Intermolcular attractions?
Answer:
Dipole-dipole attractions are only experienced by molecules that are polar...in other words, molecules that have a permanent dipole moment. The attraction is between the partially negative end of one polar molecule to the partially positive end of another polar molecule. Nonpolar molecules (like O2, N2, or CO2) don't experience these kinds of interactions.
London-dispersion forces are experienced by all molecules or atoms. Because electrons move (or more accurately, because the distribution of electron probability around a molecule or atom can temporarily shift), you might have a temporary situation where the electron distribution is slightly lopsided. For this tiny instant, the area where there's a greater probability of finding the electrons is slightly negatively charged while other regions are slightly positively charged. If this slightly negative area is close to the electron cloud around another molecule, it can cause the electron cloud around this second molecule to temporarily distort, too. This is because like charges repel one another and opposites attract...the electron cloud on the second molecule may try to adjust to account for the temporary charge in the first molecule.
This is a rough description of what's going on with these two different types of intermolecular forces. The two key differences I see are:
1) Dipole-dipole interactions only occur between polar molecules while London dispersion forces occur between all molecules
and
2) Dipole-dipole forces (when present) are typically stronger than London-dispersion forces, but this depends on a number of factors such as the size of the molecule, the strength of the dipole moment, and how polarizable the electron clouds around the molecules are.
Hope this helps!
dipole-dipole forces are attractive forces between molecules that have a permanent dipole. in other words, it's an attractive force between polar molecules. remember that polar molecules have a region that has a partial negative charge and a region that has a partila positive charge. attractive forces are established when the the positively-charged end of one molecule gets near the negatively-charged end of another molecule.
london dispersion forces exist between nonpolar molecules. nonpolar molecules don't have a permanent dipole. however, keep in mind that the electron cloud of a nonpolar molecule is not always necessarily evenly distributed around the molecule. this is because electrons are moving, and you can imagine that at some given point in time that because of the random movement of electrons, the electrons might be "bunched up" in one area of the molecule. if this happnes, the region that has the bunched up electrons will have a partial negative charge, and another region of the molecule will have partial positive charge. so the molecule has just acquired a temporary dipole. now imagine the negative end of this molecule gets near another molecule. the partial negative charge on the molecule will repel the electrons of the second molecule, and cause them to bunch up on the opposite end of the molecule. when this happens, a temporary dipole will have been induced in this second mlecule. the two molecules can now be attracted to each other because they both have temporary dipoles. these temporary dipoles that form in nonpolar molecules are much more significant and long-lasting at lower temperatures. in any case, its the ability to form these temporary dipoles that allows nonpolar molecules to liquify and solidify.
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