The shortcut formula
For a molecule with one central atom, count the electron pairs around it (the steric number):
V = valence electrons of the central atom, M = number of H or halogen atoms bonded to it, c = positive charge, a = negative charge. O and S around the centre count as 0.
For SF₄: S has 6, plus 4 for the F atoms, gives ½ × 10 = 5 pairs, so sp³d. Four are bonds, so one is a lone pair, and the shape is a see-saw.
Steric number, hybridization and shape
| Pairs | Hybridization | Lone pairs | Shape | Example |
|---|---|---|---|---|
| 2 | sp | 0 | Linear | BeCl₂ |
| 3 | sp² | 0 | Trigonal planar | BF₃ |
| 3 | sp² | 1 | Bent (V-shaped) | SO₂ |
| 4 | sp³ | 0 | Tetrahedral | CH₄ |
| 4 | sp³ | 1 | Trigonal pyramidal | NH₃ |
| 4 | sp³ | 2 | Bent (V-shaped) | H₂O |
| 5 | sp³d | 0 | Trigonal bipyramidal | PCl₅ |
| 5 | sp³d | 1 | See-saw | SF₄ |
| 5 | sp³d | 2 | T-shaped | ClF₃ |
| 5 | sp³d | 3 | Linear | XeF₂ |
| 6 | sp³d² | 0 | Octahedral | SF₆ |
| 6 | sp³d² | 1 | Square pyramidal | BrF₅ |
| 6 | sp³d² | 2 | Square planar | XeF₄ |
| 7 | sp³d³ | 0 | Pentagonal bipyramidal | IF₇ |
| 7 | sp³d³ | 1 | Distorted octahedral | XeF₆ |
Where lone pairs go
- Repulsion order: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. That's why NH₃ (107°) and H₂O (104.5°) are squeezed below 109.5°.
- Five pairs (sp³d): lone pairs take equatorial positions first, giving see-saw (SF₄), T-shaped (ClF₃) and linear (XeF₂).
- Six pairs (sp³d²): a second lone pair goes opposite the first, giving square planar (XeF₄).
Common molecules and ions
| 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 |
This shortcut works for one central atom with H, halogen, O or S around it. For molecules like C₂H₄, HCN or H₂O₂, draw the Lewis structure and count pairs on each atom. For bonding in O₂ and N₂, use the bond order calculator.
Questions students ask
How do you find hybridization quickly?
Use steric number = ½ × (valence electrons of the central atom + monovalent atoms around it − positive charge + negative charge). 2 is sp, 3 is sp², 4 is sp³, 5 is sp³d, 6 is sp³d², 7 is sp³d³.
What is the hybridization of XeF₄?
sp³d², with two lone pairs on Xe, giving a square planar shape.
What is the difference between shape and geometry?
Electron geometry counts all electron pairs, including lone pairs. Shape (molecular geometry) only describes where the atoms are. NH₃ has tetrahedral electron geometry but a pyramidal shape.
Why is the bond angle in water 104.5°?
Its two lone pairs repel the O–H bonds more strongly than the bonds repel each other, squeezing the angle below the tetrahedral 109.5°.