Practice Problems (Advanced) on VSEPR Theory - Chemical Bonding Video Lecture - Chemistry

Let's do some practice problems.
We'll look at Lewis structures where the central
atom is surrounded by either 5 or 6 things.
And then, I'll talk you through the thought
process for how to determine what the 3 dimensional
vesper shape for the molecule is and what
the bound angles are.
Okay.
If you haven't already done it, you might
want to watch Vesper videos part 2 and part
3.
They talk about the background information
for this stuff just so we're on the same page.
Okay.
So, here's our first Lewis structure.
And, this is for xc02f2.
The first thing we want to ask ourselves,
when we're figuring out the vesper structure
is how many things are there around the central
atom?
So, xenon here is the central atom.
There are these two double bonds to oxygen.
Remember, in vesper, double bonds count for
the same as single bonds.
It's just a bond to an atom.
Okay.
So, 1, 2, 3, 4, bonds and then a lone electron.
So, there are going to be 5 total thins around
this atom.
So, number 2 asks what structure is this molecule
based on.
What I mean is, if something has 5 things
around the central atom, it's based on a particular
structure.
It's going to be based on this shape here
which is trigonal bipyramidal.
So, this is the shape, trigonal bipyramidal
that we want to have in our minds.
Okay.
Now, what I mean by based on is that, if the
5 things surrounding xenon were all atoms,
then, it would be a trigonal bipyramidal shape.
But, they're not.
Okay.
They're not all 5 atoms.
So, number 3 here asks which atoms are replaced
by lone electron pairs.
So, in this molecule, there are 4 atoms and
then 1 lone electron pair which means that
1 of the atoms in this trigonal bipyramidal
molecule is going to be replaced by a lone
electron pair.
Now, if you remember from what we said earlier,
it's the equatorial atoms that get replaced
by lone electron pairs first.
So, that means that our final structure of
this molecule is going to look like this where
you can see here 1 of the equatorial atoms
has been replaced by a lone electron pair.
Okay.
But, keep in mind that they're very similar.
This shape which is called the seesaw shape,
is based on and related to the trigonal bipyramidal
shape.
It's just, it's missing one of these equatorial
atoms and it's been replaced an unshared electron
pair.
So, this molecule here has the seesaw shape.
Let's talk about angles really quickly.
Okay.
In the trigonal bipyramidal there is a 180
degree angle between these two atoms here.
And, if we look from the top down, there's
120 degrees between each of these atoms.
Okay.
Now, in the seesaw, we have this unshared
electron pair here.
And, as we said before, unshared electron
pairs push harder against atoms because they
want a little bit more space.
So, that means that these two atoms are going
to get pushed closer together like this meaning
that the angle between them is going to be
less than 180 degrees.
If we look at the top down view of this, we
see the electron pairs here and they are pushing,
pushing these atoms together, which means
that, if we look at this top down view here,
these atoms are getting pushed together.
So, the angle between them is less than 120
degrees.
So, that's what the angles are for this, for
this seesaw molecule here.
brf3, what's the vesper shape?
Okay.
Step number 1, how many things are there around
the central atom?
There are 1, 2, 3 bonds and 1, 2 lone electron
pairs.
So, 5 total things.
So then, what structure is this molecule based
on?
Well, any molecule that has 5 things around
the central atom is based on this trigonal
bipyramidal shape.
So now, question 3.
Which electrons in this are going to be replaced
by lone electron pairs?
So, this has 5 atoms.
But, brf3 only has 3 atoms then it has 2 unshared
electron pairs.
So, what it's going to look like is this right
here.
Look at how similar this is to the trigonal
bipyramidal shape.
But, it's that 2 of these equatorial atoms,
maybe say this one and this one, this one
and this one, have been replaced by unshared
electron pairs.
Here are the unshared electron pairs, these
two and these two.
Okay?
So, what that causes is this T shaped molecule.
This has a T shape.
And, in terms of angles, we just looked at
these angles a minute ago.
And the T shaped is going to have an angle
that is less than 180 degrees between these
two atoms because these lone electron pairs
are going to be pushing inward on these two
atoms.
