1. The problem statement, all variables and given/known data A hydrogen atom collides with another hydrogen atom at rest. If the electrons in both atoms are in the ground state, what is the minimum kinetic energy of the hydrogen atom such that the hydrogen atom at rest will have its electron in the first excited state after collision? 2. Relevant equations ##E=h \nu## ##E_n=E_1/n^2## 3. The attempt at a solution I know that the first excited state mean n=2 however, I am not sure how to write the energy conservation here as the question does not mention the energy of the colliding atom after the collision.