Why bond angle of hydrides of group 15, decreases down the group?

Hydride: NH₃ > PH₃ > AsH₃ >SbH₃

H-M-H angle: 107 > 92 > 90 > 91 (Degree)

This is due to the decrease in electronegativity of the central atom of hydride. As the electronegativity decrease the electrons which were more close to central atom at lower hydrides tends to more far from it and bp-bp repulsion decreases.

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