Boron

Boron – Element Profile

Symbol: B
Atomic Number: 5
Atomic Mass: 10.81 u
Position in the Periodic Table: Group 13 (Boron Group), Period 2
Electronic Configuration: 1s² 2s² 2p¹
Valency: 3
Block: p-block element
Type: Metalloid (exhibits properties of both metals and non-metals)

Occurrence

Boron does not occur freely in nature due to its high reactivity. It is found in the form of borates and borosilicates in minerals such as:

• Borax (Na₂B₄O₇·10H₂O)
• Kernite (Na₂B₄O₇·4H₂O)
• Colemanite (Ca₂B₆O₁₁·5H₂O)
• Boracite (Mg₃B₇O₁₃Cl)
• Tourmaline and borosilicate minerals

Isotopes

Boron has two stable isotopes:

• B-10 (~19.9%) – used in nuclear reactors for neutron absorption.
• B-11 (~80.1%) – stable, used in various research and medical applications.

Physical Properties

• Appearance: Brown or black amorphous powder; pure crystalline form is hard and black with a metallic luster.
• Hardness: Second only to diamond (in crystalline form).
• Melting Point: ~2300°C
• Boiling Point: ~2550°C
• Density: 2.34 g/cm³
• Poor conductor of electricity at room temperature (semi-conductor behaviour).

Chemical Properties

Boron is less reactive than metals and shows both metallic and non-metallic behavior.

1. Reaction with acids and alkalis:
   • Insoluble in water and most acids.
   • Dissolves in concentrated alkalis, releasing hydrogen gas: 2B+6NaOH→2Na3BO3+3H2↑2B + 6NaOH \rightarrow 2Na₃BO₃ + 3H₂↑
2. Reaction with oxygen:
   • Burns in oxygen to form boron trioxide (B₂O₃).
3. Covalent bonding:
   • Due to its small size and high ionization enthalpy, boron forms only covalent bonds.
4. Electron deficiency:
   • Boron compounds often have incomplete octets, making them strong Lewis acids (electron pair acceptors), e.g., BF₃, BCl₃.

Allotropes

Boron exists in:

• Amorphous form – brown powder, reactive.
• Crystalline form – very hard, high melting point, less reactive.

Important Boron Compounds

1. Boron Trifluoride (BF₃):
   • Colorless gas, strong Lewis acid.
   • Used as a catalyst in organic reactions.
2. Boron Trichloride (BCl₃):
   • Colorless, fuming liquid, hydrolyses in water to give HCl and boric acid.
3. Boric Acid (H₃BO₃):
   • Weak monobasic acid, antiseptic properties.
   • Used in eye drops, cosmetics, and as a mild disinfectant.
4. Borax (Na₂B₄O₇·10H₂O):
   • White crystalline solid, water-soluble.
   • Used in glass, ceramics, and detergents.

Uses of Boron and Its Compounds

• Metallurgy: Boron steel is extremely hard and used in armor plating.
• Glass & Ceramics: Borosilicate glass (Pyrex) is heat-resistant and used in laboratory glassware and cookware.
• Agriculture: Boron is a micronutrient essential for plant growth; borax is used in fertilizers.
• Electronics: Boron compounds used in semiconductors and as dopants in silicon.
• Nuclear industry: Boron-10 isotope absorbs neutrons, used in nuclear reactors as a control material.
• Medicine: Boric acid used as an antiseptic and eye wash; boron neutron capture therapy (BNCT) in cancer treatment.

Biological Role

• Trace amounts of boron are essential for the growth of plants, aiding in cell wall formation and nutrient transport.
• In humans, boron helps in bone health and brain function in small amounts, but excess intake can be toxic.

Special Characteristics

• Small atomic size and high ionization energy lead to non-metallic behaviour despite being in a metallic group.
• Boron forms electron-deficient molecules, leading to unique bonding such as three-center two-electron bonds (in diborane, B₂H₆).
• Shows diagonal relationship with silicon (similar properties due to similar size and charge density).

Physical and Chemical Properties

Boron is a hard, brittle, and dark-colored solid that exists in several allotropes.⁵ The most common allotropes are amorphous boron (a brown powder) and crystalline boron (black, extremely hard crystals).⁶ It’s a semiconductor, meaning its electrical conductivity increases with temperature. Due to its small atomic size and high ionization energy, boron forms strong, covalent bonds.⁷ It’s known for its ability to form electron-deficient compounds, which often act as Lewis acids (electron-pair acceptors).

A key characteristic of boron is its tendency to form complex polyhedral structures with itself. These structures are the basis for a class of compounds called boranes, which are boron hydrides. The simplest borane is diborane (⁸B2​H6​), which is a classic example of an electron-deficient molecule with “three-center, two-electron” bonds.⁹ This unique bonding pattern helps explain the stability of these seemingly electron-short compounds.

Occurrence and Production

Boron is not found in its elemental form in nature but is widely distributed in the Earth’s crust as various minerals, primarily borates.¹⁰ The most significant commercial source of boron is borax (¹¹Na2​B4​O7​⋅10H2​O).¹² Other important boron minerals include kernite and colemanite. The largest deposits of boron minerals are found in Turkey and the United States, particularly in California’s Mojave Desert.

Pure elemental boron is difficult to produce.¹³ It’s typically made by the reduction of boron trioxide (B2​O3​) with a metal like magnesium or aluminum at high temperatures. The resulting product is often an impure, amorphous powder. High-purity crystalline boron can be obtained through more complex processes, such as the thermal decomposition of boron trichloride (BCl3​) with hydrogen.

Key Applications

Boron and its compounds are incredibly versatile and have a wide range of applications:

• Glass and Ceramics: Boron is a critical component in the production of borosilicate glass (e.g., Pyrex and laboratory glassware).¹⁴ The addition of boron trioxide significantly lowers the glass’s coefficient of thermal expansion, making it highly resistant to thermal shock.¹⁵ Boron is also used in ceramics and glazes to improve their durability and appearance.¹⁶
• Agriculture: Boron is an essential micronutrient for plants, crucial for cell wall formation, sugar transport, and pollination.¹⁷ Boron-based fertilizers are used to correct soil deficiencies and improve crop yield, especially in crops like fruits and vegetables.
• Industry and Manufacturing: Boron is used in the production of high-strength, lightweight boron fibers, which are used in aerospace composites and sports equipment.¹⁸ Boron carbide (¹⁹B4​C) is an extremely hard material used for abrasive powders, bulletproof vests, and tank armor.²⁰ Boron’s ability to absorb neutrons makes it a critical component in nuclear reactors as a control rod material.²¹
• Chemistry and Medicine: Boron hydrides are used as high-energy rocket fuels. Boron compounds are also being researched for their potential in cancer therapy, particularly in boron neutron capture therapy (BNCT).

Biological Role

While essential for plants, the role of boron in animals and humans is less clear but appears to be important for bone health and brain function.²² Dietary boron intake is linked to calcium metabolism and the activity of certain enzymes.²³ Although not officially classified as an essential nutrient for humans by all health organizations, research suggests that low levels of boron may negatively impact health.²⁴

Isotopes

Boron has two stable isotopes: boron-10 (²⁵10B) and boron-11 (²⁶11B).²⁷ These isotopes have different properties that are leveraged in various applications. For instance, the high neutron absorption cross-section of 10B is what makes it so valuable in nuclear applications.