Question

why the bond angle of Nh4+ is greater than Nh3?

Answer 1

Bond angles in ammonia and ammonium ion. 

The bond angles in ammonium ion, NH4^+ are the theoretical tetrahedral bond angles of approximately 109.5 degrees. The ammonium ion is completely symmetrical. 

The bond angles in the asymmetrical ammonia molecule are less than 109.5 degrees, they are about 107 degrees. This is due to the repulsion of the bonding pairs and the lone pair.