1. Section 3.3 – Molecular Shapes and Dipoles It is time for these molecules to get in shape!
Nelson: pages

2. Molecular Shape
Lewis structures tell us nothing about how atoms in a molecule are arranged in 3-dimensional space
Could you have predicted the arrangement of atoms on the right from just seeing it’s Lewis structure?

3. VSEPR “vesper” THEORY Shapes of molecules:
Molecules take on particular shapes depending on the amount of lone pairs and bonding electrons the atom in the middle of the molecule has
Stereochemistry – study of the 3-D shape of molecules and how it affects their physical and chemical properties
VSEPR = “valence- shell-electron-pair-repulsion” theory is based on:
Electron pairs try to stay as far as possible from other e due to repulsion of negative charges
The number, type and direction of bonds to the central atom (CA) determine the shape of the molecule

4. VSEPR Continued…
VSEPR Theory is a powerful tool that helps us to guess the shape of a molecule. Such shapes are important as they determine the structure and function of the compound. For example, the arrangement of carbon atoms in a diamond help establish its unique hardness and usefulness.

5. According to VSEPR theory:
Read and summarize the points on page 91 in the space below:

6. Using VSEPR to predict molecular shapes
Bond angles – the angle formed by 2 bonds intersecting at an atom
A few rules to predicting the shape of atoms
get electron pairs as far away as possible (like charges repel)
multiple bonds are treated as 1 bond
lone pairs take slightly more room than bonding pairs

7. VSEPR Continued…
There are 6 different arrangements that you will need to memorize!!!
Let’s look at specific examples of molecules to help us come up with general formulas for each of the arrangements
Let A = Central atom
Let X = bonding electrons (bond pair)
Let E = one pair
Remember: the key concept is that all pairs of valence e repel each other and try to get as far from each other as possible
Steps to determining shape:
draw a L.D.D and consider the arrangement of valence e’s:
Determine bond pairs and lone pairs around central atom (CA)

8. Consider beryllium dihydride Total pairs = 2
Bond pairs around Be (CA) = 2
Lone pairs around CA = 0
Bond pairs repel each other and try to get as far away as possible = opposite sides of Be
Gives a linear orientation with the two bonds at an angle of 180 º
Summary:
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AX2
linear

9. VSEPR: Linear Arrangement

10. boron trifluoride Lewis dot diagram: Total pairs = 3 Lone pairs = 0
Bonding pairs = 3
Repulsion of bonding pairs causes 120º bond angles
Summary:
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AX3
Trigonal Planar

11. VSEPR: Trigonal Planar Arrangement

12. Drawing in 3-D In the plane of the slide Going back and away from you
tetrahedral
Coming out toward you

13. carbon tetrachloride Lewis dot diagram: Total pairs = 3 Lone pairs = 0
Bonding pairs = 3
Repulsion of bonding pairs causes º bond angles
Summary:
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AX4
tetrahedral

14. VSEPR: Tetrahedral Arrangement

15. nitrogen trichloride Lewis dot diagram: Total pairs = 4 Lone pairs = 1
Bonding pairs = 3
Repulsion of electron pairs causes a tetrahedral shape
If we ignore the lone pair, the shape becomes like a 3 sided (triangular) pyramid = trigonal pyramidal
We would predict that the bond angles would be 109.5º like the tetrahedral arrangement. However, lone pairs have a greater repulsion that bond pairs and therefore pushes that bond pair angles to be 107.3º
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AX3E
tetrahedral
trigonal pyramidal

16. Water Lewis dot diagram: Total pairs = 4 Lone pairs = 2
Bonding pairs = 2
Repulsion of bonding pairs causes a slightly altered (due to lone pairs) tetrahedral bonding pattern with the bond pairs having a bond angle of 104.5º
Summary:
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AX2E2
tetrahedral
Angular (V-shaped)

17. bonding-pair vs. bonding
pair repulsion
lone-pair vs. lone pair
repulsion
lone-pair vs. bonding
>

18. Hydrogen fluoride Lewis dot diagram: Total pairs = 4 Lone pairs = 3
Bonding pairs = 1
Repulsion of bonding pairs causes a tetrahedral bonding pattern. Since there is only two atoms held together by one covalent bond, the shape is linear (like all diatomic molecules)
Summary:
General Formula
Bond pairs
Lone pairs
Total pairs
Electron pair arrangement
Stereochemical formula
AXE3
tetrahedral
Linear

19. Table 7: Using VSEPR to predict molecular shape – Nelson Page 95
General Formula
Bond Pairs
Lone Pairs
Total Pairs
Geometry
Stereochemical formula
Examples:

20. General Formula
Bond Pairs
Lone Pairs
Total Pairs
Geometry
Stereochemical formula
Examples:

21. Learning Activities: Read Pages 91-97 in your textbook
Read the Learning Tips on pages 92, 94, and 96
Try Practice Problems on Page 96 # 2-4
= _____________________?
= _____________________?
= _____________________?

22. Practice Problems Page 96- Solutions

24. Everyone wants to be the center of the universe
If you have more then one CA you follow three simple steps
start with a structural or lewis diagram
identity all the CA and treat each one individually
draw the molecule going from CA to CA
Example – C2H6

25. More than one CA Continued…
Example: What are the bond pairs and lone pairs around C in the following molecule? Around N? Draw a three dimensional structure for the compound.
C: 4 bond pairs tetrahedral
N: 3 bond pairs, 1 lone pair trigonal pyramidal

26. VSEPR
For the previous compound, two realistic 3-D structures would be:

27. Multiple bonds and VSEPR
Can we predict stereochemistry for molecules with multiple bonds?
Consider ethylene (used in welding torches): C2H4 (g)
Crystallography indicates that the orientation around the C atoms is trigonal planer
Just as before with multiple CA’s:
Step 1: draw L.D.D
Step 2: count bond and lone pairs around CA (Recall: double/ triple bonds count as one)
Step 3: Determine general formula for each CA: AX3
Step 4: draw a structural diagram if necessary

28. In acetic acid, CH3COOH, there are three central atoms, and one double bond.
3-D molecule applet

29. VSEPR
Example: Draw a three dimensional structure for the following compound.
First, determine the bond and lone pairs around each CA
Trigonal planar
tetrahedral

30. More common way to draw structure.
VSEPR
Two possible 3-D structures:
More common way to draw structure.

31. Learning Activities: Read Pages 91- 97 from Nelson
Finish Practice Problems #2-4 on page 96
Try Practice Problems 6-7 on page 98
Complete Section 3.3 Questions 1-3 on page 104