In a stationary hydrogen atom, an electron jumps from n=2 to n=1, then linear momentum of the atom is

Energy of emitted photon =13.61−122ev=10.2ev∴ Momentum of the photon =Ec=102×1.6×10−193×108kg−m/s=5.44×10−27kg−m/sBy conservation of linear momentum, we conclude that momentum of the hydrogen atom  =5.44×10−27kg−m/s