1. The problem statement, all variables and given/known data There are a few problems I have with my HW that involve the characteristic size of an atom in a certain state. For example, the hydrogen atom in the ground state. 2. Relevant equations - 3. The attempt at a solution I don't know where to begin to find the characteristic size of an atom in a given state. My first thought would be to take the energy of the state and set it equal to the potential energy -Ke^2/r and then solve for r. However, alarms and bells are going off in my head doing this because this seems more like a classical method to find this so called characteristic size. Can someone push me into the right direction to find the characteristic size of an atom?