Boron Nitride

Why can boron nitride (BN) exist in forms similar to that of the allotropes of carbon, graphite and diamond?

The bond between boron and nitrogen has an analogy with the carbon-carbon bond. Both have a total of 12 electrons on the two atoms which is called isoelectronic.

They have almost the same atomic radii and a similar relationship in their electronegativities.

What are the 2 structures of boron nitride?

1. Cubic boron nitride (c-BN)

2. Hexagonal boron nitride (h-BN)

Describe the structure of cubic boron nitride.

• Consists of alternately linked boron and nitrogen atoms that together form a tetrahedral bond network.

• The lone pair of electrons on nitrogen are accepted by boron to form a co-ordinate bond, producing a giant 3D covalent lattice.

What are the properties of cubic boron nitride?

• Hardness (2nd to diamond)

• Strong

• High melting point

• Chemically inert

State some uses of cubic boron nitride.

• Industrial abrasive: used for grinding materials.

• Support for catalysts.

• Wear-resistant coating.

Describe the structure of hexagonal boron nitride.

• Has a layered structure, similar to graphite, with layers forming a 2D giant covalent network.

• The alternate boron and nitrogen atoms link together forming hexagonal rings in thin layers, held together by weak intermolecular forces.

Describe the differences between the structures of hexagonal boron nitride and graphite.

Atoms in adjacent layers of h-BN are in register (aligned). In graphite they are out of register.

Π-electrons are relatively evenly delocalised in graphite but more localised in h-BN, as each nitrogen atom in h-BN has a lone pair of electrons. This makes BN an insulator.

There is an electronegativity difference between B and N which gives polar B-N bonds, whereas there is no electronegativity difference between the carbon atoms in graphite as all of the atoms are the same.

Why is h-BN an excellent lubricant?

The hexagonal layers are able to slide past each other.

State some uses / properties of hexagonal boron nitride.

h-BN is used in electronics for semi-conductors, ceramics and in microwave windows. It can also be used as a catalyst carrier in fuel cells and batteries.

Single layers of h-BN can be wrapped to create nanotubes.