An electron in H atom makes a transition from n = 3 to n = 1. The recoil momentum of H atom will be

The recoil momentum of atom is same as that of photon but in opposite direction.Hence, recoil momentum:P=Ec=12.09×1.6×10−193×108Ns=6.45×10−27NsNote that almost whole of the energy will be carried away by the photon because it is very light in comparison to H atom.