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Tips for Identifying Intermolecular Forces - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tips and tricks for identifying intermolecular forces. Remember, the prefix inter means between. So these are forces between molecules or atoms or ions. So these are intermolecular forces that you have here.

The first type, which is the weakest type of intermolecular force, is a London Dispersion force. A London dispersion force occurs between mainly nonpolar molecules and also between noble gas atoms. They have between the noble gases. They are the weakest.

An example can be like in Methane, CH4. And so, if I drew a 3D representation of this, where the little dash line here means that this hydrogen is in the back. This hydrogen is in the front, and then this one’s over here. So this would kind of be like a 3D, like a jack that you have here. So let’s take a jack.

All the Hydrogens are spaced out evenly. And so if I drew another methane, then that means that this force in between two hydrogens. Because if you notice. all the same charges are on the exterior of that methane molecule. So since they’re all the same and the same type of charges are touching each other, then the intermolecular force would be a London Dispersion Force. That would be the weakest type of intermolecular forces that you have there. And also between noble gases like Krypton and krypton atoms, you would have London Dispersion Forces.

The second type is called a Dipole-Dipole interaction. The prefix di means 2, and so there has to be two poles. Say for example H - Br and I want to draw the electrons in lone pairs around the Bromine just to save time. So I have H – Br and H – Br, using your electronegativity value, you can figure out that Bromine is more electronegative. So that means Hydrogen would be the positive end, or the positive part of the molecule and the Bromine would be delta negative.

So, in between the Bromine and the Hydogen, you would have an intermolecular force. So it’s not a bond, like a covalent bond or ionic bond, so it’s not those. The intermolecular force that’s between those two separate atoms, would be a Dipole – Dipole Interaction, because you have opposite or different charges that are interacting between two different molecules. The charges on different parts of the two molecules, so you have different charges.

The third type that you have is called Hydrogen bonding. Hydrogen bonding is actually not a bond, remember it’s an intermolecular force. This type of intermolecular bond is the strongest. Dipole – Dipole in the middle. So hydrogen bonding is like the name says. It involves hydrogen, but it only involves 3 elements, F Fluorine, O Oxygen and Nitrogen, N. In any case you have H – F for example, and another H – F. And so in between the H and the F you would have an intermolecular force. And intermolecular force between those molecules would be Hydrogen bonding. So you have a Hydrogen bond over there.

Now there is a special case going back to London Dispersion Forces, say in H – Br, what if one of the molecules get’s flipped around so that the Bromines are actually close to each other? What happens is then since they have the same charge, that intermolecular force is actually a London Dispersion Force that could happen between those particular molecules. It depends on the orientation of your molecules. So I’ll add on London dispersion Forces could also happen when same charges interact.

These are the three types of intermolecular forces; London Dispersion Forces which are the weakest, which occur between nonpolar noble gases and same charges. So if you see any of those cases, then that will help you identify that it’s London Dispersion Force. Dipole – Dipole, generally when you have different charges like delta positive and delta negative, If you have different elect negativities, then you can probably, just remember that in between those molecules would be Dipole – Dipole. And Hydrogen bonding, only if you have Fluorine Oxygen and Nitrogen then you could probably figure out that most likely it would be Hydrogen bonding. Unless if the molecule was a non polar molecule then in that case it would be a London Dispersion Force.

Hopefully these dispersion tricks will help you identify intermolecular forces a lot easier.