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Macromolecules (IGCSE) 
• Describe the giant covalent structures of graphite and diamond
• Relate their structures to their uses, e.g. graphite as a lubricant and a conductor, and diamond in cutting tools.

Diamond is very hard and has very high melting point, because each carbon atom is bonded to four carbon atoms by strong covalent bonds in a giant molecular structure. Large amount of energy is needed to break these strong covalent bonds. Thus, diamond is used in cutting and drilling tools.
Valence electrons of carbon is used for covalent bonding. Diamond can’t conduct electricity because there are no ions or free electrons to act as charge carriers.
Graphite has a giant molecular structure. Each carbon atom forms covalent bonds with three carbon atoms. This forms rings of six atoms. The rings form layers of atoms held together by weak intermolecular forces.
Graphite can be used as lubricant or pencil lead as it is soft and slippery. Small amount of energy can overcome the weak intermolecular forces between layers of atoms and cause the layers to slide over each other easily.
Graphite is a good conductor of electricity. One valence electron of each carbon atom can move freely along the layers to form delocalised electrons. Thus, Graphite can be used as electrodes and connecting brushes in generators.

• Describe the macromolecular structure of silicon(IV) oxide (silicon dioxide)

In silicon dioxide, each silicon atom bonds is covalently bonded to four oxygen atoms and each oxygen atom bonds covalently to two silicon atoms in a giant molecular/ macromolecular structure.
• Describe the similarity in properties between diamond and silicon(IV) oxide, related to their structures.
Similar to diamond, silicon dioxide has strong covalent bonds between atoms and has giant molecular structure.  Large amount of energy is needed to break strong covalent bonds between the atoms. Silicon dioxide is very hard and is used to make sandpaper, lenses and glasses. Silicon dioxide also has high melting point thus it is also used in making bricks for lining furnaces.
Silicon dioxide also has no mobile ions or electrons thus it also cannot conduct electricity like diamond.