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[SQUEAKING][RUSTLING][CLICKING]

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CLAIRE HALLORAN: Today we're
going to do Goodie Bag 4,

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VSEPR.

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Our objective is to visualize
the three dimensional

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structure of some simple
chemical compounds.

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The only thing you'll need
is a molecular modeling kit.

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A conceptual question you
should think about today

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is, what factors determine how
bonds rotate in a molecule?

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So the first thing
we're going to do

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is draw the Lewis structure
for this molecule.

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So we know that Si, S, and CN,
since they have between three

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and five valence
electrons, make up

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the backbone of this molecule.

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And then we're going
to add all of the atoms

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around each of these
backbone atoms.

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Then counting valence
electrons, we're

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going to make sure that
we have the correct number

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of non-bonding pairs in bonds.

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And this will allow us
to identify the geometry

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at each atom in the backbone.

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So this silicon bond here
has four bonding domains,

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and all of them are
bonded to another atom.

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So we know that this is going to
be a test tetragonal geometry.

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This bond here, centered
around the sulfur,

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has five body domains, and one
of them is a non-bonding pair.

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We know that this will
be a seesaw geometry.

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And then, finally, this
carbon has two bonding domains

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with atoms in each
of those domains,

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so this will be a
linear geometry.

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And now we're ready
to build our model.

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So first, we're going to
draw the Lewis structure

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for this molecule.

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As with before, we're going
to identify the backbone

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atoms in this molecule.

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So in this case, the backward
atoms are C, B, and O.

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And now we're going to attach
the other atoms to them.

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And now we're going
to check and make sure

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that there are the proper
number of valence electrons

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around each atom.

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Next, we're going to
identify the geometry

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at each of the backbone atoms.

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So the first geometry centered
around this carbon atom here

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has four bonding
domains, all of which

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contain bonds to other atoms.

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So that's going
to be tetragonal.

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The second geometry has three
boding domains, all of which

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are bonded to other atoms.

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So this is going to be a
trigonal planar geometry.

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And, finally, the geometry
around this oxygen atom

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has four bonding domains, two
of which are bonded to atoms,

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and two of which are
occupied by lone pairs.

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So that's going to
be a bent geometry.

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And now we're ready to
construct our model.

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The Goodie Bag 4
worksheet asks us,

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whether some different bonds
can be in the same place?

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So first, it asks, whether
this H, Si, Si, S, and CN bond

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can be in the same plane?

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Remember that the CN
bond cannot rotate,

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because it's a triple bond.

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But we can rotate around this
S, Si bond, and get all three

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of the bonds to be
in the same plane.

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The plane shown
with my hand here.

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So here is our first bond.

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Here's our second bond.

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And here's our third bond.

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All on the same plane.

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Next, it asks if we can
have this H, Si, S, F,

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and CN bond in the same plane?

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And so we can try to
rotate to achieve this.

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But we notice that
this isn't possible,

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because these two bonds are
always in the same plane,

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and we can't get any of
these bonds into that plane,

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because of the seesaw geometry.

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So for our second molecule
here, the Goodie Bag worksheet

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asks a different question.

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We notice that this
central CB bond can rotate,

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and the Goodie Bag asks,
which configuration

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is the lowest energy?

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So remember that the
entire VSEPR model

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is centered around the idea that
we want to minimize repulsion

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between electron
clouds by maximizing

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the distance between them.

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So we're going to rotate
this so that the repulsion

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between these groups in the
molecules are minimized.

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So as we rotate this,
we notice that there are

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two different possibilities.

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Remember that these
green atoms are CL,

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and this yellow atom is
oxygen. These are the largest

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atoms in our model, and also
the ones that have the most

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repulsive electron clouds.

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So we're going to
want to maximize

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the distance between them.

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So one way that
this can be achieved

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is by rotating the model so
that the oxygen is aligned

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with the hydrogen, which has
a very small electron cloud,

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and thus is capable of only
very slight propulsion.

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So this configuration
is low energy,

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because the very repulsive
chlorine atoms are far

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from the repulsive oxygen atom.

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However, there's also a
different possibility.

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If we rotate, we can put
the oxygen atom in a plane

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in between the two
chlorine atoms,

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so that the oxygen is not
directly facing an atom.

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However, this configuration
puts the oxygen atom

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spatially closer to a
chlorine atom, as shown here,

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and this hydrogen atom is very
close to this chlorine atom.

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So the first configuration
is lower energy.

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Today, we used our
molecular modeling kids

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to visualize the 3D structure
of some simple molecules using

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the VSEPR model.

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By playing with these
bonds and rotating them,

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we were able to understand
how repulsion between atoms

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gives rise to the lowest
energy configurations

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of these molecules.