It's also going to mean the angle between
these two bonds are less than 90 degrees.
So, these are both less than 90 degrees.
And then, this whole thing from here to here
is going to be less than 180.
So, that's what brf3 looks like, a T shaped
molecule because 2 of these equatorial atoms
from a molecule that it's based on have been
replaced by lone electron pairs.
Okay. iof5, first thing we ask, how many things
are there around the central atom?
There are 1, 2, 3, 4, 5, 6 bonds, remember
double bones single bonds, they count for
the same, 6 bonds around this central atom.
So, what molecule is it based on if there
are 6 bonds, 6 atoms around the central atom?
It is based on an octahedral, oh that was
the wrong one.
It's, there we go, that's my octahedral molecule.
It's based on an octahedral molecule.
Now, which atoms are going to be replaced
by lone electron pairs?
They're not going to be because there are
no lone electron pairs in this molecule.
So, that means that every single one of these
atoms is going to be in place here.
And, my final structure is going to be octahedral.
And, just to brush up on the angles.
Every single one of these angles, in an octahedral,
between any 2 bond, between any 2 bonds is
going to be 90 degrees.
They're all right angles.
So, this is real easy.
6 things, 6 atoms, no unshared electron pairs,
and you have an octahedral molecule.
icl4, okay, and the whole thing is surrounded
by these brackets because it has a negative
charge which means this whole clump of atoms
has received an extra electron.
It doesn't matter if it has a charge or not,
we go through figuring out what the vesper
shape is, we go through it exactly the same
way.
Don't be thrown off at all by this charge.
It doesn't have any effect on the vesper.
Okay.
Anyway, first thing we ask, how many things
are there around the central atom?
1, 2, 3, 4, 5, 6.
6 total things.
So, what structure is this going to be based
on?
It's going to be based on octahedral shape
which looks like this which normally has 6
atoms around a central atom.
Okay.
Number 3, which atoms are going to be replaced
by lone electron pairs?
Octahedral has 6 atoms.
But, icl4 here has 2 of them replaced by lone
electron pairs.
And so, this is the structure that I'm going
to end up getting where you can see that,
if you make this comparison, the top and the
bottom atoms, if you like to think about it
that way, have been replaced by an unshared
electron pair here an unshared electron pair
here, I'm left with 4 round the middle here.
And, the shape that that gives me is square
planar because these are all kind of plane,
just like that.
And, in terms of angles, my octahedral, as
I said, has 90 degree angles between every
bond.
And the square planar, if we look at this
top view down, it's really simple too, it's
got 90 degrees between all of these bonds
here.
And that's because these unshared electrons
push down, but, these electrons on the bottom
push up.
So, there's no net movement of any of these
atoms because they're being pushed up and
down the same amount.
So, they stay exactly in the center.
And, there is a 90 degree angle between each
set of those bonds.
So, that's what we get, a square planar molecule
when we take 2 of the atoms off of an octahedral
molecule and replace them with lone electron
pairs.
sof4, okay, how many things are there around
the central atom?
There are 1, 2, 3, 4, 5.
Double bonds count the same as single bonds.
5 things around it.
So, the structure that this molecule is going
to be based on is the trigonal bipyramidal
shape which is how you would arrange 5 things
around a central atom.
Now, which atoms are going to be replaced
by lone electron pairs?
None of them because there are no lone electron
pairs in this Lewis structure.
So, that means that our sof4 is going to just
look like this.
It's going to be a full on regular trigonal
bipyramidal molecule where these are 180 between
these two bonds.
These are 90 degrees between these two bonds.
And, from the top down, each one of these
bonds has a 120 degree angle between it.
Bipyramidal for 5 atoms around a central atom.
i3 and then the whole thing has a 1 minus
charge.
So, how many things are there around the central
atom?
1, 2, 3, 4, 5.
Okay.
How do we around 5 things around a central
atom?
It's going to be based on this pyramidal,
on this trigonal bipyramidal shape that we
just looked at a middle that we just looked
at a minute ago.
