Chapter 10 · Chemical Bonding
10.7Predicting the Shapes of Molecules
13 min · two checks
Predict
Water has four electron groups around oxygen. Why is it bent, not linear?
The idea
- Use VSEPR to predict electron geometry and molecular shape.
- Include lone pairs as groups that push.
Valence-shell electron-pair repulsion (VSEPR) says electron groups around a central atom stay as far apart as they can. Two groups: linear, 180°. Three: trigonal planar, about 120°. Four: tetrahedral, about 109.5°. A group is a lone pair, a single bond, a double bond, or a triple bond — multiple bonds count as one group for shape.
The molecular geometry names atom positions only. Four groups with no lone pairs: tetrahedral (CH₄). Three bonds and one lone pair: trigonal pyramidal (NH₃). Two bonds and two lone pairs: bent (H₂O). Three groups with one lone pair: bent, as in SO₂, with an angle near 120° rather than 109.5°. Lone pairs repel a bit more than bonding pairs, so angles close slightly. Draw wedges and dashes only after you know the three-dimensional group arrangement.
Keep these
- Count electron groups, not atoms alone.
- 2 linear, 3 trigonal planar, 4 tetrahedral — that is the group arrangement.
- Lone pairs change the molecular name: pyramidal or bent.
Worked path
Predict the electron geometry and the molecular shape of H₂S. Sulfur is the central atom and has two lone pairs in the usual structure, like water.
Observe
Two bonds + two lone pairs = four electron groups.