Molecular Geometry and Polarity
VSEPR and Electron Domains
Predicting Shape
Valence shell electron pair repulsion theory predicts that electron domains arrange to minimize repulsion. Lone pairs repel more strongly than bonding pairs, often compressing bond angles.
Common Molecular Geometries
| Electron domains | Electron geometry | Example molecular shape |
|---|---|---|
| 2 | Linear | Linear |
| 3 | Trigonal planar | Bent or trigonal planar |
| 4 | Tetrahedral | Trigonal pyramidal or bent |
| 5 | Trigonal bipyramidal | Seesaw, T-shaped, linear |
| 6 | Octahedral | Square pyramidal or square planar |
Problem-Solving Tip
Count electron domains, not just atoms attached to the central atom. Lone pairs matter.
What is the molecular shape of ?
Nitrogen has four electron domains, giving a tetrahedral electron geometry and trigonal pyramidal molecular shape because of one lone pair.
Correct answer: Trigonal pyramidal
How many electron domains are around sulfur in ?
Sulfur has two bonding domains and one lone pair in the common Lewis description, for a total of three.
Correct answer: 3
Polarity and Dipole Moments
From Bond Polarity to Molecular Polarity
A polar bond arises from electronegativity differences. Molecular polarity depends on both bond polarity and geometry. Symmetry can cancel dipoles even when individual bonds are polar.
Which molecule is nonpolar despite having polar bonds?
Linear has two polar C=O bonds whose dipoles cancel.
Correct answer:
Why is water polar?
The bonds are polar and the bent geometry prevents cancellation.
Correct answer: its bent shape does not cancel the bond dipoles