Geometry: 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.
 The physical basis for the shapes of molecules can be described in terms of stable orbitals for
valence electrons, and bonds can be described in terms of orbital overlap.
 Using a qualitative model of orbitals, we can describe an atom in terms of its hybridization,
which tells us about molecular geometry and stability of electrons.
Vocabulary
 Primary, secondary, tertiary, quaternary carbons
 carbocations
 VSEPR
 Linear, trigonal planar, tetrahedral
 Wedge/dash convention
 Orbitals
 hybridization
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
 Explain what is meant by “hybridization” and “orbital”
 Given the hybridization of atoms, be able to draw orbital overlap pictures of simple molecules
and ions
Daily Problems
1. Use VSEPR to indicate the geometry of the indicated atom as linear, trigonal planar, or tetrahedral.
2. Draw an orbital picture for carbon that is sp3 hybridized, sp2 hybridized, and sp hybridized. Indicate
the angles between each orbital.
3. The carbon atom in CH3+ is sp2 hybridized. Draw an orbital overlap picture for this ion.
4. According to VSEPR, what should the bond angles be for CH3+.? Does this match your orbital overlap
picture from problem 3?
5. The nitrogen atom in NH3 is sp3 hybridized. Draw an orbital overlap picture for this molecule.
6. According to VSEPR, what should the bond angles be for NH3.? Does this match your orbital overlap
picture from problem 5?
7. Based on what you have learned about hybridization and geometry, I will expect you to be able to
draw compounds well from now on. 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.)
8. Fill in the implicit hydrogens in this molecule to indicate their proper bond angles. (You may or may
not need to use wedges/dashes.)
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Cumulative problems
9. 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.
10. Draw 2,3,3-trimethylpent-1-ene with wedges and dashes on any quaternary carbon atoms to
designate molecular shape.
11. Explain why 3-chloro-3-methylpent-2-ene does not exist.
12. Why is cyclopropene less stable than cyclopropane?
Extension problems
13. Which of these structures conveys more information? Explain.
14. Using models, explain one way in which these two compounds are the same, and one way in which
they are different.