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PROFESSOR CIMA: If you're like me, you'd say, OK, that's fine, Professor
Cima, but why does this have a boiling point of 47.5 [degrees Celsius]?
If it doesn't have a dipole moment, what's holding it together?
Well, the answer to that is the next intermolecular force that we're going
to talk about is what's called London dispersion forces.
Consider here for a second just a rare gas atom.
Argon. Argon has its nucleus and a bunch of electrons
around, and averaged out over time, they have this spherical symmetry.
But I ask you to stop for second and consider what happens in an instant.
In much shorter than a picosecond. In a very, very short amount of time, there's
a high probability that more electrons will be on this side of the atom
than they are on that side of the atom.
So if there are more electrons on the side than there are on this side of
the atom, there will be temporarily a dipole moment.
There'll be an instantaneous dipole moment. But it vanishes in the next instant of time
because where does it go? It goes over here.
It goes over here, it goes over here, and it keeps on doing that.
And of course averaged over time, the rare gas molecule doesn't have a
dipole moment. And that's true for other rare gas molecules.
They may have an instantaneous dipole moment that looks like that.
And if they're far enough apart, the random orientation that happens in
these two atoms goes on unperturbed. The situation changes, though, when they get
close. How did I draw that?
I drew this one like this. In the sense that an instantaneous dipole
on this argon atom will create an electric field that predisposes the instantaneous
dipole moment on the neighboring atom to want to be this way.
Now, these aren't permanent. In the next instant, they will be, like this
one could flip. And then, of course, what that does is it
makes that one go the other way. So in other words, what happens when these
atom get close is that the random orientations of these dipole moments
are no longer random. They start to dance.
They're correlated. And it turns out, one of the things that happens
in quantum mechanics is when electron's motions is correlated, that
lowers their energy. And so this behavior creates a net decrease
in energy, because at any instant in time, there's always two dipoles
that are oriented to lower their energy, even though it varies over time.
Now, Mr. Fritz London characterized this. He considered atoms A and B, and said what's
the dispersion force between these atoms and what is the energy of the
dispersion force. Dispersion, what I have here, A, B. It turns
out this is approximately-- and I'll define find these terms in a second.
The I's are their ionization energies, and the alphas are what's called the
polarizability. Polarizability is a property of any material,
not just a molecule. And it's basically defined as if I take a
molecule, or an atom, or any material and put it in an electric field,
which I have in my diagram this way, I will induce a dipole in that field.
The electric field causes the electrons to go to one side of the
atom on average, leaving the other side a deficit.
So that means the other side-- So that's called polarization, and it happens
in many materials too. We'll talk about that.
So the proportionality constant between the dipole created and the
electric field applied is the polarizability. So you can see why this works this way.
If I have atoms with high polarizability, it means just a little
dipole moment here induces a large dipole in the neighboring atom if it
has high polarizability. So at the heart of this interaction is the
polarizability of the atoms. It has units of Coulomb meters squared per
volt.