![]() ![]() As soon as the second molecule moves away, the induced dipole disappears. This force is sometimes called the induced dipole-induced dipole interaction since it involves the transient creation of an induced dipole within a nonpolar molecule due to the close proximity of the electron cloud of a second molecule. If the molecule is nonpolar, then no dipole-dipole interactions or hydrogen bonding can occur and the only possible intermolecular force is the weak van der Waals force. This VERY partial positive hydrogen atom will be VERY strongly attracted to the partial negative atom in the second molecule thus making the strongest intermolecular force, the hydrogen bond. Hydrogen bonds also involve the attraction of a partial positive atom in one molecule to a partial negative atom in a second molecule (and vice versa) but in this case the partial positive is a hydrogen atom which is VERY partial positive by virtue of the fact that it is covalently bonded to a small, electronegative atom (F, Cl, O, N). ![]() Hydrogen bonds are a special type of dipole-dipole interaction but are about five times stronger and thus are considered to be a distinct intermolecular force. This type of force is called a dipole-dipole interaction. If the molecule is polar, it has areas of partial positive and partial negative charges which can interact with the areas of partial negative and partial positive charges in a second molecule (since opposites are attracted to each other). How do we know which type of intermolecular forces exist between the molecules of a given substance? You must be able to figure out whether or not the molecules are polar (see the Molecular Polarity module). This means that it will take more energy to disrupt those forces between molecules and the first substance should have the higher boiling and melting points. Thus a sample of a substance capable of hydrogen bonding will have stronger forces between its molecules than those between the molecules of a substance which is only capable of van der Waals forces. Of these, hydrogen bonds are the strongest followed by dipole-dipole interactions and finally, van der Waals forces. There are a variety of these forces but the most common are: dipole-dipole interactions, hydrogen bonding and van der Waals (or London) forces. Intermolecular forces are noncovalent forces between molecules. ![]()
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