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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?

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  • $\begingroup$ "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. $\endgroup$ – ACuriousMind Apr 18 '20 at 10:32
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    $\begingroup$ He may still be in high school physics... $\endgroup$ – Buraian Apr 18 '20 at 10:43
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De-exiting of hydrogen atoms can occur in presence of an external electromagetic field. In this case we can speak about stimulated and spontaneous emission. Since we have the interaction of the atom with the external field the angular momentum of the hydrogen atom is not a conserved quantity.

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 $p = h \nu$.

It is always a no-man's-land to give classical interpretation of quantum mechanical problems, and very often they are counterproductive since they anchor us to the wrong side of nature.

Still we have some good example of classical physics applied to atoms systems. You can look at the Larmor precession (very usefull in medical applications: NMR)

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  • $\begingroup$ Doesn't it vary with the direction in which the photon emitted or is there a particular direction for which the photon is more favourable to move? and also can we conserve the momentum in the tangential direction? $\endgroup$ – An enthusiast Apr 18 '20 at 15:39
  • $\begingroup$ 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). $\endgroup$ – ACA Apr 18 '20 at 18:19

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