When an atom undergoes a transition from one level to another, the energy needed is usually provided by a photon, but it may also come from the kinetic energy of a collision. Consider the collision of two hydrogen atoms, one with an initial speed v and another initially at rest. What value of v is needed to give the moving hydrogen atom a kinetic energy of 13.6 eV? If all this energy can be transferred to one of the electrons, this collision could cause one of the atoms to ionize.