How we can add electron to atom if it is too small?

Quantum physics hydrogen atom releasing a photon?

• The following question was asked on Y! Answers by Ben O. "A hydrogen atom is initially at rest (approximately), in an excited state of unknown quantum number n. The electron drops to the next lower energy level emitting a photon. Estimate the largest possible recoil velocity of the atom." My first thought was that there couldn't be any recoil because of conservation of energy. When the photon is emitted, energy is converted from the energy stored in the electron orbital to a photon. Therefore, the atom can't recoil since this would add kinetic energy that comes from nowhere. But what about momentum. A photon has momentum. Therefore the atom must recoil to conserve momentum. Does the atom actually recoil? How are the issues of conservation of momentum and energy resolved? If the atom does recoil, what is used to determine its recoil? There's no need to actually solve the problem. I am just trying to get a better understanding of what happens.

• Answer:

  I think the key might lie in the fact that if the speed of a photon is the speed of light then the photon must be regarded as an ultrarelatavistic particle. so p = E/c The energy of a particle with a speed equal to the speed of light can be finite only if the mass is zero: so photons must be regarded as particles of zero mass. Since the energy of a photon is E = hf = hc / wavelength Momentum is therefore p = h / wavelength My quantum mechanics knowledge is not of a high standard, but if the mass is zero where does the interaction occur with the bulk of the atom, and does stopping potential enter into the problem. Another possible item for thought would be a wavelength change between absorption and emission when a photon collision occurs.