Chemistry: Demo: The Paramagnetism of Oxygen

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A prediction from molecular orbital theory that doesn't come from valence bond theory is that dioxygen, O[2], should be a paramagnetic molecule, meaning that it should have unpaired electrons; in particular, it should have 2 unpaired electrons.
Now, when we have a theory that doesn't make a correct prediction, it is superseded oftentimes by a superior theory, one that does explain something, an experimental observation. Now, that's not to say that the old theory is wrong, per se. In fact, valence bond theory is very useful for doing a lot of things, but it isn't complete. It doesn't tell the whole story. What I'd like to show you now is that the prediction for molecular orbital theory, that dioxygen should be a paramagnetic molecule, is actually correct, by pouring liquid oxygen between the poles of a magnet. First, we're going to pour liquid nitrogen between the poles of a magnet, and both molecular orbital theory and valence bond theory predict that liquid nitrogen should be a diamagnetic molecule, meaning that it should not be attracted to the poles of a magnet.
So here we have a powerful magnet. And the magnetic field lines are going in between the two ends of the nut here. And we're going to pour some liquid nitrogen in this Styrofoam cup in between the two poles. And what we're looking for is for some of the liquid to be suspended between the two poles of the magnet. And I think that you can see that that's not happening. Again, this is the nitrogen, and liquid nitrogen is not attracted to the poles of a magnet. What I'm also going to do is actually pour some more liquid nitrogen, even though this is a non-interesting result. And the reason is that what I'm trying to do is pre-cool this magnet a little bit, so that, when we pour the liquid oxygen, it hangs around for a little bit longer. Liquid nitrogen, by the way, is at 77 Kelvin and liquid oxygen is a little bit higher than that. It's something in the 80's, but I don't know what it is off the top of my head.
So what I have here is some liquid oxygen, and you can see that it's blue. Liquid oxygen is actually blue, and now I'm going to try to gently, carefully pour the liquid oxygen between the poles of this magnet. And, hopefully, you'll be able to see that it's held up between the poles of the magnet. There you go. Now you can really see that the oxygen is sitting between the poles of the magnet. It's actually attracted to the region in between the two poles. And, if it sits there for a while, until it evaporates - and, once it evaporates, it turns back into a gas - it's gone. Again, this is an indication that valence bond theory is inadequate, that shows why molecular orbital theory was necessary. And again, molecular orbital theory makes the correct prediction, that dioxygen should be paramagnetic.
Molecular Geometry and Bonding Theory
Valence Bond Theory and Molecular Orbital Theory
CIA Demonstration: The Paramagnetism of Oxygen Page [1 of 1]