According to VSEPR theory repulsion between : Lone pair-lone pair > Lone pair-bond pair > Bond pair-bond pair As the repulsion is increases then bond angle is decreases.
Molecule
Lone pair
Bond pair
Bond angle
NH2−
2
2
105°
NH3
1
3
107°
NH4+
0
4
109°28'
Therefore, the order of bond angle is NH4+>NH3>NH2−