BORON

Name: Boron (from the mineral Borax)

Symbol: B

Group: 13

Period: 2

Block: s

Category: Metalloids

Atomic Number: 5

Atomic Mass: 10.81 u

Electrons per Shell: 2.3

Electronegativity: 2.04

Density: 2.46 g/cc

Melting Point: 2075°C

Boiling Point: 4000°C

Thermal Conductivity: 27 W (m K)

Electrical Conductivity: 0.0001 S/m

Magnetic Order: Diamagnetic

Ordinary State: Solid

Oxidation States: 3, 2, 1, -1, -5

Mohs Hardness: 9.3 9.5

Vickers Hardness: 42 GPa (Gigapascals in MPa: 42,000)

Brinell Hardness: No data

Most stable isotopes: B-10 (20%), B-11 (80%)

Discoverer: Humphry Davy, English (1808)

Boron (B), a chemical element with atomic number 5, is a metalloid of group 13 with a density of 2.34 g/cm³ (in its crystalline form) and an extremely low abundance of ~0.001 ppm in the Earth's crust, making it one of the rarest elements in the observable universe. Its name derives from the mineral borax (Na₂B₄O₇·10H₂O), known since antiquity for applications in ceramics and metallurgy, and from the Arabic term buraq or Persian burah. The English terminology "boron," coined by Humphry Davy, reflects its chemical similarity to carbon and silicon, due to its ability to form covalent bonds. Discovered in 1808 by Davy in England, and simultaneously by Joseph Louis Gay-Lussac and Louis Jacques Thénard in France, boron marked a milestone in a prolific year for chemistry, in which alkaline earth metals (magnesium, calcium, strontium, and barium) were also isolated.

The discovery of boron occurred through the reduction of boric acid (H₃BO₃) with metallic potassium, a pioneering method that allowed the isolation of the element in an impure form. Davy, Gay-Lussac, and Thénard identified its unique properties, although its high reactivity and difficulty in purification limited its initial study. The cosmic scarcity of boron, along with that of lithium (Li, Z=3) and beryllium (Be, Z=4), is explained by its formation not in primary stellar processes, but through cosmic spallation, where high-energy cosmic rays collide with carbon, nitrogen, or oxygen nuclei, fragmenting them into lighter nuclei like boron. This rarity, with a stellar abundance of ~0.0001 ppm, distinguishes it even among low atomic number elements.

Historically, boron was known indirectly through borax, used in Mesopotamia and Egypt for glazes and soldering. However, its recognition as a pure element in the 19th century opened the door to modern applications. Although little known, boron is versatile, with uses that exploit its hardness (in compounds like boron nitride, BN), thermal resistance, and chemical properties in sectors as diverse as agriculture, electronics, and the nuclear industry. Its discovery consolidated the advancement of analytical chemistry, and its cosmic rarity makes it a fascinating subject of study in cosmochemistry and astrophysics.

Boron (B), a chemical element with atomic number 5, is a metalloid of group 13 with a density of 2.34 g/cm³ (crystalline) and an abundance of ~0.001 ppm in the Earth's crust, making it extremely rare. As a metalloid, it shares characteristics with carbon (C) and silicon (Si), distinguished by its structural and chemical versatility. It exists in two main allotropic forms: amorphous boron and crystalline boron, analogous to the forms of carbon (graphite and diamond). Its high hardness, chemical resistance, and unique mechanical properties make it a valuable material in industrial applications, although its high-purity extraction is challenging due to its atomic similarity to carbon.

Amorphous boron is a dark red granular solid, similar to brick, without a defined crystalline structure, making it more reactive and less hard than its crystalline counterpart. Its lack of atomic order facilitates chemical reactions but limits its mechanical strength. On the other hand, crystalline boron is a black or metallic grey material, extremely hard (9.3–9.5 on the Mohs scale, close to diamond) and rigid, with a density superior to silicon (Si, 2.33 g/cm³) despite its lower atomic number. The B-B bonds in this form are covalent, short, and very stable, resulting in a compact structure that confers exceptional compressive strength (32 GPa or 32,000 MPa on the Vickers scale), surpassed by few materials. However, as a metalloid, it is brittle and vulnerable to impacts, potentially pulverizing with repeated blows, unlike its resistance to prolonged mechanical stress.

