permanent dipole-dipole force

an unequal distribution of electrons within a molecule, leading to partially positive and partially negative regions of charge. These regions of charge interact with neighbouring molecules, and forces of attraction can arise.

• Permanent dipole-dipole interactions exist between polar molecules.

• A partially negatively charged area of one molecule is attracted to a partially positively charged area of another molecule.

• The attractions are permanent and are stronger than temporary, induced dipole-dipole forces.

• The greater the polarity of a bond or molecule, the stronger the permanent dipole-dipole interactions.

Opposite charges attract and this electrostatic attraction allows forces to arise between the polar bonds. These forces are known as permanent, dipole-dipole interactions.

induced dipole-dipole interactions

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 either be 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. These are a type of intermolecular force, which arise between all molecules. We call these forces temporary, induced dipole-dipole interactions. Sometimes they are referred to as van der Waals or London dispersion forces.

The larger the molecules, the greater the strength of temporary, induced dipole-dipole interactions. This means larger molecules have higher melting and boiling points than smaller molecules