Chemistry / Unit 4: Chemical Bonding
4.3 VSEPR and Molecular Shape
Notes
VSEPR theory says electron groups around a central atom repel and spread as far apart as possible. An electron group is a lone pair or a bond (single, double or triple each count as one).
| Bonded atoms | Lone pairs | Shape | Angle | Example |
|---|---|---|---|---|
| 2 | 0 | linear | 180° | $\text{CO}_2$ |
| 3 | 0 | trigonal planar | 120° | $\text{BF}_3$ |
| 4 | 0 | tetrahedral | 109.5° | $\text{CH}_4$ |
| 3 | 1 | trigonal pyramidal | about 107° | $\text{NH}_3$ |
| 2 | 2 | bent | about 104.5° | $\text{H}_2\text{O}$ |
- Lone pairs push harder than bonds, so they squeeze bond angles smaller.
Practice
What is the shape of a water molecule, $\text{H}_2\text{O}$?
Oxygen has 2 bonds and 2 lone pairs, which gives a bent shape.
What is the bond angle in $\text{CH}_4$?
Four bonding groups and no lone pairs give a tetrahedral shape with 109.5° angles.
$\text{BF}_3$ has three bonds and no lone pairs on boron. Its shape is:
Three groups with no lone pairs spread flat at 120°.
Ready for more? Take the Unit 4 test.