Friday, November 27, 2009

diamond and graphite / IGCSE/ GCSE CHEMISTRY NOTES

Giant structures with covalent bonds
In giant covalent structures all the atoms are bonded to each other by strong covalent bonds so they have very high melting and boiling points. They do not usually conduct electricity even if in the liquid state. Diamond and graphite are two examples, which are made from carbon atoms. These two different types of the same element are called allotropes.
Diamond: Each carbon atom forms four covalent bonds in a very rigid giant covalent structure.

Graphite: exists as layers of carbon atoms each held in place by three strong covalent bonds. Each layer is held to the one above it by weak bonds.
Physical properties of giant covalent structures

Giant molecular structures have very high melting points because all atoms are held firmly in place by strong covalent bonds. In graphite each carbon atom is held in place by three strong covalent bonds which gives graphite a high melting point. In diamond 4 strong covalent bonds holds each atom in place. This also gives diamond a very high melting point. The four bonds make diamond very hard. Graphite has weak bonds between layers so the layers slip over each other making graphite soft.
They do not usually conduct electricity even when molten because there are no charged particles to carry the current. There are free electrons between layers in graphite so it conducts electricity.
Explaining differences between propertiesSimple molecular substances like water have weak bonds between molecules so melt at low temperatures because little energy is needed to separate the molecules. Giant covalent structures like diamond have strong covalent bonds holding each atom in place. They melt at high temperatures because a lot of energy is needed to break these strong bonds.


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