Structure and bonding: 9.3.8: Giant covalent substances
Student follow up
C2.2.3 Covalent structures
a) Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.
Candidates should be able to recognise other giant structures or macromolecules from diagrams showing their bonding
b) In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.
c) In graphite, each carbon atom bonds to three others, forming layers.
The layers are free to slide over each other because there are no covalent
bonds between the layers and so graphite is soft and slippery.
d) In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.
Candidates should realise that graphite is similar to metals in that it has delocalised electrons.
e) Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, eg in tennis rackets.
Candidates' knowledge is limited to the fact that the structure of fullerenes is based on hexagonal rings of carbon atoms