This next set of
videos, we're going to predict the shapes
of molecules and ions by using VSEPR, which is an
acronym for valence shell electron pair repulsion. And really all this
means is that electrons, being negatively charged,
will repel each other. Like charges repel, and
so when those electrons around a central atom
repel each other, they're going to
force the molecule or ion into a particular shape. And so the first
step for predicting the shape of a
molecule or ion is to draw the dot structure to
show your valence electrons. And so let's go ahead and draw
a dot structure for BeCl2. So you find beryllium
on the periodic table. It's in group 2, so
two valence electrons. Chlorine is in group 7,
and we have two of them. So 2 times 7 is 14. And 14 plus 2 gives us a
total of 16 valence electrons that we need to account
for in our dot structure. So you put the less
electronegative atom in the center. So beryllium goes in the center. We know it is surrounded
by two chlorines, so we show beryllium bonded
to two chlorines here. And we just represented
four valence electrons. So here's two valence electrons. And here's another two
for a total of four. So, instead of 16,
we just showed four. So now we're down to
12 valence electrons that we need to account for. So 16 minus 4 is 12. We're going to put those left
over electrons on our terminal atoms, which are our chlorines. And chlorine is going to
follow the octet rule. Each chlorine is
already surrounded by two valence electrons, so
each chlorine needs six more. So go ahead and put six
more valence electrons on each chlorine. And, since I just represented
12 more electrons there, now we're down to
0 valence electron. So this dot structure has
all of our electrons in it. And some of you might think,
well, why don't you keep going? Why don't you show some of
those lone pairs of electrons in chlorine moving in to
share them with the beryllium to give it an
octet of electrons? And the reason you don't is
because of formal charge. So let's go ahead and
assign a formal charge to the central
beryllium atom here. So remember each of
our covalent bonds consists of two electrons. So I go ahead and put that in. And if I want to
find formal charge, I first think about the number
of the valence electrons in the free atom. And that would be two,
four-- four berylliums. So we have two electrons
in the free atom. And then we think about
the bonded atom here, so when I look at
the covalent bond, I give one of those electrons
to chlorine and one of those electrons to beryllium. And I did the same thing
for this bond over here, and so you can see
that it is surrounded by two valence electrons. 2 minus 2 gives us a
formal charge of 0. And so that's one way to think
about why you would stop here for the dot structure. So it has only two
valence electrons, so even though it's in period
2, it doesn't necessarily have to follow the octet rule. It just has to have less
than eight electrons. And so, again,
formal charge helps you understand why you can stop
here for your dot structure. Let me go ahead and
redraw our molecule so we can see it a
little bit better. And we'll go ahead and
move on to the next step. So let me go ahead and
put in my lone pairs of electrons around
my chlorine here. So we have our dot structure. Next, we're going to count
the number of electron clouds that surround the central atom. And I like to use the
term electron cloud. You'll see many
different terms for this in different textbooks. You'll see charge clouds,
electron groups, electron domains, and they have
slightly different definitions depending on which
textbook you look in. And really the term
of electron cloud helps describe the idea
of valence electrons in bonds and in lone
pairs of electrons occupying these electron clouds. And you could think about them
as regions of electron density. And, since electrons
repel each other, those regions of electron
density, those clouds, want to be as far
apart from each other as they possibly can. And so let's go ahead and
analyze our molecule here. So surrounding the central atom. So we can see that here are some
bonding electrons right here surrounding our central atom. So we could think about those
as being an electron cloud. And then over here we have
another electron cloud. So we have two electron
clouds for this molecule, and those electron
clouds are furthest apart when they point in
opposite directions. And so the geometry or
the shape of the electron clouds around the
central atom, if they're pointing in opposite
directions, it's going to give you a
linear shape here. So this molecule
is actually linear because we don't have any lone
pairs to worry about here. So we're going to
go ahead and predict the geometry of the
molecule as being linear. And if that's linear, then
we can say the bond angle-- so the angle between the
chlorine, the beryllium, and the other chlorine--
is 180 degrees. So just a straight line. All right. So that's how to use VSEPR
to predict the shape. Let's do another example. So CO2-- so carbon dioxide. So we start off by drawing
the dot structure for CO2. Carbon has four
valence electrons. Oxygen has six. And we have two of them. So 6 times 2 gives us 12. 12 plus 4 gives us
16 valence electrons to deal with for
our dot structure. The less electronegative
atom goes in the center, so carbon is bonded to oxygen,
so two oxygens like that. We just represented
four valence electrons. Right? So two here and two
here, so that's four. So 16 minus 4 gives us 12
valence electrons left. Those electrons are going to
go on our terminal atoms, which are oxygens if we are going
to follow the octet rule. So each oxygen is
surrounded by two electrons. So, therefore, each oxygen needs
six more valence electrons. I'll go ahead and put in
six more valence electrons on our oxygen. Now you might think we're
done, but, of course, we're not because carbon is going
to follow the octet rule. Carbon does not have
a formal charge of 0 in this dot structure,
so even though we've represented all of our
valence electrons now, we need to give carbon an octet. We need to give carbon
a formal charge of 0. And we can do that by moving
in this lone pair of electrons into here to share those
electrons between the carbon and the oxygen, and also with
this lone pair of electrons. So we move those in like that. And now we can see that carbon
is double bonded to our oxygen. So now our dot structure
looks like this. And each oxygen, instead
of having three lone pairs of electrons, now
each oxygen only has two lone pairs like that. So there is our dot structure. So let's go back up here
to look at our steps for predicting the
shape of this molecule. So step 1 is done,
draw dot structure to show the valence electrons. Next, we're going to count
the number of electron clouds surrounding our central atom. So we go back down here, and
we find our central atom, which is our carbon. And we think about the
regions of electron density that surround that. So we can count this double bond
as a region of electron density because we're not worried about
how many electrons are there. We're just worried
about the fact that there is a region
of electron density. So that's one electron cloud. And then over here we have
another electron cloud. So we have two regions
of electron density. We have two electron
clouds here, which are going to
repel each other. So when we look at
step 3-- predict the geometry of the
electron clouds-- predict the geometry
of the electron clouds around the central atom. Well, those electron
clouds are going to be opposite to each other. They're going to point
in opposite directions. So, once again, they're
going to force this molecule into a linear shape. So this carbon dioxide
molecule is also linear with 180
degree bond angle. So, once again, we don't have
any lone pairs of electrons on our central atom,
so we don't really have to worry about that. And we can go ahead and predict
the geometry as being linear. So that's how to approach it. Draw the dot structure. Think about electron
clouds and think about the shapes
of your molecules. In the next video,
we will look at how to approach three
electron clouds.