We’ve said that there are two ways of looking at the shape of the structure, there is the
electronic geometry and now there is the molecular geometry.
The molecular geometry is the more complex way of looking at it, but the thing is it’s
the more accurate way of describing what’s happening.
Now we’re going to say that molecular geometry treats lone pairs and surrounding elements
as different.
So remember here is the key, our central element is A, the surrounding elements around that
central element are X, and then the lone pairs on the central element are E.
So if we look at this made up compound AX3 up top with the lone pair, we’d say that
A is the central element, it’s connected to three surrounding elements and it has one
lone pair on the central element, so that would be E1.
Now technically, it around four groups so we go here to the four group section and we
look, AX3E1 will be classified as trigonal pyramidal.
So there’s a lot more shapes to remember now because of this, but, again, remember
the molecular geometry is the more accurate way of describing the shape because what we
should realize these lone pairs think of them as a raging fire or something that is out
of control.
Something like that would need a lot of room for itself.
What’s happening here is because of we have these non-bonding electrons they’re going
to need as much room as possible because they’re just pure energy, so what they do is they
push away the other guys.
They compress the bonds of those other guys, so the lone pairs are more towards self on
one end of the compound; this causes us to have a different shape.
The molecular geometry is a great way of explaining how that shape looks.
Pyramidal, you can see that this here kind of looks like a pyramid, so that’s why they
call it trigonal pyramidal.
Now if you go to this, if you have two groups, here this is AX2, there’s no lone pairs
around the central element, same thing here.
For three groups you could be AX3 where you have no lone pairs or AX2E1.
Here Sn is in the center.
It has two surrounding elements so that will be the X and it has one lone pair so that
would be E1.
This shape can come in three different names.
They all mean the same thing, so be careful your professor could say bent, angular or
V shape.
Every single one means the same exact thing.
If we have four groups around the central element, we could have AX4 where it has no
lone pairs, we could have AX3E1 or we could have AX2E2.
And, again, AX2E2 is just like AX2E1, it would be classified as bent, angular or V shape.
If you have five groups around your central element, then it goes really crazy with the
number of shapes that you could have.
Here AX5 would mean that the central element has no lone pairs around it.
AX4E1 will be seen as seesaw.
And another way we could have drawn this is we could have drawn a seesaw like this.
You may see it like that as well.
AX3E2 is T-shaped and you can see that this looks like a T, that’s why they call it
T-shaped.
Here AX2E3 is actually linear, it’s just like the AX2 up here; it’s linear as well.
All the lone pairs decide to orient themselves in this way kind of forming like a triangle
and then the surrounding elements are on either side.
Then if your AX6, well if you have six groups it can be AX6 or you’re octahedral.
If you have one lone pair your AX4E1 and your name is square pyramidal.
And then you could be AX4E2 and there you’ll be square planar kind of shaped like a square.
So, again, the molecular geometry, even though it’s more of a pain, it’s a more accurate
way of describing the real shape of these compounds.
They’re basically telling us how do the lone pairs influence the shape of our molecular
compound.
The lone pairs are like pure energy, they’re pushing all the other bonds away from them.
This actually gives them weird-looking shapes where the names come into play.
So there’s more memorizing here than with electronic geometry but if you can memorize
this entire page, it will be just like memorizing electronic geometry because we know electronic
geometry AX2 is linear, AX3 is trigonal planar, AX4 is tetrahedral, AX5 is trigonal bipyramidal
and AX6 is octahedral.
I would recommend just memorizing this page here because that way you can take out molecular
geometry and electronic geometry all at the same time.
But remember there’s a distinction, electronic geometry treats lone pairs and surrounding
elements as the same, as just X, molecular treats them as different.
We have X for surrounding elements and we have E for lone pairs.