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Studying for a test? Prepare with these 3 lessons on Chemical bonds.

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# VSEPR for 5 electron clouds

Video transcript

Let's use the VSEPR theory to
predict the structure of this molecule--
so phosphorus pentachloride So the first thing we need to do is
draw a dot structure to show our valence electrons. We find phosphorus in Group 5, So 5 valence electrons Chlorine in Group 7, So 7 valence electrons and I have 5 of them So 7 times 5 is 35 Plus 5 gives us a total
of 40 valence electrons that we need to show in our dot structure. So phosphorus goes in the center
because it is not as electronegative as chlorine. And we have five chlorines. So we go ahead and put our five chlorines around our central
phosphorus atoms like that. If we see how many valence
electrons we've drawn so far, this would to be
2, 4, 6, 8, and 10. So 40 minus 10 gives us 30
valence electrons left over. And remember, you start putting
those leftover electrons on your terminal atom. So we're going to put
those on the chlorines Each chlorine's going to
follow the octet rule. So that means each chlorine
needs 6 more electrons. Now, each chlorine is surrounded
by 8 valence electrons like that. So if I'm adding 6 more
electrons to 5 atoms, 6 times 5 is 30. So I have now represented
all of my valence electrons on my dot structure. Notice that phosphorus is
exceeding the octet rule here. There are 10 valence
electrons around phosphorus. And it's Ok for the phosphorus to
do that because it's in Period 3 on the periodic table. I like to think about formal charge. And so if you assign a
formal charge to phosphorus, you'll see it has a formal charge of 0. And that helps to explain- for
me, anyway- the resulting dot structure. Now, step two We're going to count the
number of electron clouds that surround our central atom. Remember, an electron cloud is just a region of electron density. So I could think about these
bonding electrons in here as a region of electron
density around my central atom. I could think about these
bonding electrons, too. So here's another electron cloud. And you can see we have
a total of five electron clouds around our central atom. The next step is to predict
the geometry of the electron clouds. Those valence shell electrons
are going to repel each other. All right. So that's a VSEPR theory--
Valence Shell Electron Pair Repulsion Since they're all negatively charged, they're going to repel
and try to get as far away from each other as
they possibly can in space. When you have five electron pairs, it turns out the furthest they
can get away from each other in space is a shape called a
trigonal bipyramidal shape. So let me see if I can draw
our molecule in that shape. We're going to have our
phosphorus in the center, and we're going to have three
chlorines on the same plane. So let me attempt to show three
chlorines on the same plane here. These are called the equatorial positions because they're kind of along
the equator, if you will. So three chlorines in the
same plane, one chlorine above the plane, and one
chlorine below the plane. Those are called axial positions. All right. So there's a quick sketch Let me see if I can draw
a slightly better shape of a trigonal bipyramial shape here. So let me see if I can
draw one over here so you can see what it looks
like a little bit better. So we could have one pyramid
looking something like that. And then, down here, let's see if we can draw another pyramid in here like that So that's a rough drawing, but we're trying to go for a trigonal
bipyramidal shape here So let's focus in on those chlorines that are on the same plane first. If I'm looking at these three chlorines and I go over here to my
trigonal bipyramidal shape, you could think about those 3 chlorines as being at these corners here. So it's a little bit easier to see. They're in the same plane. So those are the equatorial chlorines. When I think about the
bond angle for those- so those chlorines
being in the same plane, you have these three bond angles here. And so when we did trigonal planar, we talked about 360
degrees divided by 3- giving us a bond angle of 120 degrees. So you could think about that
as being a bond angle of 120. All right. So same idea, Those bonding electrons are
going to repel each other. When we focus in on our axial
chlorines- so this one up here and this one down here. You could think about those
as being here and here on your trigonal
bipyramidal shape like that. And if you draw the axis, if
you draw a line down this way connecting those, it's easy to see those are 180 degrees from each other. So you could think about a bond angle of 180 degrees between
your chlorines like that. And then, finally, if we think
about the bond angle between, let's say, this axial
chlorine up here at the top and then one of these
green chlorines right here, I think it's a little bit easier
to see that's 90 degrees here. So this bond angle right
here would be 90 degrees. And so those are your
three ideal bond angles for a trigonal bipyramidal situation here. It's important to understand
this trigonal bipyramidal shape because all of the five electron
cloud drawings that we're going to do are going to
have the electron clouds want to take this shape. So it's important to
understand those positions. For step four, ignore any lone pairs and predict the geometry of the molecule. Well, there are no lone pairs
on our central phosphorus. So the electron clouds take
a trigonal bipyramidal shape and so does the molecule. Let's go ahead and do another example. Sulfur tetrafluoride here. So we're going to start by
drawing the dot structure, and we need to count our
valence electrons, of course. So sulfur's in Group 6,
so 6 valence electrons. Fluorine is in Group 7, so 7 valence electrons. I have 4 of them. 7 times 4 is 28. 28 plus 6 is 34 valence electrons. We know sulfur is going
to go in the center because fluorine is much
more electronegative. We put sulfur in the center here. We know sulfur is bonded to 4 fluorines. So we put our fluorines around like that. And let's see how many valence
electrons we've shown so far- 2, 4, 6, and 8. So 34 minus 8 gives us
26 valence electrons we still need to account
for on our dot structure.