8-1 Molecular shape
The Lewis diagram we learned to make in the last chapter are a way to find bonds between atoms and lone pais of electrons on atoms, but are not intended to say anything about the three-dimensional shape of the molecule. IN fact they can completely mislead you!
Take CH Cl
2
H ::Cl:
| |
:::Cl-C-Cl::: H-C-Cl:::
| |
H H
I II
The above Lewis diagram leads you to believe that there are two structures for atoms. Further, structure I is nonpolar because the polarity of the C-Cl bonds cancel out, but structure II is polar because the polar bonds are oriented so they can add together instead of canceling out.
Key Concept:
Molecules with the same chemical formula but a different 3-D arrangement of atoms are called stereoisomers
Thus you would think that there are two stereoisomers for CH Cl
2
WRONG, on so many levels!!
8-2 The Tetrahedron
What is it then?
A tetrahedron.
What is a tetrahedron?
Models and Figure 8.2
2 2 that contains a C with 4 bonds to it
So how do we bridge the gap between the flat, two-dimensional Lewis formula and a realistic 3-D structure?

8-3 VSEPR Theory
VSEPR stands for Valence-Shell Electron-Pair Repulsion
A big mouthful of words, but a simple enough concept
Key Concept:
VSEPR means that you arrange electron pairs around a central atom in such a way as to minimize their contact with each other. This gives rise to linear, trigonal planar, tetrahedral, trigonal bipyramid and octahedral electron geometries
2
1.
BeCl
2
Lewis structure:
: :
:Cl:Be:Cl:
: :
There are only 2 pairs of electrons around the Be atom. So there are 2 electron regions How can we get them as far apart as possible from each other? Place
We call this structure linear, and, since there are no lone pairs, the molecular and electronic structure are both the same, linear.
2. BF
3
:
:F:
: : : Note here is the example of an electron deficient B we had earlier
:F:B:F: There are three electron regions, and the way to get these 3 bonds
We call this structure Trigonal planar . Again there are no lone pairs so the molecular and electronic structures are the same.
8-4 Prediction of Molecular Geometry
3.
Those two were easy. Now on to the tetraheral
CH
4
H 4 single bonds, 4 electron regions.
| If the molecule was planar like this piece of paper, you get four 90's.
H-C-H
| spread. We call this structure a Tetrahedral
H So whenever 4 pairs of electrons around an atom, should be
Tetrahedral

4. PCl
5
5 electron regions. How do we get these away fro each other?
One straight up, one straight down, 3 even about the equator
We call this structure a trigonal bipyramid
There are 90 angles between the up and down Cl’s and the one at the equator, and there are 120 angles between the atoms at the equator.
3
5.
6 electron regions octahedral electron and molecular geometry

4
8-5 Lone pairs and shape
In all of the above structures we only had bonding pairs of electrons around the central atom. What happens when we mix in lone pair of electrons?
Key Concept:
A lone pair counts as 1 electron region, but distorts the geometry slightly. Also lone pairs of electrons DO NOT appear in the molecular structure
6.
NH
3
H
I the lone pair counts as an electron region, so 4 electron regions
H-N: This makes our electronic structure tetrahedral, but what about the
I Molecular structure? In the molecular structure we ignore the lone
H pairs (We can only see atoms, not electrons) That makes this molecular structure a trigonal pyramid . Technically the lone pair ‘pushes’ a little harder (about fine point.
7. H O
:
H-O-H
4 electron regions, electronic structure = tetrahedral
Ignoring the lone pairs, the molecular structure is V-shaped
Before we look at lone pairs in other geometries, let’s look as how we handle multiple bonds
8-6 VSEPR and multiple bonds
Key Concept:
In VSEPR Theory we count double or triple bonds as 1 electron region
8.
9.
CO
2
::O=C=O:: 2 double bonds count as 2 regions
Linear electron and molecular structure
HCN
H-C / N: 1 double bond, 1 triple bond, both count as a single region
Linear electron and molecular structure

5
10.
SO
2
: : O =S-O:::
:
Looking at the S, the double bond counts as one electron region, the double bond as another electron region, and the lone pair as a third electron region, for a total of 3 electron regions. Again our electronic structure is a trigonal pyramid.
However, what is the molecular structure? In the molecular structure we ignore the lone pairs (We can only see atoms, not electrons) That makes this molecular structure V-shaped.
Due to the extra electrons in a double bond, double bonds will ‘push’ extra hard on electrons around them and distort the geometries slightly. For example the
H
\
C=O::
/
H
3 electron regions
Trigonal planar geometry
Bond angle should be 120 but is actually 116 o
Again, I don’t fret about this small difference
8-7 Trigonal Bipyramids
Lone pairs in trigonal bipyramids present an interesting problem. Where do yu put them? Above and below or around the equator?
preferred configuration because it minimizes electron repulsion
Key Concept:
Always put lone pair of electrons around the equator of a trigonal bipyramid

