Van Der Waals Force
A Van der Waals force between molecules is a relatively weak intermolecular attraction. All neighboring molecules in liquids and solids attract each other. The nature and strength of these interactions depends on the types of atom groups or functional groups that comprise the molecules. Some molecules are polar and some have hydrogen bonds. These relatively strong intermolecular interactions require specific structural features. Polar interactions require a nonsymmetric arrangement of bonds with atoms of different electronegativity--polar bonds. Hydrogen bonding requires that one species have a hydrogen atom bonded to a highly electronegative atom such as fluorine, oxygen, or nitrogen. The other species must have a highly electronegative atom without a hydrogen atom bonded to it. However, all molecules interact with other molecules through Van der Waals interactions.
Van der Waals forces are the attractive forces of one transient dipole for another. A transient dipole is a temporary imbalance of positive and negative charge. At particular instances, even atoms that are spherical on average, such as those of the noble gases, will have greater electron density on one side of the atom than another. At that instant, the atom will possess a temporary dipole with a negative charge concentration on the side of the atom with greater electron density.
If this happens in the case of an argon atom in liquid argon, for example, the argon atoms next to the one with temporary dipole would feel the effect of the dipole. An atom near the negative end of the dipole would have its own electrons slightly repelled from the negative concentration of charge, developing a dipole with its positive end near the negative charge of the original atom. An argon atom on the other side of the original temporary dipole would feel its electrons attracted to the positive end of the dipole, developing a dipole with the opposite orientation. In this way, temporary dipoles are propagated through a liquid or solid. The motion of the molecules in the liquid or solid soon disrupts the pattern, but similar events take place continually. The larger the size of atoms and the more electrons they possess, the greater the probability of forming substantial transient dipole interactions. Molecules which are non-polar and non-polar functional groups of molecules only experience Van der Waals interactions with other molecules or functional groups.
To understand the differences in properties of larger molecules, the additivity of intermolecular interactions becomes important. In effect, the interaction of each group of atoms of a molecule with a group of atoms of a neighboring molecule can be considered to be independent of the interactions of other groups of atoms of the molecules. The total energy required to move two molecules apart is the sum of all the energies of the individual interactions. The more groups and the stronger each individual interaction, the greater the sum of energy of interactions. Among the non-polar linear alkanes, the boiling point for a molecule with many -CH groups, such as liquid octane--CH(CH)CH--is higher than that of gaseous propane--CHCHCH--because of the greater number of Van der Waals interactions between the octane molecules. For large molecules such as the higher alkanes (heavy oils and waxes) and polymers such as polyethylene, the total attractive energy due to Van der Waals forces can be greater than the polar interactions or hydrogen bonding interactions of other, smaller molecules. Hence, molecules that have only Van der Waals interactions may still melt or boil at high temperatures.
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