🟣 This is Higher Level (HL) content.
Five Electron Domains (Trigonal Bipyramidal Base)
The base geometry is asymmetric – 3 equatorial positions (120° apart) and 2 axial positions (90° from equator). Lone pairs always occupy equatorial positions to minimise repulsion.
| BP | LP | Molecular Shape | Example | Bond Angles |
|---|---|---|---|---|
| 5 | 0 | Trigonal bipyramidal | PCl₅ | 90° & 120° |
| 4 | 1 | See-saw | SF₄ | ~102° & ~173° |
| 3 | 2 | T-shaped | ClF₃ | < 90° |
| 2 | 3 | Linear | XeF₂, I₃⁻ | 180° |
Six Electron Domains (Octahedral Base)
All six positions are equivalent (all 90° apart). When lone pairs are present, they adopt trans positions (opposite each other, 180° apart) to minimise LP–LP repulsion.
| BP | LP | Molecular Shape | Example | Bond Angles |
|---|---|---|---|---|
| 6 | 0 | Octahedral | SF₆ | 90° |
| 5 | 1 | Square pyramidal | BrF₅ | < 90° |
| 4 | 2 | Square planar | XeF₄ | 90° |
⚠️ Examiner Trap – Lone Pair Placement
In trigonal bipyramidal systems, lone pairs always go equatorial. In octahedral systems with 2 LP, they go trans (180° apart, not adjacent). Getting this wrong leads to incorrect molecular geometry predictions.
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