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PROFESSOR CIMA: Now what makes this polar covalent bond so different,
other than the difference in bond energy? And that's hidden in the name.
It's polar. What does that mean?
Well it means it has a dipole moment. Now a dipole moment arises from two charges.
So let's do it here. Just to keep it consistent with my notes,
I'm going to put a negative charge here and an equal and opposite positive
charge here. And the convention is to draw the position
of the two charges as a vector from the positive charge to the negative
charge. So a distance r.
So we say that the separation of these two charges creates this electric
field, that we've pictured before. It kind of looks like the magnetic field at
the two ends of two magnets. It's the same electric field as what we call
a dipole, where it's a vector that starts at the positive and goes to the
negative. And I've drawn on the same link but that's
not quite right. They're just drawn in the same direction.
And numerically what happens is, is that the dipole is the product of the
charge times the charge separation. And it's in the same direction as r.
Now obviously if this is-- now this is the absolute magnitude of the charge.
If this is an electron and that's a proton, it's just the charge on the
electron, the fundamental charge. The units of this, obviously, are going to
be coulomb times distance: coulomb-meters.
It was so important early on in chemistry, that a special unit was
derived called the debye to measure. And it turns out it's 3.3 x 10^ -30 coulomb-meters.
And it's basically of the order of the magnitude-- not completely-- but of the order of the magnitude
of the dipole moment created in a hydrogen atom, an instantaneous
dipole moment in a hydrogen atom.
And It's called the debye unit. And it's handy because, as we'll see, the
dipole moments that we deal with in molecules are of the order of the debye.
Now why is this important for this polar covalent bond?
Well we said here that this extra energy was because the electron was
not being shared equally between the two atoms. So as a result, a polar covalent bond has
to have a dipole moment. Yes sir?
STUDENT: Which q is the one being used in the equation?
You've got p vector equals q times r vector. PROFESSOR CIMA: So the net charge is zero.
So this q is equal and opposite in magnitude from this one.
So whatever this q is, that absolute magnitude goes in there.
That's all it is. Good question.
OK. So by the nature of this, the fact that we're
sharing electrons unequally between A and B--
let's say B has the greater electronegativity. They'll be some q negative over here and q
positive over here. And there's some distance, there's some bond
length between them. So the dipole moment, at least numerically,
is going to be b times q. Now q, in this case, doesn't have to be in
units of electrons charge, fundamental charge.
Right? It's how much on average does the electron
go there? So it can be some sort of partial charge on
one end of the molecule separated by this.