# Why are dipoles "permanent/induced dipole permanent/induced dipole" and not just "permanent/induced dipole" once?

My teacher would always say "induced dipole induced dipole" and while it annoyed my slightly (as you were saying the same word twice) it didn't really bother me, but now looking through some exam papers it's called the same thing "induced dipole induced dipole", so why is it repeated twice? Would I be "wrong" for just saying "induced dipole" once? If so, what's the difference between the two?

(Sorry if this seems like a simple or stupid question, just I'm always curious and the internet doesn't really work for searches like this.)

• The correct typographical syntax would be "induced dipole–induced dipole", which should help to clarify why it is being repeated twice. Dropping the adjectives, you get "dipole–dipole", which is easily understood: two dipoles are interacting. Unfortunately, in verbal speech, this kind of nuance isn't there to provide the additional clarification, and many people are sloppy in written grammar, too. May 26 '17 at 7:25
• "Sorry if this feels like a simple or stupid question" In the words of a certain someone from the Physics.SE "There are useful questions, and there are questions that aren't so useful...but there's no such thing as a stupid question". This is a pretty good question, and I for one, appreciate that you took the time to post it here. All questions are welcomed at Chem.SE (except poorly formatted HW questions), so feel free to post here. Looking forward to your future contributions to Chem.SE. Cheers! ;) May 26 '17 at 10:48

Because it takes two to tango.

Dipoles interact with each other. A Lone dipole has nothing to interact with (other than an electric field, but if we ignore some externally applied macro field, there is nothing for a lone dipole to interact with).

So molecules with an inherent dipole (like water or chloroform) interact with each other. One molecule's dipole interacts with the other molecule's dipole. So you would never say "dipole interaction" only "dipole-dipole interaction". The repeated word is because it takes two dipoles to interact.

The same pattern applies to non-polar molecules with little or no inherent dipole. For example, benzene. Benzene has no built-in dipole, but the electrons in its bonds are fairly polarisable (which basically means it is easy to induce a dipole in them). So benzene molecules do interact but via London or van der Waals forces which are much weaker than the reactions of molecules with inherent dipoles. But one way to describe those weaker interactions is (simplifying a lot) to see them as forces created by fluctuation and temporary quantum-mechanical induced dipoles. Imagine the electrons in one benzene molecule stray from perfect symmetry in their distribution and create a temporary dipole; this may interact with the electron clouds in other benzene molecules an induce other temporary dipoles (or interact with other existing temporary dipoles). Either way the interaction is an "induced dipole-induced dipole" interaction.

So however you understand the interactions, to describe them properly you have to repeat the phrase twice.

• You can also have "permanent dipole-induced dipole interactions", where, for example, the permanent dipole on the water molecule interacts with the electrons on the benzene and causes an induced dipole, which then has a dipole-dipole interaction with the water dipole. (The fact that the interaction can be mixed means that it's not redundant to specify "permanent" or "induced" twice - it's providing valuable information that the type of dipole is the same.)
– R.M.
May 25 '17 at 23:27

Because it takes two items to have an interaction.

Two permanent dipoles interact.

Two induced dipoles interact.