1. The Shapes of Molecules
VSEPR Theory

2. Two Dimensions, Three Dimensions
Electron dot diagrams and structural formulas are not complete representations of molecules.
They do not take into account the three-dimensional shapes of molecules.
The 3D shapes of molecules are very important when considering the physical and chemical properties of substances.

3. Any theory of chemical bonding must be valid in 3 dimensions, not just the 2D representations shown on paper.
Spectroscopic and x-ray crystallographic data of molecules must be accounted for in any valid theory of chemical bonding.

4. X-Ray Crystallography

5. Valence Shell Electron Pair Repulsion
VSEPR theory – when electron pairs repel, molecules adjust their shapes so that the valence electron pairs are as far apart as possible.

6. The classic tetrahedral shape of methane is often used as a demonstration
CH4 has four bonding pairs of electrons and no unshared pairs.
Bond pairs are furthest apart when the angle between the central carbon and all four of its hydrogens is 109.5.
This theoretical result has been confirmed experimentally countless times.

7. If we substitute N for C as the central atom, we gain one more valence electron.
What effect does this have on the structure of ammonia (NH3)?
Ammonia has an unshared pair of electrons
the unshared pair has no bonding atom, and is held more closely to the nucleus.

8. the unshared pair repels the bonding pairs, pushing them closer to each other (H – N – H bond angle = 107)
the ammonia molecule has a pyrimidal shape.

9. Substituting O for N as the central atom, we gain a valence electron and another unshared pair of electrons.
the two bonding pairs and the two unshared pairs form a tetrahedral arrangement about the central oxygen
the molecule is planar, and bent
the two unshared pairs strongly repel the bonding pairs (H – O – H bond angle = 104.5)

10. Carbon Dioxide
In carbon dioxide, the central carbon has no unshared pairs. What is the best bond angle for those O=C=O double bonds?
the electrons are furthest apart from each other when the bond angle equals 180, which is exactly what is found in nature. CO2 is a linear molecule.

11. Other Interesting Shapes (These do NOT follow octet rule!)

12. Molecular Model Lab
In this lab, you will experimentally observe the molecular models of some sample molecules.
Some models may be two-dimensional, others will be three-dimensional.
When drawing the three dimensional models on paper, use the following key:

13. Drawing 3D Structures
Use an empty triangle to represent a bond below the plane of the paper (going in to the paper).
Use a darkened triangle to represent a bond above the plane of the paper (coming out of the paper).
Use a straight line to represent a bond in the plane of the paper.

14. Materials
The molecular model kit should come with multiple colored balls, short stubby sticks (single bonds) and long flexible sticks (double and triple bonds).
Atom Ball Color
Hydrogen yellow
Carbon black
Oxygen red
Sulfur red
Chlorine green
Nitrogen blue
Phosphorus blue

15. Electron Pair Geometry
Two electron pairs will arrange themselves linearly with an angle of 180°.
Three electron pairs will arrange themselves in a trigonal planar arrangement with an angle between them of 120°.
Four electron pairs will arrange themselves tetrahedrally with an angle between them of 109.5°.
In Regents Chemistry we only work with compounds that follow the octet rule: 4 electron pairs around the central atom.

17. Procedure For each molecule you will:
Draw a Lewis dot diagram for the species. If possible, obey the octet rule.
Construct a model of the molecule. For double and triple bonds, use long flexible sticks. For single bonds, use the short stubby sticks.
Considering the electronegativity of the atoms, the shape of the molecule, and the pure negativity of the unshared electrons, answer the questions on the chart.

18. Data Table Dot Diagram Picture Shape Polarity Notes and/or Questions
Molecule
Dot Diagram
Picture
Shape
Polarity
Notes and/or Questions
CH4
(bp -160°)
Is this molecule symmetrical?
CH3Cl
(bp -24°)
Which atom is negatively charged compared to the others?
CF4
(bp -128°)
The polarity of the molecule depends on the shape, not the bonds.
NH3
(bp -33°)
Remember the pair of unbonded electrons. How will it affect its shape?

19. Notes and/or Questions
Molecule
Dot Diagram
Picture
Shape
Polarity
Notes and/or Questions
H2O
(bp 100°)
Why is this molecule extremely polar? O2
(bp – 183°)
Double bonded molecule N2
(bp -196°)
Triple bonded molecule
PH3
(bp – 90°)
Is this more or less polar than NH3? Why?

20. Notes and/or Questions
Molecule
Dot Diagram
Picture
Shape
Polarity
Notes and/or Questions
H2O2
(bp + 150°)
You have this substance in your medicine cabinet
CO2
(bp -57°)
Two double bonds
CH3OH
(bp +65°)
An alcohol
C2H4
(bp -89°)
Double bond between the carbon atoms
What is the shape of this molecule?
C2H2
(bp -84°)
Triple bond between the C atoms HF
(bp +20°)
This compound is nearly ionic
H2S
(bp -60°)
The red ball can be used for sulfur...Why?

21. A Tale of Three Compounds
Methane CH4
BP -160°C
Methanethiol CH3SH Boiling Point 6°C
Methanol
CH3OH
Boiling Point 70°C

22. Water and Hydrogen Sulfide
BP -60°C
No hydrogen bonds

23. Phosphine PH3 Boiling Point - 87°C
Two More to Ponder
Phosphine PH3 Boiling Point - 87°C
Ammonia NH3
Boiling Point -33°C

24. Questions What is a dipole?
Which molecular shapes will make a molecule with polar covalent bond(s) become a non-polar molecule?
Why do molecules with four single bonds with carbon have a tetrahedral shape?
Methanol (CH3OH) has a boiling point of 70◦C. Methane (CH4) has a boiling point of –160◦C. Explain this difference in physical properties in terms of electronegativity differences between bonded elements.
Which element should have a higher boiling point, ammonia (NH3), or phosphine (PH3)? Explain your reasoning.

25. Make a Number Line for Boiling Points