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THE VAN DER WAALS' FORCES:

    THE FORCE IS WEAK IN THIS ONE
  1. DIPOLE DIPOLE FORCE
  2. DISPERSION FORCE
  3. HYDROGEN BONDING

 

THE FORCE IS WEAK IN THIS ONE

       The van der Waals' forces (or interactions), named after Dutch scientist Johannes Diderik van der Waals (November 23, 1837 - March 8, 1923), are the attractive or repulsive forces between atoms, molecules, (or between parts of the same molecule) and surfaces, other than those due to covalent bonds or electrostatic interactions between ions with themselves or neutral molecules. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations of nearby particles (a consequence of quantum dynamics). The term includes forces between permanent dipole and a corresponding induced dipole, and instantaneous induced dipole-dipole forces (London dispersion force). "Van der Waals' forces" is also used loosely as a synonym for the totality of intermolecular forces. Compared with normal chemical bonds, van der Waals' forces are relatively weak, but play a fundamental role in diverse fields such as supramolecular chemistry, structural biology, surface science, polymer science, nanotechnology, and condensed matter physics.

There are three van der Waals' forces:

        Dipole-Dipole forces occur in polar molecules (those that have unequal sharing of electrons around their atoms, leading to part of the molecule being more positively charged, and part being more electronegative.) In a solution where there are billions of molecules with a slight charge variance on each side, the negative part of one molecule will orient itself with the positive side of a neighboring molecule. These intermolecular forces cause the molecules to 'stick' together.

        Dispersion forces exist between non-polar molecules where, on average, there is equal sharing of electrons. Electrons are not stationary, however, and their constant movement is probabilistic, meaning that at one particular instant there might be a small charge variance on one side of the molecule or the other. This temporary charge lasts only for an instant before disappearing as quickly as it formed, but while it exists, it creates a weak intermolecular dipole force.

        Hydrogen bonding is exactly the same as a dipole-dipole force, just stronger*, so it gets a special name. Hydrogen bonds occur between any molecule having both hydrogen and either oxygen, fluorine, or nitrogen. H is extremely good at losing electrons, and O, F and N are extremely good at attracting electrons, so the hydrogen bond results in an extreme dipole situation, whereby the very positive side of one molecule will orient itself with the very negative side of another.

        *All of the three van der Waals' forces are very weak, so the 'strong' hydrogen bond is only strong compared with the other two types.

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