How to work it out
- Central atom: P, with 5 valence electrons.
- Add 1 for each H or halogen around it: 3. O and S add nothing.
- Steric number = ½ × (5 + 3) = 4.
- 3 atoms are bonded, so 4 − 3 = 1 lone pair.
- 4 electron pairs → sp³, tetrahedral arrangement; the lone pairs make the shape trigonal pyramidal.
Why PCl₃ has this shape
One lone pair on P pushes the three P–Cl bonds together, so the angle is about 100°, less than the tetrahedral 109.5°.
PCl₃ is polar: its shape means the bond dipoles don't cancel.
Other molecules with the same trigonal pyramidal shape: XeO₃, NH₃, H₃O⁺.
Hybridization of common molecules
| Molecule or ion | Hybridization | Shape | Lone pairs |
|---|---|---|---|
| XeF₄ | sp³d² | Square planar | 2 |
| XeF₂ | sp³d | Linear | 3 |
| XeF₆ | sp³d³ | Distorted octahedral | 1 |
| XeO₃ | sp³ | Trigonal pyramidal | 1 |
| XeOF₄ | sp³d² | Square pyramidal | 1 |
| SF₄ | sp³d | See-saw | 1 |
| SF₆ | sp³d² | Octahedral | 0 |
| ClF₃ | sp³d | T-shaped | 2 |
| BrF₃ | sp³d | T-shaped | 2 |
| BrF₅ | sp³d² | Square pyramidal | 1 |
| IF₅ | sp³d² | Square pyramidal | 1 |
| IF₇ | sp³d³ | Pentagonal bipyramidal | 0 |
| PCl₅ | sp³d | Trigonal bipyramidal | 0 |
| PCl₃ | sp³ | Trigonal pyramidal | 1 |
| NH₃ | sp³ | Trigonal pyramidal | 1 |
| H₂O | sp³ | Bent (V-shaped) | 2 |
| CH₄ | sp³ | Tetrahedral | 0 |
| BF₃ | sp² | Trigonal planar | 0 |
| BeCl₂ | sp | Linear | 0 |
| CO₂ | sp | Linear | 0 |
| SO₂ | sp² | Bent (V-shaped) | 1 |
| SO₃ | sp² | Trigonal planar | 0 |
| H₂S | sp³ | Bent (V-shaped) | 2 |
| NH₄⁺ | sp³ | Tetrahedral | 0 |
| H₃O⁺ | sp³ | Trigonal pyramidal | 1 |
| NO₃⁻ | sp² | Trigonal planar | 0 |
| NO₂⁺ | sp | Linear | 0 |
| CO₃²⁻ | sp² | Trigonal planar | 0 |
| SO₄²⁻ | sp³ | Tetrahedral | 0 |
| ClO₄⁻ | sp³ | Tetrahedral | 0 |
| I₃⁻ | sp³d | Linear | 3 |
| ICl₄⁻ | sp³d² | Square planar | 2 |
| PF₆⁻ | sp³d² | Octahedral | 0 |
| SnCl₂ | sp² | Bent (V-shaped) | 1 |
Questions students ask
What is the hybridization of PCl₃?
sp³. The central P has 4 electron pairs around it: 3 bonds and 1 lone pair.
What is the shape of PCl₃?
Trigonal pyramidal. The electron pairs are arranged tetrahedral, but the 1 lone pair is not counted in the shape.
What is the bond angle in PCl₃?
100°, smaller than the ideal tetrahedral angle because lone pairs repel the bonds more.
Is PCl₃ polar or nonpolar?
PCl₃ is polar: its shape means the bond dipoles don't cancel.