VAN DER WAALS -- Johannes Van Der Waals

These forces are much weaker than covalent bonds and can themselves vary in strength.

They are weak attractive forces caused by the movement of electrons within a molecule. Electrons move randomly within a bond so that at a particular point in time they may be nearer to one atom than the other. This creates a temporary polarity (temporary dipole) in the molecule.

If two molecules with similar temporary dipoles happen to be oriented with opposite charges directed at each other, an attractive force will exist between the two molecules.

Sometimes, a temporary dipole in one molecule will induce a similar dipole in a neighbouring, with an effect similar to the above. The combined effect of these temporary and induced dipoles is to produce weak attractive forces between neighbouring molecules, resulting in increased boiling points.

The grerater the number of electrons in a molecule, the greater the number of possible temporary dipoles. Consequently, the extent of the intermolecular attraction is greater. This explains why oxygen gas (with 16 electrons per molecule) has a higher boiling point than hydrogen (with only 2 electrons per molecule).

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Leaving Certificate (Shapes of molecules and intermolecular forces) Chemistry Apunte sobre Shapes of molecules and intermolecular forces, creado por eimearkelly3 el 06/08/2013.