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London Forces or van der Waals Forces Dipole-Dipole Attraction Hydrogen Bonding Intermolecular forces(IMF's) are forces of attraction that hold groups of covalently bonded atoms called molecules to other molecules. ***Therefore Ionic compounds are technically not held together by IMF's.
Generally, the weakest IMF is called London Forces or van der Waals Forces. This is the only force between 2 nonpolar molecules. It is found in all molecules, but usually not acknowledge to it being so weak. An example is Nitrogen (N2). It is nonpolar and can only liquefy by these weak forces. A little energy and liquid nitrogen molecules will overcome these attractive forces and vaporize.
Stronger than van der Waals is a Dipole-Dipole Attraction. Yes, that is the interaction of two dipoles (aka a polar molecule). Since polar molecules have a slightly positive and slightly negative end they attract very nicely. Much stronger force of attraction than van der Waals, which is present but insignificant in comparison to dipole-dipole forces.
The strongest of the IMF's is hydrogen bonding. It is NOT what holds 2 hydrogen (H2) molecules together (that is van der Waals). Hydrogen bonds occur between molecules that have a permanent net dipole resulting from hydrogen being covalently bonded to either fluorine, oxygen or nitrogen.
***2 Important Notes- 1. If all 3 of these IMF's are present in the substance, this order holds true.
2. When comparing 2 different substances, the stronger the IMF's has the higher the boiling point. ***Here is the how we separate the top chemistry students from the rest.** The type of IMF depends on the polarity of the molecule. van der Waals forces can be stronger than hydrogen bonding if you are comparing 2 different substances.
London Forces or van der Walls Forces Dipole-Dipole Attraction Hydrogen Bonding On to Like Dissolve Like |
