C3.6 Structure of Simple Molecules.

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Using Models

Models are used in everyday life to help us understand things. For example, the map of London Underground is a simplified model of the maze of tunnels carrying trains beneath the streets. Like all models, each one is useful but has some limitations. Dot and cross diagrams show which atom the electrons in the bonds came from originally, but in reality, all electrons are identical. Like all models drawn on paper, the electrons are in fixed positions between two atoms. However, scientists believe that the electrons in covalent bonds are constantly moving, but on average are found most of the time between the two nuclei of the atoms they are bonding together. 

In giant structure, the models can never accurately reflects the many millions of atoms (or ions) bonded together in the giant lattices. However, they can represent a tiny fraction of a structure. They can indicate the chemical formula of a compound by the simplest ratio of the atoms or ions in models of their giant structures.

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Intermolecullar Forces.

Covalent bonds are very strong. So the atoms within each molecule are held very tightly together. However, each molecule tends to be quite separate from its neighbouring molecule. The force of attraction between the individual molecules in a covalent substance is relatively small - there are weak intermolecular forces between molecules. Overcoming these forces does not take much energy. Intermolecular forces increase with the size of the molecules, so larger molecules have higher melting points and boiling points. For examples, polymers are made from very long chain molecules, so the intermolecular forces are relatively high compared with smaller molecules. These stronger intermolecular forces make polymers solids at room temperature. Intermolecular forces increase with the size of the molecules, so larger molecules have higher melting points and boiling points. For examples, polymers are made from very long chain molecules, so the intermolecular forces are relatively high compared with smaller molecules. These stronger intermolecular forces make polymers solids at room temperature. 

Polymers are mad up of many small reactive molecules that bond to each other to form long chains. The simplest example is polyethene made up from thousands of small ethene molecules, reacting together, We can represent the long polymer chains in poly(ethene) like this: instead of showing all the covalent bonds, this can be abbreviated using the repeating unit of the polymer chain. You have seen that ionic compounds will conduct electricity when they are liquids. Although a substance that is made up of simple molecules may be a liquid at room temperature, it will not conduct electricity. 

Compounds made of a simple molecule do not conduct electricity, even when they are molten or dissolved in water unless they react with the water to form aqueous ions, as acid molecules do. This is because there is no overall charge on the simple molecules in a compound like sucrose. So in there, neutral can't carry an electricical charge, unlike ions in molten or aqueous ionic compound.

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