Boron nitride is the second hardest substance known (after diamond). it has a very high melting point, but does not conduct electricity. What types og bonding do you think are present in boron nitride? A) covalent bonding in two-dimensional layers of alternating boron and nitrogen molecules, with LDFs between layers. B) Covalent bonding in boron nitride molecules (BN), with LDFs between the molecules C) A three dimensional lattice of ionic bonding between boron and nitrogen D) an extended network of boron and nitrogen atoms held together by covalent bonds E) it is not possible to predict the type of bonding considering its bulk properties
The correct answer and explanation is :
The correct answer is D) an extended network of boron and nitrogen atoms held together by covalent bonds.
Explanation:
Boron nitride (BN) exhibits remarkable hardness and a high melting point, properties that are indicative of its strong bonding and structural integrity.
Bonding in Boron Nitride:
- Covalent Bonding: In both its hexagonal (h-BN) and cubic (c-BN) forms, BN consists of boron and nitrogen atoms connected by strong covalent bonds.
- Hexagonal Boron Nitride (h-BN): This form has a layered structure similar to graphite, where each layer consists of alternating boron and nitrogen atoms bonded covalently in a hexagonal lattice. The layers are held together by weaker van der Waals forces, allowing them to slide over each other, which contributes to h-BN’s lubricating properties.
- Cubic Boron Nitride (c-BN): Under high-pressure and high-temperature conditions, h-BN transforms into c-BN, which adopts a diamond-like cubic structure. In this form, each boron atom is tetrahedrally bonded to four nitrogen atoms, creating a three-dimensional network of strong covalent bonds. This structure imparts exceptional hardness to c-BN, making it one of the hardest known materials.
Why Other Options Are Incorrect:
- A) Covalent bonding in two-dimensional layers of alternating boron and nitrogen molecules, with London Dispersion Forces (LDFs) between layers: While h-BN does have a layered structure with alternating boron and nitrogen atoms, the layers are held together by van der Waals forces, not just LDFs, and the bonding within layers is covalent, not molecular.
- B) Covalent bonding in boron nitride molecules (BN), with LDFs between the molecules: BN does not exist as discrete BN molecules; instead, it forms extended networks with continuous covalent bonding.
- C) A three-dimensional lattice of ionic bonding between boron and nitrogen: BN forms covalent bonds, not ionic bonds. The electronegativity difference between boron and nitrogen is not sufficient to result in ionic bonding.
- E) It is not possible to predict the type of bonding considering its bulk properties: The bonding in BN can be predicted based on its structural characteristics and the known chemistry of boron and nitrogen.
Structural Representation:
Below is a schematic representation of the hexagonal boron nitride (h-BN) structure, illustrating the alternating layers of boron and nitrogen atoms:
Conclusion:
The exceptional hardness and high melting point of boron nitride are attributed to its extended network of covalently bonded atoms, forming robust structures in both its hexagonal and cubic forms.