Why bond angle of trihalide increase down the group in 15 group while of hydride decrease down the group.

Explanation:

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 electronegativity 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.

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