B · Element 5 · Metalloid
A metalloid of extremes: harder than most metals, essential to plants, and the element that makes borosilicate glass survive a flame.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where boron sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All boron atoms have the same number of protons, 5, and differ only in their number of neutrons: those variants are its isotopes. The most common, B-11, has 6 neutrons and makes up 80.35% of natural boron.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| B-10 | 5 | 19.65% | Stable | — |
| B-11 most common | 6 | 80.35% | Stable | — |
Here the classroom shortcut, rounded atomic mass (10.81) minus atomic number, gives the same answer: 6.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 87 pm — that is 0.087 nm, so about 5.75 million of them side by side would span a millimetre.
Thermal range
Boron is liquid over a 1925 K window, from 2348 K to 4273 K.
The story
Hard metalloid essential for plant growth. Forms extremely hard compounds.
The discovery of boron emerged from the golden age of chemistry when brilliant minds raced to unlock the secrets of mysterious compounds. This story involves French revolutionaries, English experimenters, and a substance that had puzzled humanity for over a thousand years.
Long before its discovery, boron compounds tantalized civilizations across the globe. Borax reached Europe via the ancient Silk Road, traded as a precious flux for metalworking and a component in Chinese ceramics. Arab alchemists called it "burak," while Venetian glassmakers paid premium prices for this mysterious white powder that made their glass crystal clear.
Marco Polo wrote about borax mines in Tibet, and medieval European alchemists tried desperately to understand why this salt could clean, preserve, and transform other materials. They had no idea they were working with compounds of an unknown element.
The breakthrough came during one of history's most turbulent periods. Joseph Louis Gay-Lussac and Louis-Jacques Thénard, working in Napoleon's France, were investigating the mysterious properties of boric acid. These brilliant chemists had survived the French Revolution and were now pushing the boundaries of chemical knowledge under the patronage of the French Academy of Sciences.
On June 21, 1808, they achieved the impossible - they isolated pure boron by heating boric acid with potassium metal in an iron tube. They had created element number 5, though they didn't yet understand its true significance.
Simultaneously, across the English Channel, Sir Humphry Davy was conducting his own experiments at the Royal Institution in London. Just nine days after the French success, Davy achieved the same result using a different method - electrolysis of boric acid. The scientific rivalry between significant France and Napoleonic-era Britain drove both teams to new achievements.
Davy's approach was more elegant but equally dangerous. He passed electric current through molten boric acid, depositing tiny amounts of brown material at the electrode.
Both French and English chemists initially struggled to name their discovery. The French team proposed "bore" from the Arabic "burak" (borax), while Davy suggested "boracium." The modern name "boron" emerged from international scientific cooperation, combining the Arabic root with the Greek suffix "-on" meaning "forming."
Boron remained a laboratory curiosity until the borax boom in the American West. In 1872, Francis Marion Smith discovered rich borax deposits at Teels Marsh, Nevada, earning him the nickname "Borax King." The twenty-mule teams of William Tell Coleman's Harmony Borax Works (1883-1889), later part of Smith's Pacific Coast Borax Company, became an American legend, hauling borax 165 miles across the Mojave Desert to the nearest railroad.
The real breakthrough came in 1909 when Ezekiel Weintraub developed a method to produce pure crystalline boron, revealing its true properties as one of the hardest elements known. This discovery launched boron into the modern age of advanced materials and nuclear technology.
Applications
Boron is a little-noticed but widely used element in modern technology, enabling everything from bulletproof armor to smartphone screens. Its unique chemical properties make it central in advanced applications where ordinary materials fail.
