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PROFESSOR: So now we're going
to start Part (b).

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First we're going to
read the question.

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To which does an atom of argon
form a stronger bond: another

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argon atom or an atom
of krypton.

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Justify your choice with an
explanation, using narrative

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or cartoons or both that makes
reference to the operative

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chemical bonding.

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So again, he's not just asking
you for one or the other, but

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that you give an explanation
to show that you understand

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the concepts that are
being tested.

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Here we're asked: does argon
bond stronger with another

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argon or krypton.

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And some students were confused
by the usage of the

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term bond here.

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But remember that bond doesn't
necessarily mean a covalent or

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an ionic bond, it can also mean
intermolecular bonds,

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which we have been using in
Part (a), so it might seem

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logical that we would think
about intermolecular

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bonds in Part (b).

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Now we need to look at the
specific species we are asked

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to consider here.

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Argon and krypton are both Nobel
gases, and what do we

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know about Nobel gases?

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They have complete octets; they
don't bond very often,

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and so, especially
to each other.

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Therefore.

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we will probably be looking at
intermolecular bonds, as I

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said before, but also
the weakest form,

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which is Van der Waals.

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We're looking at Van der Waals,
because each of these

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are a mono-atomic species.

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That is, they just are gases as
one atom of argon and one

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atom of krypton.

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And so, Van der Waals is the
only type of interaction they

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can have. As I said in Part (a),
Van der Waals has to do

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with the movement of electrons
in an induced dipole.

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And what does that mean?

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So, we're going to talk
a little bit about

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polarizability.

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And polarizability is basically
how easy is it to

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induce a dipole into the
atom or molecule.

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And because these are non-polar
atoms, obviously, we

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want to think about what can
cause a difference in

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polarizability between
argon and krypton.

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So if you think about it,
we're going to do a very

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simplistic model of the atom.

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We have a cloud of electrons,
and to get intermolecular

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interactions, we want to have
a partial negative and a

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partial positive induced
in the atom.

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And this can be due to just
spontaneous fluctuations in

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the electron density or to an
external charge or another

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molecule or something.

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But here we have two non-polar
atoms, so we want to see which

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one will inherently be
easier to polarize.

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And so if we go back to our
two species here, we have

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argon, which has 18 electrons
and krypton,

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which has 36 electrons.

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So based on our discussion of
polarizability, we can see

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that krypton is going to have
a greater amount of

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polarizability, because it has
a larger cloud of electrons.

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Those electrons are bound
less strongly.

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As we know from our periodic
trends, krypton is a larger

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atom, and so we are going to say
that the polarizability of

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krypton is greater than argon.

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As we talked about in Part (a),
even though these had the

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same type of intermolecular
bonds, if one of them exhibits

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a higher difference in partial
charges as krypton will, then

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it results in a stronger bond.

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And so the answer here would be
that krypton has a stronger

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interaction because it
is more polarizable.

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And that because is important.

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Remember, when you're answering
a question, you

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don't just want to put down the
answer, you want to show

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that you know why that's the
answer, and that you

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understand the concepts.