10−42 N−m The angular momentum of the electron for the nth state is given by Ln​=2πn h​ Angular momentum of the electron for n=3,Li​=2π3h​ Angular momentum of the electron for n=2,Lf​=2π2h​ The torque is the time rate of change of the angular momentum.Torque, τ=tLf​−Li​​ ​=10−8(2 h/2π)−(3 h/2π)​=10−8−(h/2π)​=10−8−10−34​[∵2πh​≈10−34 J−s]=−10−42 N−m​ The magnitude of the torque is 10−42 N-m.