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A problem in Bohr's day was understanding why an orbiting electron does not continuously radiate an EM field. An orbiting electron is a moving charge and according to Maxwell, this should generate an electric and magnetic field. Bohr said that photons (packets of EM energy) are only emitted when an electron jumps from a high quantum state to a lower one. Does that mean that an electron in a stable atom really isnt moving? Conversely, does Bohr's model mean that all motion requires energy transitions from one qunatum sate to another?


marked as duplicate by Dilaton, Dan, Ben Crowell, BebopButUnsteady, Alan Rominger Jul 29 '13 at 17:11

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  • $\begingroup$ Hi aquagremlin. Try looking at physics.stackexchange.com/q/68381/26076 . "Does that mean that an electron in a stable atom really isn't moving?": You're altogether on the right track - excellent thinking! It is not moving in the everyday sense we think of it - in an orbital it is a point particle in an unchanging superposition of localized states - so you could say it is standing still at all the points in the orbital at once. $\endgroup$ – WetSavannaAnimal Jul 24 '13 at 12:32
  • $\begingroup$ @WetSavannaAnimalakaRodVance I don't think OP is asking about modern theories of atomic structure. Bohr model is almost classical with just few assumptions, so, electrons in his model are indeed nearly classically orbiting around a nucleus. $\endgroup$ – Wildcat Jul 24 '13 at 12:37
  • $\begingroup$ @Wildcat Good point - but I was so thoroughly impressed that aquagremlin seemed to have come up with the idea on his/her own that the electron really isn't moving that I just had to comment. It was a long time before I really appreciated this, and in exactly what sense, and then only after I had been told it many times! $\endgroup$ – WetSavannaAnimal Jul 24 '13 at 12:40
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    $\begingroup$ possible duplicate of Why don't electrons crash into the nuclei they "orbit"? $\endgroup$ – Dilaton Jul 24 '13 at 13:27
  • $\begingroup$ @Wildcat i was thinking about the bohr model's contradiction that led to my question. i was not restricting my thinking to the bohr model nor questioning the details of that model. rather i am trying to understand the concept of quantum motion. $\endgroup$ – aquagremlin Jul 24 '13 at 23:57

Does that mean that an electron in a stable atom really isnt moving?

No. Bohr just postulated the existence of stationary orbits, which have a property that an electron orbiting around the nucleus in this orbits doesn't radiate the energy.

Conversely, does Bohr's model mean that all motion requires energy transitions from one qunatum sate to another?

No. The restriction on the motion of an electron is that it can orbit a nucleus only in the above mentioned stationary orbits. In addition to that type of motion, which does not require any energy transitions, an electron can also jump from one stable orbit to another, in which case the will be some energy transfer.

Details can be found in Wikipedia article.

  • $\begingroup$ tnx for the wiki reference. Also the reference at the top of my question (physics.stackexchange.com/questions/20003/…) had an interesting comment:'All the s-states have an anti-node at the center, and predict that the electron spends a small but non-negligible fraction of the time in the nucleus.'.This leads me to believe that the electron orbital is the lowest energy state. That an electron when paired with a proton behaves differently than when alone. It also raises the question of the orbitals of the protons in the nucleus. $\endgroup$ – aquagremlin Jul 25 '13 at 0:04
  • $\begingroup$ @aquagremlin to begin with you should realize that orbitals are not observable, they are just a mathematical approximation. This approximation is indeed quite useful in understanding of the multitude of chemical phenomena at the simplest and yet sufficient for practical applications level, but still it is nothing but an approximation. $\endgroup$ – Wildcat Jul 25 '13 at 6:54

Well aquagremlin is indeed on the right track. Heinrich Hertz and Maxwell established that a varying current in a conductor (antenna) radiates EM waves. A varying current, is just another way of saying "accelerating electric charge"

After Rutherford established that the atom was indeed a compact nucleus, surrounded by other charges (electrons), Niels Bohr made a completely arbitrary assumption, that the electrons orbited the nucleus, and that only when they did so in orbits that set up standing wave patterns at particular frequencies, did the electrons not radiate due to their constant acceleration. Sommerfeld added to the mess, by introducing elliptic orbits as well. So this proposal, simply abandoned the classical result of Maxwell and Hertz.

Quantum mechanics saved the day by saying that the "orbits" of Bohr/Sommerfeld, are really "orbitals" , just regional maps of the probability of finding the electron in that location. So the orbital motion disappears, replaced by a probability map.

So Maxwell-Hertz was completely restored, and is to this day. The only three fundamental physical constant, which have exact numerical values, with zero error, are (c) the velocity of propagation of Hertzian EM waves, and epsilon naught, the permittivity of free space, plus mu naught, the permeability of free space. Those two values are used in the Hertz derivation of the velocity of EM waves as c = 1/ sqrt(mu0 x epsilon0), and also in deriving the characteristic impedance of free space = 120.pi Ohms (roughly = 377 Ohms = sqrt (mu0 / epsilon0).

Also, according to quantum mechanics, one of the allowed orbitals includes the nucleus itself, so the electron could be in the same location as the nucleus.

So accelerated charges ALWAYS RADIATE, no exceptions. Neutral atoms that tend to have zero dipole moments, become distorted during (thermal) collisions, since the nucleus and electron cloud have equal charges, but the nucleus has at least 1875 times the electron mass, and usually 3750 times, for neutrons = protons.

So all the momentum of the atom is in the nucleus, so the nuclei continue on, after the electron clouds in collisions have stopped. So during the collision encounter, the distorted atoms, have a non-zero electric dipole moment, and therefore do radiate as a consequence of Temperature. That is the source of the thermal black body like radiation.

Since the collision velocities vary widely, and also the trajectories (of the nuclei), a broad range of radiation frequencies will be generated. The collision encounter times are like all day, in the time scales of em radiation events.


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