AS-Level Bonding

  • Intermolecular forces exist between molecules

  • When molecular substances change state, it is the arrangement of intermolecular forces that change (not the covalent bonds inside the molecules) 

  • Temporary induced dipole-dipole forces (Van der Waals, London forces, Dispersion forces) exist between all molecules

    • Unequal electron distribution around a molecule creates an instantenous dipole that can induce a dipole on a neighbouring molecule.

    • The two opposite dipoles from two molecules are attracted to each other and a weak force occurs

    • They are the weakest intermolecular forces

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QUICK NOTES

Temporary, induced dipole-dipole forces

 

Electrons surround atoms and exist between atoms in the form of covalent bonds. Think of a straightforward covalent bond between H and H, (H ).

 

 

 

 

 

 

 

 

 

 

 

We can see electrons exist around the whole molecule as they are shared between the bonding atoms. Electrons move very quickly so, at any one time, the electrons in this covalent bond may not be perfectly opposite one another.  As a result, they would not be equally shared around the hydrogen atoms.

 

Electrons have a negative charge. If we have more electron density on one side of the H molecule than the other, then one side of the H becomes very slightly negatively charged, leaving the other side of the H becoming very slightly positively charged.

 

 

 

 

 

 

This is called a temporary dipole. It is instantaneous! This means it does exist for very long as the electrons are constantly moving. But, if you have millions of molecules in a small area, you will always have some dipoles arising at any one time.

 

 

 

The bigger the molecule or bonding atoms, the larger the dipole will be. This is because there are more electrons moving around, and if more electrons move to one side than the other, a greater negative effect will be felt on one side and a greater positive effect on the other side.

 

If a molecule with a dipole is lined-up with another molecule, it can ‘induce’ a dipole on that neighbouring molecule. This is because electrons held within the neighbouring molecule will be either attracted to the very slightly positive end of the first molecule or repelled by the very slightly negative end.

 

This electrostatic attraction gives rise to very weak forces between molecules. It is a type of intermolecular force and arises between all molecules.  We call these forces temporary, induced dipole-dipole interactions . Sometimes they are referred to as Van der Waals forces.

 

 

 

The larger the molecules, the greater the number of temporary, induced dipole-dipole interactions.

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