Bohr's third postulate states than an electron can make a transition from one stationary state of lower energy to another of higher energy if the required amount of energy is provided by means of a photon of appropriate energy (required to make the transition).

However does it say anything about why an excited electron will become de-excited? Why will an excited electron eventually come back to the ground state? After all it does have the energy to stay in the particular stationary state of higher energy. And by the definition of a stationary state it won't lose any energy in that state...

  • $\begingroup$ It's a good question with no chance of an answer explaining the mechanism behind this phenomenon. I tried to give an explanation in a very dry written elaboration about Complex one-dimensional structures in space. $\endgroup$ Apr 13, 2017 at 4:47
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    $\begingroup$ I once read that when Bohr explained his theory to Rutherford, Rutherford (thinking classically) objected that only when the photon had been emitted did the atom reach its 'final' state, so how could the atom its initial state emit just the right energy of photon for the atom to reach its final state? [Loosely: how would the atom know what energy of photon to emit?] $\endgroup$ Sep 3, 2022 at 18:37

2 Answers 2


Spontaneous emission cannot be explained with the Bohr model. QED is needed. You can read more about spontaneous emission on the wiki page.

  • $\begingroup$ Plain old quantum mechanics works just fine without resorting to QED mind you. Or else I've got a bunch of quantum books that I must be mis-remembering. $\endgroup$
    – Jon Custer
    Apr 11, 2017 at 18:00
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    $\begingroup$ QED is definitely needed, for decay to occur the EM field needs to be quantized. I guess finding the rate of decay can be done with only quantum but it does not explain the fundamental process. $\endgroup$
    – jmadden
    Apr 11, 2017 at 18:21

Well, the relevant postulate in this link is

When an electron moves between stationary states, it is accompanied by the emission or absorption of a photon. This photon's energy is given by ΔE=hf

Also here, where they are not numbered:

When an electron jumps from a higher energy level to a lower one, the amount of energy absorbed or emitted is given by the difference of energies associated with the two levels.

So it is a part of the postulates that had to be assumed in order to model the data of the hydrogen spectrum.

  • $\begingroup$ So he couldn't explain why every electron jumps back to the ground state eventually? $\endgroup$ Apr 12, 2017 at 1:59
  • $\begingroup$ correct. Postulates are extra axioms in physics theoretical models that pick up the subset of solutions that are relevant to fitting observations and predicting new data. $\endgroup$
    – anna v
    Apr 12, 2017 at 2:57

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