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.
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.
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)
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)