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Question about energy states

  1. Nov 30, 2014 #1
    Ok, so what I considered to be true for quite some time now has been somewhat tarnished after something that was said recently in a lecture, so I am looking for some insight.

    Basically, I was told several times by several teachers/professors over the years that when an electron absorbs a photon, and moves to a higher energy state, the distance between the electron and the nucleus increases. (Obviously there is a probability, and I know of the electron cloud and such, but basically the higher energy state makes it more likely that the electron is further out) This was then used to demonstrate that at a high enough energy state, the electron would be "shot" out, and become a free electron, and the atom would become ionized. The logic being that the energy added by the photon was so high that the distance between the nucleus and electron was too great for the nucleus to "pull it back in." (again, we are talking figuratively) This was also used to show why heavier elements were typically easier to rip electrons from, as the valence shell was much further away from the nucleus.

    Now then, fast forward to the last week. It was stated that the electron was not capable of physical movement, and although the electron could be anywhere in the cloud, the probability was rather fixed into set orbitals. When I inquired about the above statement, it was flat out said that the increase in energy does not change the charge of the electron, so would not change the distance, and that a change in charge that WOULD change the distance, would have an acceleration and therefore a movement, which wasnt possible.

    So now I am quite confused on which is actually correct. Thanks in advance to anyone that can help with this, as it has sort of messed with my head, being that something I "knew" for a long time might actually be very false.
  2. jcsd
  3. Nov 30, 2014 #2


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    This is the correct view of things. For example, on the graph at the bottom of this page, you can see the probability of finding the electron a certain distance r away from the nucleus Pnl(r) for the various orbitals. As you can see, the electron, on average, is farther from the nucleus for orbitals with higher energies. This must intuitively be true because as you move two oppositely charged objects farther apart, you increase their potential energy.

    I have no idea what the person who offered the second explanation was trying to say, but it makes no sense to me.
  4. Nov 30, 2014 #3
    Their point didnt make much sense to anyone, but they were basically looking at it from a charge point of view. Someone else mentioned that the charge hadnt changed, just the energy, and that an increase in charge would have actually pulled them closer. But I guess their point was something about how to move further away, the electron would have to move, meaning an application of a force, and therefore an acceleration. He stated that no such acceleration was possible for an orbiting electron, and that all movements within the cloud were instantaneous. (ie. without acceleration)
  5. Dec 1, 2014 #4


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    Many experts in quantum mechanics would not agree with the statement that electrons do not move; they clearly have non-vanishing kinetic energy, and there even are interpretation of quantum mechanics (e.g. Bohm mechanics) which are experimentally indistinguishable from mainstream-interpretations, and in which they do fly around like classical billard balls.
    Also, whether or not the eletronic transition upon absorption of a photon happens instantaneous is a non-trivial question which cannot be answered in the context of vanilla molecular quantum mechanics (without QED), where the eletromagnetic field is treated classically. This process is simply not within the realm of the model you are working in.

    But, anyway, neither of these two aspects has not much to do with electrons being farther away from the nucleus or not. The different orbitals clearly have different <r^n> expectation values, with valence electrons being much further away from the core. Your first explanation fits to this. This has nothing to do with the charge of electrons changing, and I cannot see how anyone would think that. Claiming that is like saying that you need to change your own body mass in order to climb up a large building in an elevator.
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