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VSEPR Theory
Valence-Shell Electron-Pair Repulsion (VSEPR)
The geometry and shape of an atom in a molecule is dictated by the atom's hybridization (number of σ-bonds and π-bonds). It is important to consider lone pairs as they also occupy space! Some examples with carbon are below.
Wize Tip
In organic chemistry the magic numbers are 109.5° (around sp3 atoms), 120° (around sp2 atoms) and 180° (around sp atoms).
Imperfect Bond Angles
Sometimes atoms are forced to adopt a less-than-ideal geometry (due to the presence of a ring). This helps explain why five- and six-membered rings are more stable than their smaller and larger analogues.
The Effect of Lone Pairs
As mentioned above, lone pairs occupy space similar to atoms. In fact, the repulsion of lone pairs is greater than other electron pairs - this makes the "perfect angle" between atoms less than perfect.
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Example: VSEPR
What are the bond angles at each of the atoms labelled (C1 and N2)?
- C1 is 180o
- N2 is 120o (beware resonance structures)
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Example: VSEPR
What are the bond angles (assuming no lone pair distortion) at the following atoms (N3, C4, C5, B6):
- N3: 109.5
- C4: 120
- C5: 180
- B6: 120 (remember, boron is electron deficient)
Mark Yourself Question
- Grab a piece of paper and try this problem yourself.
- When you're done, check the "I have answered this question" box below.
- View the solution and report whether you got it right or wrong.
Practice: VSEPR
Draw a Lewis structure and provide the bond angles at the central atom(s) for the following molecules:
- NCl3
- SO2
- BBr3