In group 15 trihalides,bond angle increases …

All these hydrides are covalent in nature and have pyramidal structure. These involve sp³ hybridization of the central atom and one of the tetrahedral positions is occupied by a lone pair.Due to the presence of lone pair, the bond angle in NH₃ is less than the normal tetrahedral angle. It has been found to be 107 degrees. Down the group the bond angle decreases as:

In all these hydrides, four electron pairs, three bond pairs and one lone pair surround the central atom. Now, as we move down the group from N to Bi, the size of the atom goes on increasing and its electro negativity decreases. Consequently, the position of bond pair shifts more and more away from the central atom in moving from NH₃ to BiH₃. For e.g., the bond pair in NH₃ is close to N in N-H bond than the bond pair in P-H bond in PH₃. As a result, the force of repulsion between the bonded pair of electrons in NH₃ is more than in PH₃. In general, the force of repulsion between bonded pairs of electrons decreases as we move from NH₃ to BiH₃ and therefore, the bond angle also decreases in the same order.