Editing 2019-2020 Updated IMF Material
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The links below need to be modified due to more specific explanation of Intermolecular Forces (IMF) as will be discussed now.<br> <br> <u><b>van der Waals forces</b></u> is the name people give to both <b>dipole-dipole IMF (D - D)</b>, between two polar molecules, and <b>London dispersion forces (LDF)</b>, between 2 non-polar molecules. <br> <br> Therefore, <b>Hydrogen bonding (H - B)</b> is not considered a <b>van der Waal force </b>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). <br> <br> With respect to London dispersion forces (LDF), actually all compounds and <b>even single atoms</b> 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. <br> <Br> <b>Therefore, all species have LDF (or some times dispersion forces) but most of the time, the other IMF make the LDF force insignificant.</b><br> <br> 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.<br> <br> 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.<br> <br> 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.<br> <br> [http://tmcleod.org/cgi-bin/chem1/wiki.cgi?Intermolecular_Forces_And_Physical_Properties IMF and Physical Properties] <br> <br> *Link to 1st year Bond polar, Molecular polarity, IMF, [http://tmcleod.org/cgi-bin/chem1/wiki.cgi?Polarity_And_Intermolecular_Force_(IMF) Polarity and Intermolecular Force (IMF)] *Other Physical Properties and IMF link **[http://tmcleod.org/cgi-bin/chem1/wiki.cgi?Physical_Properties_Or_Application_Of_Intermolecular_Force_(IMF)_-_Vapor_Pressure_And_Boiling_(Part_Of_Phase_Of_Matter) Physical Properties or Application of Intermolecular Forces (IMF) - Vapor Pressure and Boiling] **[http://tmcleod.org/cgi-bin/chem1/wiki.cgi?Physical_Properties_Or_Application_Of_Intermolecular_Force_(IMF)_-_Solubility_And_Miscibility Physical Properties or Application of Intermolecular Force (IMF) - Solubility and Miscibility] <br> *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.)<br> <br>
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