Boric acid, once used in foods, is prohibited as a preservative due to its cumulative toxicity.
Environment
Borate mining is open-pit and alters desert landscapes, with dust and residual brines. Borates are soluble and in excess they saline soils and waters; boron is also a micronutrient with a narrow margin between plant deficiency and toxicity.
Frequently asked questions about Boron
What is boron for in the human body?
Boron is a micronutrient that participates in calcium, magnesium and vitamin D metabolism, and can influence bone health and inflammatory processes. However, there is no universally established daily need and no clearly defined essential function as for other minerals. It is obtained mainly from fruits, nuts, legumes and vegetables. Supplements should only be used with professional judgment, as excess can be toxic.
Is boron good for plants?
Yes, boron is an essential micronutrient for many plants. It participates in cell wall formation, sugar transport, pollination, fruit set and root and growth point development. Its deficiency can cause deformed fruits, thick or brittle leaves and death of growth points. However, the window between deficiency and toxicity is narrow: excess boron burns leaf edges and harms the crop, so soil analysis is advisable.
Is boron used in glass and ceramics?
Yes. Boron is key in the manufacture of borosilicate glass, a material resistant to sudden temperature changes and thermal expansion. Therefore it is used in laboratoryware, cooking pots, pharmaceutical ampoules and optics. In ceramics and glazes, borates help lower melting temperature, improve shine and increase durability. It is also used in detergents, fertilisers, flame retardants and insulating materials, according to the specific compound.
Why is boron used in fertilisers?
Boron is added to fertilisers when soil does not provide enough available boron for the crop. It is especially important in horticultural crops, fruit trees, sugar beet, sunflower and vines, where it influences flowering, fruit set and fruit quality. It can be applied to soil or by foliar route, depending on crop and agronomic diagnosis. Because excess is easily toxic to plants, the dose must be based on soil or leaf analysis and technical recommendations.
Is boron the same as borax?
No. Boron is the chemical element, while borax is a mineral and compound that contains boron, specifically a hydrated sodium borate. Borax has been used in cleaning, preservation, ceramics and as a boron source for plants, but it should not be confused with a safe product for human consumption. Handling borax requires caution: it can irritate skin and eyes, and its ingestion is not recommended. For domestic or agricultural uses, follow the product instructions.
What foods contain boron?
Boron appears naturally in plant foods, especially nuts such as almonds and walnuts, legumes, avocados, dried plums, grapes, apples, pears and some vegetables. The amount depends on soil, irrigation water and food processing. In a varied diet based on fruits, vegetables, legumes and nuts, intake is usually sufficient without supplements. People with dietary restrictions or medical treatments should consult before taking boron products.
Is boron dangerous to health?
Boron in usual dietary amounts is not considered dangerous for most people, but boron compounds can be toxic in high doses. Excessive exposure, especially by ingestion of borax or boric acid, can cause digestive irritation, nausea, vomiting and effects on kidney or nervous system. It can also irritate skin and eyes. Borax should never be used as a supplement or applied without following the manufacturer's safety instructions.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.