Structure of amines

Nitrogen orbitals of all amines are sp3 hybridised and the geometry of amines is pyramidal.

Each of the three sp3 hybridised orbitals of nitrogen overlaps with the orbital of hydrogen or carbon depending upon the composition of amines. The fourth sp3 orbital contains an unshared pair (lone pair) of electrons.

According to VSEPR theory

Therefore, the bond angle between any two adjacent H-atoms or alkyl groups decreases from the tetrahedral angle of 109.5º to 107º in primary (1º) and secondary (2º) amines and ammonia.

In case of tertiary (3º) amines, the bond angle may increase from 107º due to steric hindrance between the three bulky alky groups. For example, it is 108º in case of trymethylamine as shown in following figure.

Pyramidal shape of trimethylamine