Boron
Atomic Data
| Atomic Number | 5 |
| Symbol | B |
| Atomic Weight | 10.81 u |
| Density (STP) | 2.34 g/cm³ |
| Melting Point | 2075.85 °C (2349 K) |
| Boiling Point | 3926.85 °C (4200 K) |
| Electronegativity | 2.04 (Pauling) |
| Electron Config. | 1s² 2s² 2p¹ |
| Oxidation States | +3 |
| Phase at STP | Solid |
| Category | Metalloid |
| Period / Group | 2 / 13 |
| CAS Number | 7440-42-8 |
Electron Configuration
1s2 2s2 2p1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 1 | 5 |
| Total | 3 | 5 | ||
Isotopes of Boron
Boron has two stable isotopes: boron-11, which makes up about 80% of natural boron, and boron-10, which accounts for the remaining 20%. Boron-10 is of particular technological importance because of its high thermal-neutron capture cross-section.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Boron-10 | ₁₀B | 5 | 5 | 19.9% | Stable |
| Boron-11 | ₁₁B | 5 | 6 | 80.1% | Stable |
Abundance & Occurrence
Boron is a moderately rare element, averaging about 10 parts per million in Earth's crust. It does not occur in its elemental form in nature; it is always found in oxidised form, predominantly as borate minerals. The most commercially significant are borax (Na2B4O7·10H2O) and kernite, mined primarily in Turkey, the USA, and Argentina. Turkey alone holds roughly 70% of the world's economically extractable boron reserves.
World Boron Reserves by Country (approx. %)
Global Boron Demand by Sector (approx. %)
Discovery & History
Safety & Handling
- Low acute toxicity: Elemental boron and simple boron compounds such as boric acid and borax have low acute toxicity for humans. Lethal doses in animal studies are relatively high — far above any likely environmental or occupational exposure.
- Skin and eye irritation: Boron dust or boron compounds in large quantities can irritate eyes, skin, and mucous membranes. Flush with water if contact occurs; standard PPE (safety glasses, gloves) is recommended when handling boron powders.
- Inhalation: Prolonged inhalation of boron dust or boron trioxide fumes may irritate the respiratory tract. Use dust masks or respirators when working with fine boron powders or at elevated temperatures.
- Reproductive and developmental hazard (borax): Animal studies show boron compounds can act as reproductive toxicants at high doses. The EU classifies borax as a reproductive toxicant category 1B. Pregnant women should limit exposure.
- Flammability: Fine boron powder is combustible and can form explosive dust–air mixtures. Keep boron powder away from open flames and oxidising agents.
- Storage: Store in dry, sealed containers away from moisture (boric acid absorbs water). Keep separate from strong oxidisers and reactive metals.
Boron in the Real World
Real-World Uses
- Borosilicate glass (Pyrex) — Adding boron trioxide (B2O3) to silica glass lowers its coefficient of thermal expansion by roughly a third. This makes borosilicate glass resistant to thermal shock, which is why laboratory glassware, cookware, and pharmaceutical containers are made from it.
- Detergent bleach (borax and perborate) — Sodium tetraborate (borax) is a key ingredient in many laundry powders, softening water by chelating calcium and magnesium ions. Sodium perborate releases hydrogen peroxide in wash water, providing an oxygen-based bleach for colour-safe fabric whitening.
- Semiconductor dopant — In silicon and germanium semiconductor manufacturing, boron is the primary p-type dopant. Implanting trace amounts of boron creates electron "holes" that are essential to transistors, solar cells, and integrated circuits.
- Nuclear shielding and reactor control — Boron-10 has a very high thermal-neutron capture cross-section (3,840 barns). Boron carbide (B4C) control rods are used in many reactor types to regulate fission. Boric acid is dissolved in the coolant of pressurised-water reactors as a chemical shim for reactor power control.
- High-strength boron fibres — Boron filaments deposited on tungsten or carbon cores are used in high-performance structural composites for aircraft and aerospace applications, offering higher stiffness than carbon fibre at comparable weight.
- Agriculture (micronutrient) — Boron is an essential micronutrient for plant cell wall formation and reproduction. Boron deficiency causes hollow stem in brassicas, cracked stem in celery, and poor fruit set in many crops. Borax is used as a soil amendment at very low application rates.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is boron used for?
Boron has a wide range of uses. Borax (sodium tetraborate) is used in detergents, cleaning products, and as a flux in metallurgy. Borosilicate glass (Pyrex) is made with boron oxide to give it low thermal expansion. Boron is also used as a neutron absorber in nuclear reactors, as a dopant in semiconductor manufacturing, and in the production of high-strength boron fibre composites for aerospace applications.
Is boron a metal or nonmetal?
Boron is a metalloid — it sits on the boundary between metals and nonmetals on the periodic table. In its crystalline form it is a very hard, dark, semiconducting solid with metallic lustre but poor electrical conductivity. Its chemistry is dominated by covalent bonding, which is more typical of nonmetals.
Why is boron used in nuclear reactors?
Boron-10 (one of boron's two stable isotopes, making up ~20% of natural boron) has an exceptionally high cross-section for absorbing thermal neutrons. Control rods in nuclear reactors are often made from boron carbide or boron steel. Boric acid is also dissolved in the coolant water of pressurised-water reactors as a soluble neutron absorber that can be varied to control reactor power.
How was boron discovered?
Boron was isolated simultaneously in 1808 by two independent teams. Humphry Davy in England and Joseph Louis Gay-Lussac with Louis Jacques Thénard in France both reduced boric acid with potassium metal to obtain an impure form of the element. Davy named it boracium; the name boron was later derived from the mineral borax combined with the suffix -on used for carbon.
What is borax and is it safe?
Borax (sodium tetraborate decahydrate, Na2B4O7·10H2O) is a naturally occurring boron mineral mined mainly from dried lake beds in the USA, Turkey, and Argentina. It has low acute toxicity and has been used safely for decades in laundry boosters, cleaning products, and as a fire retardant. However, it is classified as a reproductive toxicant at high doses in animal studies, and its use in food and cosmetics is restricted in several countries.