Organic molecules

Carbon forms a huge variety of both synthetic and natural molecules. These organic molecules have a structure based upon carbon chains and/or carbon rings. A 'family' of organic compounds with the same functional group is called a homologous series.

The alkanes are a homologous series with a structure based on a carbon chain - each carbon atom forms four covalent bonds

The properties of organic molecules depend on their structure being made of simple molecules. The atoms in an individual molecule are joined together by strong covalent bonds. The intermolecular forces between molecules are weaker.

The intermolecular forces vary between molecules, so different organic compounds have different melting points and boiling points.

The strong covalent bonds joining together atoms in a molecule plus the weaker intermolecular forces

Simple molecular substances have no overall charge, and their electrons are not free to move. So these substances are poor conductors of electricity and heat.

Polymers

Polymers consist of very long molecules that contain chains of carbon. They too are held together by very strong covalent bonds. There are greater intermolecular forces between the long chains compared with smaller simple molecules. This means that polymers have a higher melting point than many other organic molecules.

A model of a short section of a poly(ethene) molecule, a simple polymer. Polythene molecules contain thousands of carbon atoms joined together in a chain

The intermolecular forces affect:

• hardness
• melting point
• behaviour on heating (whether the polymer returns easily to shape after heating)
• flexibility