Boron

Boron is a non-metallic element characterised by its extreme hardness and black colour. It exists in several allotropic forms and has an unusually high melting point due to its crystalline solid lattice. Although Boron is relatively rare, it is primarily found in nature as an orthoboric acid. 

1.0Introduction

Boron is the fifth element in the periodic table (Z=5) and is found in Group 13. Due to its properties, which exhibit characteristics of both metals and nonmetals, it is classified as a metalloid.

The name "Boron" is derived from the Arabic and Persian words for borax, its primary ore. Boron is relatively rare in the Earth's crust, with an abundance of about ten ppm, similar to lead. The pure element is shiny, black, and extremely hard—nearly as hard as a diamond in its purest form, though too brittle for practical use.

Boron is a good conductor at high temperatures but an insulator at room temperature and below. These properties and others contribute to its classification as a metalloid. In addition to its crystalline form, Boron is an amorphous dark brown powder.

2.0Properties of Boron

Physical Properties

• Melting Point: 2365 K
• Boiling Point: 4275 K
• Atomic Mass: 10.811 u
• Electronic Configuration: 1s²2s²2p¹
• Electronegativity: 2.04 (on the Pauling scale)
• Ionisation Potential: 8.298 eV
• Isotopes: Boron has two naturally occurring and stable isotopes: ¹¹B (80.1%) and ¹⁰B (19.9%)

Chemical Properties

1. Oxidation Rate:

• The oxidation rate of boron depends on factors such as particle size, crystallinity, temperature, and purity.
• Boron does not react with air at room temperature, but at elevated temperatures, it burns to form boron trioxide (B2O₃​).

2. Reactivity:

• Halogenation: When boron reacts with halogens, it forms trihalides. For example, the reaction with bromine is:

2B+3Br_2​→2BBr₃​

• Acid Resistance: Crystalline boron is chemically inert and resistant to attack by most acids, including boiling hydrofluoric and hydrochloric acids.
• Reactivity with Strong Oxidizers: Boron is slowly attacked by hot concentrated hydrogen peroxide, a hot mixture of sulfuric and chromic acids, as well as hot concentrated nitric acid, mainly when finely divided.

3.0Boron and its Compounds

Many boron compounds are electron-deficient, meaning they lack a complete octet of electrons around the central boron atom. This electron deficiency is why boron acts as a strong Lewis acid, readily accepting electron pairs from other molecules. Boron-hydrogen compounds are known as boron hydrides or boranes.

1. Borane (BH₃)

• Three hydrogen atoms are symmetrically bound to a central boron atom in the trigonal planar structure of borone (BH₃).
• The boron atom is electron-deficient because it only has six electrons in its valence shell—three from boron and three from hydrogen. 

2. Diborane (B₂H₆)

 Diborane consists of two boron atoms and six hydrogen atoms.

• Four hydrogen atoms are bonded directly to the boron atoms in a planar configuration.
• By building bridges between the two boron atoms, the remaining two hydrogen atoms create "three-center two-electron bonds."
• Three atoms share two electrons in this unusual bonding, resulting in an electron-deficient structure.

4.0Other Boron Compounds

1. Boron Ores:

• Boron compounds are found in Earth's crust, with concentrated ores in regions like Italy, Russia, Tibet, Turkey, and California.
• Borax is the most common boron ore that produces various boron compounds.

2. Sodium Perborate (NaBO_3​⋅4H₂​O):

• It is formed by crystallising a solution of borax and hydrogen peroxide.
• It contains two peroxy groups that bridge boron atoms, giving it bleaching properties, especially in colour-safe bleaches.

3. Boric Acid (B(OH)₃​):

• Weakly acidic and electron-deficient.
• When mixed with water, it accepts an OH⁻ ion to form the complex ion [B(OH)₄]⁻.

5.0Uses of Boron and Its Compounds

1. Boric Acid:

• It is also known as orthoboric acid, hydrogen borate, or boracic acid.
• A weak, monobasic Lewis acid widely used as:
• • Eye lotions and antiseptic for minor burns or cuts.
  • Food preservative.
  • Insecticide: First registered in the United States in 1948 to control various insects.

2. Borax: A vital boron compound used as:

• Cleansing agent.
• Borax bead test in analytical chemistry.
• Ingredient in medical soaps.

3. Borates:

• Applied as brighteners in washing powders.
• Used as a bleaching agent.

4. Boron Nitride: Utilised as a lubricant in the growth of nanotubes.
5. Borazine: Used in the production of carbon-based explosives.
6. Ceramics and Glass: Boron compounds are widely used in manufacturing ceramics and glass.
7. Boron Trifluoride (BF₃)

• Catalyst in organic reactions. 
• Precursor for boron compounds. 
• Etching Agent in semiconductors, 
• Used in gas-phase reactions.