NH3 molecule: According to VSEPR theory, NH3 should have tetrahedral shape due to presence of 4 electron pairs. The presence of lone pair on nitrogen causes greater repulsion to the bond pairs. As a result of which the three N-H bonds move slightly closer. So it decreases the normal tetrahedral bond angle of 109.50 to 1070 . Therefore, ammonia has irregular geometry. Since one of the tetrahedral position is occupied by lone pair, the shape of ammonia molecule is pyramidal.