ApchemWiki | RecentChanges | Preferences
The links below need to be modified due to more specific explanation of Intermolecular Forces (IMF) as will be discussed now.
van der Waals forces is the name people give to both dipole-dipole IMF (D - D), between two polar molecules, and London dispersion forces (LDF), between 2 non-polar molecules.
Therefore, Hydrogen bonding (H - B) is not considered a van der Waal force because Hydrogen bonding IMF is larger (or stronger) IMF than van der Waals forces. Remember, H - B is the special very large dipole - dipole IMF caused when by dipoles that contain H - F bond, H - O bond and/or H - N bond since these bonds have high Δ EN (electronegativity difference) but more importantly the non-hydrogen atom are small radii atoms (causing higher Coulombic force).
With respect to London dispersion forces (LDF), actually all compounds and even single atoms have LDF. Remember, LDF is caused by the fact that the electrons in atoms and/or molecules are in constant motion so there is always times where electrons are not equally distributed over the element and/or compound causing an Instantaneous dipole (unevenly distribution causes a partial negative and partial positive area of the species making instantaneous Coulombic forces). This instantaneous Coulombic forces (we called Instantaneous dipole) only exist of a short period of time (billionth of a second) so it is a very weak force.
Therefore, all species have LDF (or some times dispersion forces) but most of the time, the other IMF make the LDF force insignificant.
This is the reason if you compare two different substance with significantly different molar mass / Molecular weights (MW) ( i.e. very large difference in number of atoms/electrons), the substance with the higher molar mass has a higher IMF (can often be seen with a high boiling point for example) not matter what type of IMF the two substances have. Of course, it has to be significantly different.
The information below will say that LDF are when two non-polar molecules interact. This is NOT inconsistent with the explanation above. Remember, non-polar molecules due to either bonding type and/or symmetrical geometry, on average, the electrons are evenly distributed throughout the compound. So these substance do not exert Coulombic force ( thereby IMF) in any other way except for Instantaneous dipoles (i.e. London Dispersion Forces). Therefore, LDF for these compounds are the only force being exerted.
Please keep the above discussion in mind when go to the links below that contains Youtube video, lecture notes. and graphs/figures on IMF and the Physical properties that they cause.
[IMF and Physical Properties]
- Other Physical Properties and IMF link
- For AP students, there are more physical properties that are caused by IMF that will be discussed in a different webpage (i.e. Surface tension, capillary action/effect, cohesion, adhesion.)