Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- VSEPR: electron domains around an atom repel (Coulombic repulsion) and get as far apart as possible.
- Domain count: each lone pair = 1; each bonded atom = 1, whether the bond is single, double or triple.
- Shape name uses atom positions only. Lone pairs change the shape and squeeze angles.
- Repulsion: lone pair–lone pair > lone pair–bond > bond–bond. Multiple bonds also push a little harder than single bonds.
- With 5 domains, lone pairs go equatorial. With 6 domains, a second lone pair goes opposite the first.
- σ bond: head-on overlap on the axis. π bond: side-on, above and below. π is weaker than σ and stops rotation, which allows cis/trans isomers.
Key relationships
| Domains | Lone pairs | Shape | Angle | Hybridization |
|---|---|---|---|---|
| 2 | 0 | linear | 180° | sp |
| 3 | 0 / 1 | trigonal planar / bent | 120° / just under 120° | sp² |
| 4 | 0 / 1 / 2 | tetrahedral / trigonal pyramidal / bent | 109.5° / about 107° (NH₃) / 104.5° (H₂O) | sp³ |
| 5 | 0 / 1 / 2 / 3 | trigonal bipyramidal / seesaw / T-shaped / linear | 90°, 120°, 180° | shape only |
| 6 | 0 / 1 / 2 | octahedral / square pyramidal / square planar | 90°, 180° | shape only |
| Bond property | Rule |
|---|---|
| Higher bond order (same two elements) | shorter, stronger |
| Larger atoms | longer, usually weaker |
| C–C 347, C=C 614 kJ mol⁻¹ | π adds about 267 kJ mol⁻¹, less than the σ bond |
Dipole moment = polar bonds + shape that does not cancel them.
Assumptions behind the model
- You start from the best Lewis diagram; resonance and expanded octets do not change the domain count.
- Angles quoted are ideal; real angles shift with lone pairs, multiple bonds and atom size.
- Hybridization labels are only used for 2–4 domains. How hybrid orbitals form, d-orbital hybridization and molecular orbital diagrams are outside the course.
Mistakes to avoid
- Counting a double bond as two domains.
- Naming the electron-domain geometry when the question asks for the molecular geometry.
- Forgetting lone pairs on the atom when naming its hybridization.
- Putting trigonal bipyramid lone pairs on the axis.
- Saying a double bond is twice as strong as a single bond.
- Assuming polar bonds always mean a polar molecule.
Quick self-check
- Shape and angle of SCl₂? (4 domains, 2 lone pairs: bent, under 109.5°)
- σ and π bonds in HCN? (2 σ, 2 π; carbon is sp, 180°)
- Shape of SF₆, and does it have a dipole moment? (octahedral; no, the six bond dipoles cancel)
Next: practice questions.