Boron carbide (B₄C) is the third-hardest material known to humanity, surpassed only by diamond and cubic boron nitride. Military contractors like BAE Systems and General Dynamics use boron carbide in:
Boron's strong neutron absorption makes it a standard nuclear control material. Boron-10 has a neutron absorption cross-section 1,000 times greater than most materials:
Borosilicate glass transformed laboratory science and consumer products. Corning Inc. transformed glassmaking with boron-enhanced formulations:
Boron is an essential plant micronutrient that enables cellular wall formation and reproductive processes. Modern agriculture relies on boron compounds for:
Where it comes from
10 mg/kg of Earth's crust · more abundant than 69% of elements
Boron is one of the universe's rarest elements, with an abundance of only 10 parts per million in Earth's crust. This scarcity tells a fascinating story of cosmic destruction and terrestrial concentration.
Unlike most elements, boron cannot survive inside stars - it's destroyed at temperatures above about 5 million K. Instead, boron forms through cosmic ray spallation, when high-energy particles smash into carbon, nitrogen, and oxygen nuclei in space, fragmenting them into lighter elements including boron. This violent process makes boron a "fossil" of cosmic ray activity over billions of years.
Despite its cosmic rarity, boron becomes concentrated in specific geological environments through unique processes:
Evaporite deposits contain the world's major boron resources:
Turkey dominates world boron production with 73% of global reserves, primarily from the Kirka and Emet mines. The United States ranks second with California's Boron mine (owned by Rio Tinto), which has been mined since the 1920s and as an open pit since 1957. Other significant deposits exist in:
Seawater contains about 4.5 mg/L of boron, making oceans a virtually infinite but dilute source. Some geothermal springs concentrate boron to economically viable levels - Italy's Larderello geothermal field was the world's first source of industrial boron in the 1800s.
Certain plants act as "boron accumulators," concentrating the element up to 1,000 times above soil levels. Marine algae and some desert plants evolved this ability to tolerate high boron environments that would kill other organisms.
Handling
Boron and most boron compounds are relatively low-toxicity materials compared to many industrial chemicals. However, proper handling procedures are essential for safe use in laboratory, industrial, and household applications.
Acute toxicity: Boron compounds have low acute toxicity. The LD₅₀ (lethal dose for 50% of test subjects) for boric acid is 2,660 mg/kg in rats, about the same acute toxicity as table salt (rat oral LD₅₀ about 3,000 mg/kg).
Chronic exposure: Long-term exposure to high levels may affect reproductive health and development. OSHA has not established specific exposure limits for elemental boron, but recommends treating it as a nuisance dust.
Eye contact: Flush with clean water for 15 minutes, seek medical attention if irritation persists
Skin contact: Wash with soap and water, remove contaminated clothing
Inhalation: Move to fresh air, seek medical attention if breathing difficulties occur
Ingestion: Rinse mouth, drink water, seek medical attention for large amounts
Boric acid and borax are classified in the EU as toxic to reproduction (may damage fertility or the unborn child), so use household products only as directed:
Note: Boron is an essential micronutrient for plants and may have beneficial effects for human bone health, but dietary supplements should be used only under medical supervision.
Quick answers
Boron (symbol B) is element 5 on the periodic table, a metalloid in period 2, group 13. A metalloid of extremes: harder than most metals, essential to plants, and the element that makes borosilicate glass survive a flame. At room temperature it is a solid.
[He] 2s² 2p¹, giving 2 occupied shells with the distribution 2, 3. Its outer shell holds 3 electrons, which is what sets its bonding behaviour.Boron melts at 2348 K (2074.9 °C) and boils at 4273 K (3999.9 °C). That puts it among the most refractory elements — it stays solid at temperatures that vaporise most metals.
The standard atomic weight of boron is 10.81 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common boron atom, B-11, has 6 neutrons: its mass number, 11, minus its atomic number, 5. It makes up 80.35% of natural boron. The classroom shortcut, rounded atomic mass (10.81) minus atomic number, gives the same 6.
Boron has a density of 2.37 g/cm³. Water is 1.0 g/cm³, so a block of boron is about 2.4× heavier.
Boron has a Pauling electronegativity of 2.04. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Boron was discovered in 1808 by Davy, Gay-Lussac & Thenard. Origin of the name: from borax, via Arabic buraq.
Boron makes up about 10 mg/kg of the Earth's crust — uncommon, but not rare.