Intermolecular Forces of Attraction are forces that exist between two molecules that cause their attraction to each other. Every molecule has at least one force of attraction. There are three forces of intermolecular attraction. These are: London Dispersion Forces, Dipole-Dipole Attraction, and Hydrogen Bonding, and are defined as follows:
London Dispersion Forces: Because electrons are constantly in motion, one end of a (non)polar molecule is temporarily either positive or negative. Because of these temporary dipoles, two (non)polar molecules have a very weak force of attraction between them.
Dipole-Dipole Attraction: An electrostatic attraction between the positive or negative end of one molecule and the oppositely charged end of another dipole.
Hydrogen Bonding: A special case of dipole-dipole attraction, hydrogen bonding is a temporary covalent bond between two dipoles containing hydrogen and oxygen, fluoride, or nitrogen.
Above is a lewis structure model of the attraction between two molecules of POCl3. This attraction can occur by either London Dispersion Forces or Dipole-Dipole Attraction. It can not, however, occur by the force of hydrogen bonding because the molecule does not contain any hydrogen to bond to the oxygen at the negative end of the molecule.
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