Explain the following observations giving appropriate reasons :The HEH bond angle of the hydrides of Group 15 elements decreases as we move down the group.

Explain the following observations giving appropriate reasons :The HEH

1.	Explain the following observations giving appropriate reasons :The HEH bond angle of the hydrides of Group 15 elements decreases as we move down the group.
Answer» Solution : The elements in Group 15 are N, P, As, Sb and Bi. They form hydrides with the formula `EH_3`. The CENTRAL atom E is sp hybridised. One of the hybridised orbitals contains a lone pair of electrons while the other three hybridised orbitals overlap with the s-orbitals of three hydrogen atoms to form three E-H bonds. Normal angle in sp HYBRIDISATION is `109^(@)-28.` but due to greater lone pair-BOND pair repulsion, the angle is decreased to `107^(@)` in the case of `NH_3`. However the `angleHEH` decreases as we move from N to P to Sb. This is because as we move DOWNWARDS in to Bi Group 15, the bond length EH increases, the repulsion between bond pairs decreases and consequently the angle `angleHEH` decreases.