Chemically, boron is highly resistant to corrosion, even against aggressive acids like hydrofluoric acid (HF), which does not attack it even at high temperatures, a rarity among pure elements. It also resists oxidation at elevated temperatures, which likens it to high-performance ceramic materials, although it is a pure element, not a compound. Its atomic radius (85 pm) is similar to that of carbon (77 pm), which causes carbon to be a common impurity in pure boron, hindering its purification. Boron forms ultrahard compounds, such as boron carbide (B₄C) and boron nitride (BN), with hardnesses close to diamond (9–10 Mohs) and high melting points (B₄C: ~2,350 °C; BN: ~2,970 °C). These compounds are resistant to abrasion, thermal shock, and impacts, being essential in industrial applications. Rhenium diboride (ReB₂), another notable compound, is an ultrahard material (40 GPa), but its high cost, due to the rarity of rhenium (Re), limits its production. The properties of boron, including its hardness, chemical resistance, and ability to form stable covalent compounds, distinguish it as a niche material. Although brittle in its pure state, its compounds are fundamental in advanced technologies, from armor to cutting tools, highlighting its importance despite its scarcity and difficulty of purification.

Boron (B), a chemical element with atomic number 5, is a metalloid of group 13 with a density of 2.34 g/cm³ (crystalline) and an abundance of 0.001 ppm in the Earth's crust, making it extremely rare. Due to its high purification cost (100–500 USD/kg) and fragility in its elemental state, pure boron has limited applications, with its compounds being the most widely used in industry. With a global production of ~2,000 tons annually in 2025, more than 50% of boron is used in the form of boric acid (H₃BO₃) and derived compounds, excelling in sectors such as ceramics, glass, electronics, and agriculture for its chemical resistance, hardness, and thermal properties.

In the chemical industry, boric acid is the base for producing structural and ornamental ceramics, such as tiles and glazes, which utilize its resistance to high temperatures (~1,700 °C for boron compounds) and chemical stability. In the manufacture of specialized glasses, boron oxide (B₂O₃) is a key component, especially in borosilicate glass, which contains ~8–13% B₂O₃ along with sodium (Na₂O), potassium (K₂O), or calcium (CaO) oxides. Unlike soda-lime glass (Na₂O-CaO-SiO₂), which accounts for ~90% of global glass production, borosilicate is more resistant to thermal shock, chemical corrosion, and heat-induced cracking. Its toughness and ability to withstand rapid temperature changes (up to ~500 °C without fracturing) make it ideal for applications such as laboratory glassware (beakers, flasks), cookware (e.g., Pyrex), and optical components in headlights or telescopes. Although more expensive than common glass, borosilicate is commercially accessible, unlike more exotic compounds like boron nitride (BN).

Boron is also used as a dopant in fiberglass, improving its fluidity during manufacturing and its resistance to heat and corrosion, which is crucial in composites for construction, automotive, and aerospace industries. In electronics, pure boron or its compounds, such as boron carbide (B₄C), are employed in semiconductors and shielding for neutron detectors in nuclear reactors, due to the high capacity of the B-10 isotope (20% natural abundance) to absorb thermal neutrons. Boron nitride (BN), with a hardness close to diamond (9–10 Mohs) and a melting point of ~2,970 °C, is used in cutting tools, abrasives, and wear-resistant coatings. In agriculture, boric acid and borates (such as Na₂B₄O₇) are essential micronutrients in fertilizers, correcting boron deficiencies in soils for crops like citrus and cotton. Although elemental boron is rare in applications due to its fragility and cost, its compounds are fundamental in advanced technologies. The low toxicity of boron, compared to heavy metals like lead (Pb), and its chemical resistance make it valuable in a context of strict environmental regulations, such as RoHS, which promote safe materials. Its versatility in glasses, ceramics, and electronics underscores its importance, despite its scarcity and purification challenges.