11. SeF
4
6
One lone pair, so 5 electron regions, again a trigonal bipyramid electron structure. But what about the molecular structure? We have two nonequivalent places to put the lone pair. Top/bottom or around the equator. If we put the lone par at the top or bottom, then it has 3 have only 2 F’s at 90, and 2 at 120. This is slightly better. So as a rule you always put the lone pars around the equator in a trigonal bipyramid .
We call this molecular structure a See-Saw
(You need to see a demo to see this!)
12. ClF
3
Again 5 electron regions for a trigonal bipyramid electronic structure, but with 2 lone pairs around the equator, the molecular structure is called T-shaped
(Oops error in figure One of the equitorial F’s should be :)
13. I
3
-
Note: the entire structure has a - charge.
5 electron regions, trigonal bipyramid electronic structure, but a linear molecular structure!

7
8-8 Octahedra
Putting the first lone pair in octahedra is no problem because all the positions are equivalent. But where do you put the second pair?
Since lone pairs of electrons ‘push’ harder than bonding pairs, we want to put them as far apart as possible. That puts them on opposite sides in the octahedral structure.
Key Concept:
If there are two lone pairs of electrons in an octahedral, the pairs are placed opposite each other in the structure
14. IF
5
6 electron regions octahedral electron geometry the lone pair can go any place, there is no difference.
The molecular geometry is call square pyramid (Put the lone pair on the bottom to it looks flat, what you have left is a pyramid)
15.
XeF
4
6 electron regions
Octahedral electron geometry
The lone pairs go on opposite sides so they don’t have to contact each other. The resulting molecular structure is called square planar
Lucky for you no octahedral molecules have been found with >2 lone pairs!

8-9 Structure and Dipoles
At the end of the last chapter we talked about polar bonds and polar molecules
Just to remind you
Key Concept:
In order to has a molecular dipole (be polar) a molecule must:
1. Have polar bonds
2. Have an asymmetric arrangement of these polar bonds
Go over the structures we just did and decide if the molecules are polar or not
2. BF
3
3. CH
4
4. PCl
5. PCl
5
-
6
6. NH
3
8. CO
2
9. HCN
10. SO
2
11. SF
4
12. ClF
3
13. I
3
-
15. XeF
4
Polar bonds - linear symmetric structure - nonpolar
Polar bonds - symmetric structure - nonpolar
Nonpolar bonds - Molecule can’t be polar
Polar bonds - symmetric structure - nonpolar
Polar bonds - symmetric structure - nonpolar
Polar bonds - asymmetric molecule - polar
Polar bonds - asymmetric molecule - polar
Polar bonds - symmetric structure - nonpolar
Borderline polar bonds - asymmetric arrangement - polar
Polar bonds - asymmetric molecule - polar
Polar bonds - asymmetric molecule - polar
Polar bonds - asymmetric molecule - polar
Nonpolar bonds - Molecule can’t be polar
Polar bonds - asymmetric molecule - polar
Polar bonds - symmetric structure - nonpolar
8-10 Isomers
We have already discussed resonance, where you keep the atoms in place, but arrange electrons differently.
Another thing you will see is the ability to make different structures be re-
H
|
H H H H :O: H
| | | | | |
H-C-C-C-O::-H H-C-C-C-H
| | | | | |
H H H H H H
8

Key Concept:
Structural isomers are molecules with the same chemical formula but a different arrangement of bonds connecting the atoms. One structural isomer will have different physical and chemical properties than another structural isomer, so they are truly a different molecules
9
It is also possible to isomers in which the spatial arrangement of atoms is
Molecules with different spatial arrangements of the same chemical bonds are called stereoisomers.
Here we will talk about two kinds of stereoisomers, Geometric Isomers and
Optical Isomers
Key Concept:
Geometric isomers are molecules with the same chemical connectivity between atoms, but a different geometric arrangement of these bonds. One geometric isomers will have different physical and chemical properties than an other geometric isomer, so they are truly a different molecules
Optical Isomers occur when one stereoisomer is the mirror image of another stereoisomer . The physical and chemical properties of two optical isomers are usually almost identical, but the biochemical properties in a living cell can be radically different.
For instance all but 1 of the 20 different amino acids that make up the proteins of your body are stereoisomers. And ALL terrestrial organisms use the L-form of these amino acids
Figure 8-26 D and L isomers of Alanine
Added figure Thalidomide The upper isomer is a sedative, and is a great treatment for morning sickness. The bottom isomer is a tetratogen and cause birth defect and bone cancers