Cross-Group VSEPR Connections
The Universal Pattern: Electron Pairs Determine Geometry
All molecular geometries in Groups 16-18 follow VSEPR:
| pairs | Bond pairs | Lone pairs | Base geometry | Molecular geometry | Example |
|---|---|---|---|---|---|
| 3 | 3 | 0 | Trigonal planar | Trigonal planar | SO3 |
| 3 | 2 | 1 | Trigonal planar | Angular/bent | SO2, O3 |
| 5 | 2 | 3 | Trigonal bipyramidal | Linear | XeF2 |
| 5 | 3 | 2 | Trigonal bipyramidal | T-shaped | ClF3, XeOF2 |
| 5 | 4 | 1 | Trigonal bipyramidal | See-saw | SF4 |
| 6 | 4 | 2 | Octahedral | Square planar | XeF4 |
| 6 | 5 | 1 | Octahedral | Square pyramidal | BrF5, IF5 |
| 7 | 7 | 0 | Pentagonal bipyramidal | Pentagonal bipyramidal | IF7 |
The LONE PAIR PREFERENCE RULE:
- In TBP (5 pairs): lone pairs go to EQUATORIAL (fewer 90° repulsions)
- In Octahedral (6 pairs): lone pairs go to TRANS (maximally far apart)
Connection to Group 15:
- NH3 (sp3, 3BP + 1LP = pyramidal)
- PCl5 (sp3d, 5BP + 0LP = trigonal bipyramidal) → Same VSEPR principles apply across all groups
Why XeF2 is NOT angular (unlike H2O):
- H2O: sp3, 2BP + 2LP → angular
- XeF2: sp3d, 2BP + 3LP → linear (lone pairs go equatorial in TBP, bond pairs go axial)
- The extra d-orbital makes all the difference!