Anomalous Properties of Boron

Boron (B), chemical element, semimetal of periodic table Group 13 essential for plant growth and widely used in industry. Pure crystalline boron is a black, lustrous semiconductor, which means it conducts electricity like a metal at high temperatures but is nearly an insulator at low temperatures. It is hard enough to scratch some abrasives, such as carborundum, but too brittle to be used in tools (9.3 on the Mohs scale). It makes up about 0.001% of the Earth’s crust by weight. Boron is found in combination with the major commercial boron minerals, borax, kernite, and tincalconite (hydrated sodium borates), which are particularly concentrated in the arid regions of California, and colemanite, ulexite, and tourmaline are minerals that are widely distributed. Sassolite, or natural boric acid, is found primarily in Italy. Boron is a chemical element with the symbol B and the atomic number 5. It is a brittle, dark, lustrous metalloid in its crystalline form and a brown powder in amorphous form. Because it is the lightest element in the boron group, it has three valence electrons for forming covalent bonds, resulting in a wide range of compounds such as boric acid, the mineral sodium borate, and boron carbide, an ultra-hard crystal. Boron is a low-abundance element in the Solar System and the Earth’s crust because it is entirely synthesized by cosmic ray spallation and supernovae rather than stellar nucleosynthesis. It makes up about 0.001% of the Earth’s crust by weight. It is concentrated on Earth due to the water solubility of its more prevalent naturally occurring constituents, the borate minerals. These evaporites, such as borax and kernite, are mined industrially. Turkey, the world’s largest producer of boron minerals, has the largest known deposits.

The group IIIA or 13th group elements from the first series of p block elements, with atomic radii and other properties varying as we move down the group. Boron, due to its small size, exhibits unusual behaviour and distinct properties when compared to the other elements in the group.

Overview

The chemical behaviour of group 13 elements exhibits certain vital patterns. Because these elements are covalent in nature, their trichlorides, iodides, and bromides are hydrolyzed in water. With the exception of boron, species such as tetrahedral [M(OH)]^4– and octahedral[ M(OH)6]³⁺ exist in the fluid medium.

Because they lack an electron, monomeric trihalides are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases, such as NH₃, to complete an octet around the boron.

F₃B+:NH₃ →F₃B -NH3

Because of the absence of d-orbitals, the greatest covalence of B is 4. Because d-orbitals are accessible with Al and other elements, the maximum covalency can be normal beyond 4. Halide bridging dimerizes a large portion of other metal halides (for example, AlCl₃)) (e.g., Al₂Cl₆). With the help of electrons accepted from halogen in these halogen bridged particles, the metal species completes its octet.

Members of the Boron family are better known as group 13 elements in the modern periodic table. Members of this family have a diverse set of physical and chemical properties. These elements’ electronic configuration is given by ns2np1. This family’s members are as follows:

Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), Thallium (Tl), and a radioactive synthetic element known as ununtrium, Nihonium (Nh).

The chemical and physical properties of members of the boron family are found to follow a consistent pattern. Boron’s properties differ from those of other members of the group due to its smaller size and lack of the d orbital. These boron property deviations result in the classification of anomalous boron properties.

Anomalous properties of boron

Boron family members react with halogens to form tri-chlorides, bromides, and iodides. These halides have covalent bonds and are hydrolyzed in water. Because of the electron deficiency, these trihalides are strong Lewis acids as we move from boron to thallium, the metallic character increases. Electronegativity of the elements decreases first down the group from B to Al, then increases marginally due to differences in atomic size.

• While boron is non-metal, aluminium is a metal.
• Boron, on the other hand, is not a good conductor of electricity.
• Boron is found in two forms: amorphous and crystalline. Aluminum is a fragile metal that does not exist in many structures.
• Boron’s boiling and melting points are significantly higher than those of aluminium.
• Boron only shapes covalent compounds, whereas aluminium shapes some ionic compounds.
• Boron oxides and hydroxides are acidic in nature, whereas aluminium oxides and hydroxides are amphoteric.
• Boron trihalides BX3 exist as monomers, whereas aluminium halides exist as dimers Al2X6.
• Boron hydrides are quite inert, whereas aluminium hydrides are quite brittle.
• Boron is a metalloid, whereas the other members of the family are post-transition metals.
• Boron oxides and hydroxides are acidic in nature, whereas the other elements in the family form amphoteric oxides and hydroxides.

Anomalous behaviour of boron

Boron exhibits the following anomalous behaviours:

(a) Boron is a harder element than the other elements in its group.

(b) It has a higher melting and boiling point than the other members of the group.

(c) It can only form covalent and ionic compounds.

(d) Boron oxide is a weak acid, whereas amphoteric aluminium and gallium oxides. The oxides of indium and thallium are acidic in nature.

(e) Boron hydrides, or boranes, are quite stable, whereas aluminium hydrides are unstable.

(f) While dilute acids have no effect on boron, others liberate H2 from it.

(g) Borates have higher stability than aluminates. Boron is non-metal, whereas the other members are metals.

(h) Boron is a poor electrical conductor, whereas other metals are excellent conductors.

Important trends and anomalous properties of boron

The following are some of the most important trends in group 13 elements:

• Trichloride, tri bromides, and triiodides are covalent elements that can be hydrolyzed in water.
• These elements’ monomeric trihalides are strong Lewis acids.

BF₃+:NH₃→NH₃-BF₃

• Halogen bonding is commonly used to dimerize metal halides of group 13 elements.

Boron has the following unusual properties:

• Boron has significantly higher melting and boiling points than the other elements in group 13.
• Boron can only form covalent compounds, whereas the other elements in group 13 can form both ionic and covalent compounds.
• Boron is a metalloid, whereas the other elements in group 13 are metals.
• Boron oxides and hydroxides are acidic, whereas oxides and hydroxides of other elements in the group are amphoteric and basic.

Also read: Anomalous Properties of Boron Hydrides

FAQs

What causes boron's unusual behaviour?

Boron has atomic radii that are smaller and a maximum covalency of four. As a result, it behaves differently than the other elements in its group and has distinguishing properties, which explains its anomalous behaviour.

Why is 4 boron's maximum covalency?

The boron's outer shell, which is available for bonding, has a total of four orbitals (one s-orbital and three p-orbitals). As a result, the boron's maximum covalency is only 4.