London vs. Dipole

[ElizabethP1L]

Hi everyone! I was just wondering what the main difference is between London forces and dipole-dipole. I thought they were essentially the same thing...Thanks in advance! :)

[Chem_Mod]

So London Dispersion forces are slightly different in that they exist even when there is no net dipole moment in the molecule, ie something that is non-polar will only have London dispersion forces. This intermolecular force is the result of instantaneous dipoles that will happen for a fleeting moment and then disappear. You can think of this like a chandelier that flickers, it is on the whole time but then will randomly flicker. However, when a molecule is polar it has a net dipole moment, meaning that any there is an uneven distribution of charge. This localized charge on specific atoms is what will cause attractions between molecules with opposite charged regions. You can think of this as if the chandelier has several lights on one side of it that are out. So a molecule like H₂O will have dipole-dipole forces because it is polar due to its net dipole moment. Water will also have London dispersion forces, since all molecules have London dispersion forces, but only polar molecules can have dipole-dipole interactions. London dispersion forces are the weakest, and so you can think that this is the point for all molecules, but then other characteristics of a molecule, like a net dipole moment or the ability to hydrogen bond, will potentially add on more intermolecular forces to attract molecules within the same physical state.

[Nina_A_Section1E]

https://socratic.org/questions/5a64204ab72cff7b83554c78 This site has a really good summary!

London forces/Van der Waal's forces is completely random and unrelated to electronegativity. Sometimes electrons happen to be closer to one molecule at a given time, creating contemporary partial negative charges near one atom and positive near another. This creates partial negative and positive charges on the molecules next to them, leading to a chain reaction of weak partial charges.

All other forces have to do with the electronegativity of an atom, so how strongly it pulls e- toward it, which also creates partial charges, but stronger than the random London dispersion forces.

[Nicole Shak 1L]

In one of the homework problems from chapter 6, it asks what possible intermolecular forces occur in the molecules given, and for each of the molecules, London forces can always occur. London/Vander waals forces are small/weak temporary interactions and can occur quite frequently, whereas dipoles occur when there is a stronger difference in electronegativity.