So when we look at our geometry this is an example of some of

the geometries we're going to look at and I want to talk about how we name them.

But the key thing we're worried about here is how these

angles form to get the maximum distance between these electrons groups.

And so these are all examples where we're only dealing with bonding groups.

We're not going to deal with non bonding groups just yet,

we'll get to that shortly ,but here we're looking at bonding groups.

So if I have my central atom, and I have two groups around my central atom,

the way that they can be as far apart from one another as possible.

Is for them to be 180 degrees from one another.

When I look at my next geometry,

where I have three groups, now, I'm looking at 120 degree angle.

Because that's how these groups that can be as far apart from one

another as possible.

When I go beyond having three groups, when I go to four groups,

things change a little bit.

Because here looking at two groups and

three groups, I'm looking at situations where I can do them in a single plane.

Now when I get to the four groups around the central atom, now I have to look at

my bond angles but I have to really go into that sphere shape, okay.

So, I imagine a kind of a sphere encircling this molecule.

And now, when I look at these bond angles the way that they get

as far apart from one another is possible is to a 109.5 degrees.

And that angle’s a little bit harder for us to kind of picture in our head.

We're used to 90 and 120 or

180, but 109.5, it doesn't quite kind of stick out in our head very well.

We'll talk a little bit more about the other two with five groups and

six groups when we get to those later.

Now we're going to spend a little bit more time looking at what happens when we

have lone pairs, so we have four electron groups.

But what happens when they're not all bonding groups around that central atom?