I have just learned the Bohr model of the atom that electrons can be viewed as standing waves at discrete energy levels. However standing waves on, say, a string are created by 2 waves in opposite directions, but in an atom there may be just 1 electron travelling in 1 direction, so how does it create a standing wave? Thanks in advance!

  • $\begingroup$ +1 You are really trying to square the circle with the (100 year old ) Bohr model. Don't get me wrong, I went through it all myself, but it was dead in the water the day after its paper was published. It's a terrible, imo, visual and physical intro to QM. $\endgroup$ – user167453 Oct 7 '17 at 16:30
  • $\begingroup$ Considerations: (a) The electron in a ground-state hydrogen atom has no orbital angular momentum, only spin. (b) Indeed our understanding is that this electron is intrinsically in a state which is spread out over space. (c) While there is a probability distribution associated with this spread, we know that this information is not sufficient to give us the "wavy" parts of the theory, which also require a phase angle at each point. Otherwise one does not predict e.g. the tetrahedral angles that carbon sometimes forms. (d) Secretly the electron is just an excitation of a quantum field. $\endgroup$ – CR Drost Oct 7 '17 at 16:43
  • $\begingroup$ As Countto10 has mentioned, this model is just an illustration to grasp the physical meaning of quantum fields producing these modes. So, as an illustration, just use a circular string, forget the sphere. In reality, it's not a sphere, but a cloud of mathematical probabilities that don't exist, but define where the electron may appear if detected. And it is always detected as a particle, not wave. Back to your question, a wave is long and can loop around the circle string many times interfering with itself. A whole number of wavelengths in a circle for a standing wave. $\endgroup$ – safesphere Oct 7 '17 at 16:47

I wrote a comment that might be unfair on you, as to pass exams etc., you have to jump through the hoops you are given in your introductory physics lectures, so please stick to your text and learn that.....but:

The history of physics is a long list of models, and attempts to get physical pictures that we can relate in the real world with what goes on at the smaller and smaller levels.

This is ok, until we get to the atomic level, and then it fails completely. Trying to imagine something "weird" in ordinary terms, such as standing waves is never going to work and Bohr and his colleagues knew this, that's why the Bohr model of the atom is called semi classical. It tries to take physics as it was understood at the time, and make it fit atomic and molecular structure, which it couldn't.

Today, and actually since Bohrs' model, very few physicists even try to imagine a physical picture of what an atom, or an electron, "look" like. Instead, math is used to make predictions of how the electron will behave rather than "what" an electron actually is.

After all, when we measure the mass or electric charge of an electron, we are providing a better and better description and measurement of it's physical properties, but we are not really any closer than Bohr was to pinning down what an electron is, because there is nothing like it in the world around us.

  • $\begingroup$ Gasp!! Do you mean that [this picture][commons.wikimedia.org/wiki/File:Atom_diagram.png] is wrong? Then why do people keep using it!? :) $\endgroup$ – Bill N Oct 7 '17 at 17:33
  • $\begingroup$ @BillN I can't see the image, (bad link) but I can guess what you mean, and everybody (with any curiosity , as I am delighted the OP has) wants bedrock, but personally, I ain't optimistic it's available, or ever will be. Still, jobs for the physics boys , and girls, for a while yet.... $\endgroup$ – user167453 Oct 7 '17 at 17:44
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    $\begingroup$ @Countto10 Just remove the last character from the URL and the picture is there. $\endgroup$ – safesphere Oct 7 '17 at 20:09
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    $\begingroup$ @Countto10 I think the OP's question was about understanding the logic in the Bohr's model, mot about the validity of this model. On this site one always should start his question with, "I'm not asking about that", and only then proceed with, "I'm asking about this". Otherwise people here tend to reflect what's in their mind rather than what the question is really about. Good answer though +1 :) $\endgroup$ – safesphere Oct 7 '17 at 20:24
  • $\begingroup$ @safesphere. Yeah, Planck doubted existence of atoms, after looking at that mini solar system, so do I. Thanks $\endgroup$ – user167453 Oct 7 '17 at 20:26

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