Okay.
But now, which of the atoms from this 5 atom
shape are going to be replaced by lone pairs?
Well, there are going to be 1, 2, 3 lone pairs
instead of 3 atoms.
So, this one's going to go, this one's going
to go, and this one's going to go.
And, here is the shape that we get left with.
Okay?
You can see this atom stays, this atom stays,
obviously the central atom stays.
Here they are.
But now, if you look at it from the top down,
these 3 unshared electron pairs have been
replaced with 3 equatorial atoms in this trigonal
bipyramidal structure.
Okay.
So, this leaves us with a linear structure
where the 3 atoms are all just I a row.
Okay.
And, the way this relates to what we saw before
is, normally, we have our 180 degree angle
between these two bonds.
And, it turns out that, in a linear structure,
we have the exact same thing because, sure,
there are all of these unshared electrons.
But, they're all pushing equally against this
one and this one in all directions.
So, nothing gets bent up or down.
They're all just bam, bam, bam in a straight
line just like that, 180 degrees between them
with 3 unshared electron pairs around the
central atom on, replacing the equatorial
atoms.
Okay?
Last one, tef5 and the whole thing has a negative
charge.
Okay.
How many things around the central atom?
1, 2, 3, 4, 5 bonds and 1 lone electron pair,
6 things total.
So, that means that the structure of this
is going to be based on the octahedral molecule
which is this right here, octahedral.
Which atoms are replaced by lone electron
pairs?
So, instead of having 6 atoms, like I do in
a regular octahedral, 1 of these atoms is
going to be replaced by this lone electron
pair.
So, that means that my structure is going
to look like this.
Check it out.
I just cut its head off and replaced it with
a lone electron pair.
This atom is gone and it's been replaced by
these electrons right here.
Okay?
So, this is going to give me the structure
that we call square pyramidal because if you
look at it like this, sits up, looks like
this, kind of looks like a square pyramid.
So, really quickly, bond angles in a regular
octahedral molecule are all 90 degrees.
Bond angles are all 90 degrees.
And, in the square pyramidal, we have these
unshared electrons that are pushing down,
they're pushing down on all of these atoms
a little bit.
So, that means that the bond between this
and this, between this and this, this bond
is going to be less than 90 degrees because
these are getting pushed down.
And, since these two are getting pushed down,
they end up a little closer together which
means that the bond between these two is a
little less than 90 degrees as well.
So, take 1 of the 6 atoms away from an octahedral
molecule and you end up with something that
has a square pyramidal shape.
Okay.
So, that's how you go through and look at
a Lewis Structure and figure out what its
3 dimensional vesper shape should be.
A lot of people really freak out about vesper.
And, they think there are all these different
structures that they have to memorize.
You know, they make the flash cards and they
have all the bond angles and everything.
But, really, vesper's not that hard if you
think about it along these lines like you're
going through these steps here.
You figure out how many things are around
the central atom, what structure is that based
on?
Like, maybe it's based on the octahedral.
And then, you just think about which of the
atoms are going to get replaced when, with
unshared electron pairs.
And, that will tell you the structure you
end up with.
Just 2 things to keep in mind.
When you have a trigonal bipyramidal molecule,
you always replace the equatorial atoms one
at a time.
So, if all 3 of them are replaced, you end
up with this linear molecule.
Other hand, when you have an octahedral molecule
like this, the first atom you replace, well
it doesn't really matter, but, it looks like
it's at the top.
And then, the second atom you replace, instead
of being one of these, the second one is on
the other side.
And, you get this structure here where this
has been replaced and this has been replaced.
But, anyway, you don't have to memorize a
whole lot with vesper.
All you have to do is understand, really well,
how you start with the structure that the
molecules are based on.
And then, you just figure out which atoms
to remove.
And, always remember that unshared electron
pairs push atoms closer together.
So, if there's an unshared electron pair pushing
atoms, their angle is going to be less than
it is in the perfect structure that the molecule
is based on.
Okay.
So, that's how you can figure out a vesper
structure based on a Lewis structure.