# In the Bohr theory, how does excitation and relaxation effect angular momentum?

When single electron atoms is de-excited from higher orbit to lower orbit there is a change in angular momentum. As,

$$L=n\frac{h}{2\pi}.$$

I don't see any reason why it should happen as there is no external torque. Can energy substitute for the external torque which happens? What exactly is the properties of this torque?

• "Bohr" theory is wrong, and no one really uses it in practice anymore. Quantum mechanics is widely accepted as the correct description of energy levels in atoms and many other phenomena. Why are you asking about an obsolete theory? Inconsistencies in it are precisely why we don't use it anymore. – ACuriousMind Apr 18 '20 at 10:32
• He may still be in high school physics... – Buraian Apr 18 '20 at 10:43

This process is well described by a semi-classical treatment, but, if you are looking for a (classical) phenomenological intuition you can imagine the atom like a rotating disk. When an emission occur, it lose angular momentum because of the emission of a photon with momentum $$p = h \nu$$.
• This is why you can't trust of classical analogue. The change in angular momentum depends on the polarization of emitted photon. This has no classical analogue and trying to give you a classical picture i bring you out of the way. Not only you have the quantum number $n$ and orbital angular momentum but also a magnetic quantum number, and all of them can change because of the emission. As you can see there are plenty of ingredients that can't be explained with classical mechanics (torque and so on). – ACA Apr 18 '20 at 18:19