Diamond, graphite, graphene, fullerenes and silicon dioxide: how their structures explain their very different properties, plus polymers.
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1.What is a giant covalent structure?
A structure in which all the atoms are joined to their neighbours by strong covalent bonds, forming one enormous network.
2.Give three examples of giant covalent substances.
Diamond, graphite and silicon dioxide.
3.Why do giant covalent structures have very high melting points?
Melting requires many very strong covalent bonds to be broken, which takes an enormous amount of energy.
4.Describe the structure of diamond.
Each carbon atom forms four covalent bonds to other carbon atoms, in a rigid giant three dimensional structure.
5.Why is diamond very hard?
Its rigid network of strong covalent bonds extends throughout the whole structure, so it cannot be deformed.
6.Does diamond conduct electricity, and why?
No. All four outer electrons of every carbon atom are used in bonding, so there are no delocalised electrons.
7.Describe the structure of graphite.
Each carbon atom forms three covalent bonds, producing layers of hexagonal rings with no bonds between the layers.
8.Why is graphite soft and slippery?
There are only weak forces between the layers, so the layers can slide over one another easily.
9.Why does graphite conduct electricity?
Each carbon atom uses only three of its four outer electrons in bonding, so one electron per atom is delocalised and free to move.
10.What is graphite used for and why?
Pencil leads and lubricants because the layers slide, and electrodes because it conducts and has a very high melting point.
11.Which structure does graphite resemble in its bonding and conduction?
A metal, because it too has delocalised electrons carrying charge.
12.What is graphene?
A single layer of graphite, one atom thick.
13.Give two properties of graphene.
It is extremely strong for its mass and it conducts electricity very well, which makes it useful in electronics and composites.
14.What are fullerenes?
Molecules of carbon atoms with hollow shapes, based on hexagonal rings but sometimes containing rings of five or seven carbon atoms.
15.What was the first fullerene discovered?
Buckminsterfullerene, C60, which has a hollow spherical shape.
16.Give two possible uses of fullerenes.
Delivering drugs into the body, and use as lubricants or as catalysts because of their large surface area.
17.What are carbon nanotubes?
Cylindrical fullerenes with very high length to diameter ratios.
18.Give two properties of carbon nanotubes.
They have very high tensile strength and conduct both heat and electricity well, which makes them useful in nanotechnology and to reinforce materials.
19.Describe the structure of silicon dioxide.
A giant covalent structure in which each silicon atom is bonded to oxygen atoms in a rigid three dimensional network.
20.Why does silicon dioxide have a high melting point?
Many strong covalent bonds must be broken to separate the atoms.
21.What is a polymer, in terms of bonding?
A very large molecule in which many small units are joined by strong covalent bonds.
22.Why is a polymer solid while a small molecule is often a gas?
The intermolecular forces between the very large polymer molecules are much stronger, so far more energy is needed to separate them.
23.Compare the bonds in diamond and graphite.
Both have strong covalent bonds between carbon atoms. Diamond has four per atom in a rigid network; graphite has three per atom in layers with weak forces between them.
24.Why is diamond used for cutting tools?
It is extremely hard because of its rigid giant covalent structure.
25.Does graphene conduct electricity?
Yes, very well, because like graphite each carbon atom has one delocalised electron.
26.Why can fullerenes be described as simple molecular rather than giant?
They are individual molecules of a fixed size, held to each other by weak intermolecular forces, unlike diamond which is one continuous network.
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