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Intermolecular Forces - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Intermolecular forces are forces that hold two molecules together. Dispersion forces (also called Van der Waals Forces) act on all molecules and are the only forces between two non-polar molecules. Two polar molecules are held together by the electrostatic attraction between their dipoles in dipole - dipole force. The strongest type of intermolecular force is hydrogen bonding.

Alright so we're going to talk about intermolecular forces, you might also have heard that as IMF That is the attraction between 2 or more molecules, I hope you don't get that confused with intramolecular forces. So an easy way like you think, when you think internet like the internet is how you interact with other people in the out of the whole entire world. But if you have intranet off your school or your maybe you have a job has something called intranet that is just within that particular school or within that particular job. It's not, you can't communicate with people everywhere, so there are differences with if intermolecular forces and intramolecular forces. Intramolecular forces you might be familiar with, ionic bonds are intramolecular forces, how the cations and ions are attracted to each other. Covalent bonds are also intramolecular forces and they're actually the bonds between the sharing of the valence electrons, there's the covalent bond that's actually intramolecular forces.

And then lastly the metallic force, it's the metal cations with mobile electrons kind of flowing between the cations the sea of electrons. Those 3 are intramolecular forces they're within a compound. Intermolecular forces are between a compound and another compound, so let's talk about those guys, okay so the 3 main IMFs the first one being called hydrogen bonds this is deceiving because hydrogen bonds are not really bonds, they're not bonds but they're called bonds because they're very strong in comparison to the rest of them.

These guys are really strong you might have noticed them as called H bonds and what they are is just when a hydrogen atom of one molecule is attracted to a highly electronegative atom of another molecule. The highly electronegative atom that I'm talking about are F fluorine, oxygen and nitrogen these are the only ones that can be, hydrogen can be bonded to, to make hydrogen bonding. So for example what I mean by that let me grab a pen, what I mean by that is let's take water for example, here is what water looks like and it's one water molecule and another water molecule. These are the covalent bonds not the IMF okay these are the covalent bonds within the water molecule.

Okay and we know that oxygen is much more electronegative than hydrogen. So the electrons are going to flow a lot more around the oxygen atom than around the hydrogen atom. So this is going to be slightly negative and we're going to denote that with a delta with a minus charge and these guys are going to be slightly positive. Okay same goes with this guy, electron is going to flow towards the oxygen making this slightly positive and this slightly negative. Okay and we know that negative charges and positive charges attract each other, so we're going to have a very strong attraction here, this is our H bond okay this guy is responsible for the boiling point of water being pretty high, it's responsible for the helping freeze and the way the crystal structure is. It's responsible for a lot of things within water, that makes water, water. This hydrogen bond is pretty important and it's pretty strong, it's a very strong high attraction one water molecule to another.

Another type of IMF is dipole-dipole, this is when attraction between oppositely charged regions of a polar molecule. Now don't forget in a polar bond we have one that's highly electronegative atom bonded to another non-electronegative atom. So not electronegative as fluorine, oxygen and nitrogen but pretty close. So electrons here are going to flow towards the chlorine okay making this positive, this negative, same thing goes over here making this positive and this negative. And again we know that negative and positive attract, very similarly to hydrogen bonds but these guys are much more different in electro negativity values than these guys. So this is not as strong, so this guy here is the dipole-dipole interaction.

Okay so these guys are the strongest and these guys are strong but not as strong. Okay the last one is London dispersion or you might have known that as Van Der Waals forces it's or induced dipole that might be something that you've heard before. Induced dipole when something is induced we kind of force it to be a certain way, so it has been called induced dipole between two non-polar molecules. So here's nitrogen that's non-polar, electrons are going to be equally shared among this nitrogen as they are in this nitrogen. However, we do know that electrons are constantly moving right, they're always in motion. So some point or sometimes the electrons are going to be over here at some particular time. So we're going to just for a second this might be slightly negative and this might be slightly positive. Which then induces this molecule to then, say electron I don't like being around other electrons so this guy is going to flow over here making this guy positive and this guy negative making an attraction here.

Here's the induced, we induce this one to actually change, so here is the induced dipole, this is very, very weak because electrons are constantly moving, this will break and electrons will flow back here because we know this is non polar there's no reason that it's over here versus over here this is non-polar electrons are going to be equally shared. So it'll break, so this is very, very weak, things that are non-polar are typically gasses because they're not going to be, it's not going to take a lot of, hardly any energy actually to make them into gasses because these electrons, I mean sorry this bond is very, very weak and the attraction is very, very weak so its going to be all over the place. So typically gasses then liquids and these are typically solids of room temperature because of the IMF, so that basically is IMF.