Molecular Shape: VSEPR and VB theory
Major concepts
 The shape of a molecule can be determined if the geometry around central atoms is known.
 Valence shell electron pair repulsion can often be used (in simple molecules) to determine
whether an atom’s electron geometry is linear, trigonal planar, or tetrahedral.
 Wedged and dashed lines can be added to bond-line convention to show shapes.
Vocabulary
 Primary, secondary, tertiary, quaternary carbons
 carbocations
 VSEPR
 Linear, trigonal planar, tetrahedral
 Wedge/dash convention
Students should be able to:
 Use VSEPR to determine geometry of an atom
 Draw bond-line structures including wedges and dashes to show appropriate shapes
 Determine approximate bond angles for compounds
Daily Problems
1. Use VSEPR to indicate the electron geometry of the indicated atom as linear, trigonal planar, or
tetrahedral.
2. According to VSEPR, what should the bond angles be for CH3+.?
3. According to VSEPR, what should the bond angles be for NH3.?
4. Based on what you have learned about geometry, I will expect you to be able to draw compounds
well from now on, and this means that you will have acceptable geometries. Here are some very
common mistakes that I see. Explain why these drawings are not well drawn, then draw the correct
structure. (Hint: Use models to check out the bond geometries.)
5. Fill in the implicit hydrogens in these molecules to indicate their proper bond angles. (You may or
may not need to use wedges/dashes.)
Cumulative problems
6. The shape of products of these reactions is partially incorrect. A. Use VSEPR to determine which
atom has the wrong geometry, then draw the product in the correct shape using wedges and dashes as
necessary. B. Indicate whether this reaction leads to greater stability or less stability of the compound.
7. Draw 2,3,3-trimethylpent-1-ene with wedges and dashes on any quaternary carbon atoms to
designate molecular shape.
8. Explain why 3-chloro-3-methylpent-2-ene does not exist.
9. Why is cyclopropene less stable than cyclopropane?
Extension problems
10. Which of these structures conveys more information? Explain.
11. Using models, explain one way in which these two compounds are the same, and one way in which
they are different.