C3.6 – Giant Structures

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1. Describe the giant covalent structures of graphite and diamond.

In some covalently bonded substances, there is a network of covalent bonds throughout the whole structure. Such substances are called giant covalent molecules or macromolecules. Diamond and graphite are two such macromolecules.

Diamond and graphite are two forms of carbon that exist (different forms of the same element are called allotropes; don’t confuse this with isotope which is the an element with the same proton number but a different neutron number).

In diamond, carbon atoms form four covalent bonds with the other carbon atoms. These carbon atoms link together to form a giant lattice.

In graphite, each carbon atom is joined by three other carbon atoms. These atoms are arranged in hexagonal layers

2. Relate their structures to the use of graphite as a lubricant and of diamond in
   cutting.

Diamond has very high melting and boiling points because of its closely linked network of carbon atoms (it is an exception to the general rule that covalently bonded structures have low melting and boiling points) It is also very hard, it doesn’t scratch easily. This is why diamond is used for cutting and drilling metals and glass.

Graphite’s bonding between it’s layers is very weak. This means that layers can slide over each other. This makes graphite very slippery and can be scratched easily. Because of this, graphite is used as a lubricant (substance used to reduce friction; oil is another lubricant) and in pencil leads.

3. Describe the structure of silicon(IV) oxide (silicon dioxide).

In the silicon dioxide structure, each silicon atom is bonded to four oxygen atoms but each oxygen atom is bonded to two silicon atoms.Silicon dioxide has a similar structure to diamond. It forms very hard colourless crystals and has a  high melting and boiling point.

Notes submitted by Lintha

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