Question

Boron nitride has a graphite-like structure with \(\mathrm{B}-\mathrm{N}\) bond distances of \(1.45 \AA\) within sheets and a separation of \(3.30 \AA\) between sheets. At high temperatures the \(B N\) assumes a diamondlike form that is almost as hard as diamond. Rationalize the similarity between \(\mathrm{BN}\) and elemental carbon.

Short answer

Graphite-like BN has a similar structure to graphite, with sheets of B and N atoms bonded covalently in a hexagonal arrangement. At high temperatures, BN forms a diamond-like structure where B and N atoms are tetrahedrally bonded, exhibiting diamond-like properties such as hardness. The similarity between BN and elemental carbon arises due to their electronic configurations and bonding characteristics, which result in comparable geometries and bonding strength.

Step-by-step solution

Structure of graphite-like BN and comparison to graphite (carbon)

Graphite-like BN consists of sheets of B and N atoms bonded covalently to each other in a hexagonal arrangement, similar to the way carbon atoms form layers in graphite. The covalent B-N bond lengths within the sheets are 1.45Å, and their interlayer separation is 3.30Å, which is also quite comparable to the structure of graphite.

Structure of diamond-like BN and comparison to diamond (carbon)

At high temperatures, BN adopts a diamond-like structure, in which B and N atoms are tetrahedrally bonded to each other, forming a 3-dimensional network—just like in the case of diamond, where all carbon atoms have similar tetrahedral arrangements. Due to this structural similarity, diamond-like BN exhibits properties comparable to diamond, such as hardness.

Explanation for BN's similarity to elemental carbon

The reason why boron nitride (BN) closely resembles elemental carbon in both the graphite-like and diamond-like forms lies in their electronic configurations and bonding characteristics. Both boron (B) and nitrogen (N) possess three valence electrons each, similar to the four valence electrons of carbon (C). Upon forming compounds, B and N atoms can effectively share electrons with each other to form covalent bonds, comparable to carbon both in geometry and bonding strength. Thus, due to similarities in their electronic configurations and the subsequently formed bonds, BN displays structures and properties akin to elemental carbon.

Key concepts

Graphite Structure

In its graphite structure, boron nitride (BN) mirrors the arrangement we see in carbon-based graphite. Here, sheets of boron and nitrogen atoms link together through strong covalent bonds, creating a hexagonal pattern. This is similar to layers of carbon in graphite, which are comprised of interconnected carbon atoms. Within each BN layer, the B-N bond measures about 1.45 Å while the sheets themselves are spaced at 3.30 Å apart.
This spacing is akin to that found in graphite, allowing the layers to glide over one another easily during physical interactions. This layer bonding is responsible for graphite's soft and slippery feel, a characteristic that BN shares in this form.

• The hexagonal layer structure is significant for both materials.
• The weak forces between layers let them slide, contributing to their lubricating properties.
• The strong covalent bonds within a layer result in high intra-layer strength.

Covalent Bonds

Covalent bonds form when atoms share pairs of electrons, creating a tightly bonded structure. In BN's graphite-like form, each boron atom bonds covalently with a nitrogen atom, forming a robust network within each layer. These covalent B-N bonds are comparable to the carbon-carbon bonds in graphite, delivering similar stability and strength.
This bond strength is a result of both boron and nitrogen having three valence electrons, enabling them to effectively share and create a stable electron arrangement. The strength and nature of these covalent bonds account for the distinct properties of boron nitride and its resemblance to carbon-based materials in its various forms.

• Covalent bonds are essentially the glue holding atoms firmly together in a structure.
• The electron-sharing nature of these bonds results in highly stable layers.
• The significant bond strength is crucial in both graphite and diamond-like structures.

Diamond Structure

At high temperatures, boron nitride transforms into a diamond-like structure. In this form, the arrangement of atoms becomes three-dimensional, forming a lattice of tetrahedrally bonded B and N atoms. This structure is identical to that of diamond, where all carbon atoms share a similar arrangement.
This intricate network results in BN exhibiting properties comparable to diamond, such as exceptional hardness. The tetrahedral bonding pattern requires each atom to form four strong covalent bonds with its neighbors, creating a compact and resilient material.

• Tetrahedral structures maximize space utilization and create a rigid framework.
• This structure provides the material with significant durability and strength.
• Similar to diamonds, diamond-like BN has high resistance to scratching and cutting.

Hardness Properties

Hardness is a material's ability to resist deformation or scratches. Both diamond and diamond-like BN exhibit high hardness due to their strong bonds and three-dimensional structures. The tetrahedral bonding in these materials makes them extremely durable and difficult to break or damage.
In boron nitride's diamond form, the robust covalent bonds and compact lattice contribute to these remarkable mechanical properties. This structural integrity means that, like a diamond, diamond-like BN can cut or scratch nearly any other substance.

• The hardness increases as the structural integrity of the bonds and the lattice is improved.
• Materials with diamond-like structures have significant applications in cutting tools.
• Beyond hardness, these materials are also valued for their high thermal stability.