1. The problem statement, all variables and given/known data Ground-state hydrogen atom with 12 ev kinetic energy collides head-on with another ground-state hydrogen atom at rest. Using principles of conservation of energy and momentum, show that an inelastic collision cannot occur. Therefore the collision must be elastic. 2. Relevant equations Sum of energy before equals sum of energy after Vector sum of momentum before equals vector sum of momentum after 3. The attempt at a solution If the collision is inelastic, then the total kinetic energy after the collision will be less than it was before. Conservation of energy requires that the atoms absorb some of the initial kinetic energy and change their internal state. Conservation of momentum requires that the vector sum of velocities be unchanged by the collision (masses cancelling out). There is no way for this to occur, as I see it, if the total kinetic energy has been reduced by an inelastic collision. Therefore the collision must be elastic. This logic seems to say that all collisions between like atoms are elastic. Correct? Regardless of the initial kinetic energy?