The bond angles depend on the number of bonding areas and the number of
lone pairs.
A molecule with 4 bonding pairs and no lone pairs is tetrahedral with a bond angle of 109.5°.
For each lone pair the bond angle decreases by 2.5°
This is because the lone pair occupies a broader orbital as it is not shared by two atoms. So lone pair repulsion is greater than bonding pair repulsion.
Ammonia has a lone pair and 3 bonding pairs and is trigonal pyramidal. The lone pair pushes the bonds closer together giving a bond angle of 107°
Water has 2 lone pairs so the bond angle is pushed further to 104.5° and the shape is non-linear or bent.
| bonding pairs | lone pairs | shape and bond angle | example |
| 2 | 0 | linear 180° | BeCl2 |
| 3 | 0 | trigonal planar 120° | BF3, AlCl3 |
| 4 | 0 | tetrahedral 109.5° | CH4 |
| 5 | 0 | trigonal bipyramidal 120° 90° | PCl5 |
| 6 | 0 | octahedral 90° | SF6 |
| 3 | 1 | trigonal pyramidal 107° | NH3 |
| 2 | 2 | non-linear 104.5° | H2O |
| 1 | 3 | linear 180° | HCl |