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    How can we tell what type of inter molecular forces exists between molecules and atoms (excluding ionic, covalant and metallic) like the dispersion, permanent dipole - dipole bonds. I know what they are, but how do we tell whether or not they exist in a given molecule?
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    (Original post by Uranium Potato)
    How can we tell what type of inter molecular forces exists between molecules and atoms (excluding ionic, covalant and metallic) like the dispersion, permanent dipole - dipole bonds. I know what they are, but how do we tell whether or not they exist in a given molecule?
    Hydrogen bonding is present in any molecule where hydrogen atom(s) are bonded to Oxygen, Nitrogen, or Fluorine. A permanent dipole is present when the difference between the electronegativities of two atoms is at least around 0.7 or 0.8(?) (I have seen some numbers like this somewhere before, I think). But with permanent dipoles, I think you get a feel for whether a molecule has one after a bit. Dispersion (van der Waals) forces are present between all molecules, but increase as the proton number of the atom increases due to the increased strength of the instantaneous dipole, as more electrons are present. Hope this helps a bit!
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    Permanent dipole-dipole interactions will only exist in atoms of elements where there is a relatively large difference in electronegativity. This means there is a polarity and a permanent dipole-dipole interaction will be present, for example between molecules of HCl as hydrogen is on one side of the periodic table (delta positive) and Cl is fairly electronegative as it's on the far right side (delta negative).

    Instantaneous dipole-induced dipole interactions (also called London dispersion forces) hold together molecules with a simple covalent structure. For example, fluorine (F2) has a relatively low melting point because there is no covalent bond broken, but the weak London forces are overcome, and this requires little energy.
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