Opposite Charges Attracting - Distance of Electrons from Nucleus

Discussion in 'Physics' started by Jake1234, Jul 27, 2009.

  1. Jake1234

    Thread Starter Member

    Oct 14, 2007
    I have a question about the separation between electrons and protons in the atomic structure. It is sort of a fundamental question, but I searched online and didn't find anything and I also searched the site and wasn't able to locate an answer.

    I was wondering why electrons are so far apart from the proton and neutron inhabited nuclei when opposite charges are suppose to attract? Shouldn't electrons be bonded to the nuclei as well instead floating in dense clouds around atoms?

    If anyone could provide input, I would gladly appreciate it.
  2. KL7AJ

    Senior Member

    Nov 4, 2008
    Hi Jake:

    Good questions.

    The classical model of an atom is perhaps a bit misleading. If you take a hydrogen atom, expand the proton to the size of a grapefruit, the electron would be the head of a pin about 3000 miles away! There's a LOT more emplty space in an atom than the normal "picture" indicates.

    Just as in a solar system, however, the centrifugal force of the electron orbiting the nucleus is in equilibrium with the attractive force, barring any external forces.

    The forces bonding the protons and neutrons together are not normal electric forces...they are MUCH stronger. In the big picture, electrical charges are MINUSCULE. But they are the more tangible forces for most of what us humans experience. :)

    Hope this helps.

  3. steveb

    Senior Member

    Jul 3, 2008
    Yes, it's definitely a good question that can be addressed at few levels.

    First, consider the earth and moon. Why doesn't the moon just crash into the earth. After all, they are both attacted by gravity. So, KL7AJ talked about that. The concept of orbiting should make intuitive sense.

    Next, another problem with the classical model is that the electron is accellerating when it is in orbit and classical electromagnetic theory says that it should radiate energy and go crashing into the nucleus. Once people understood that electrons were in orbit (so to speak) the realized there was a big problem with classical theory, and they were baffled. To answer your question completely you need to come to grips with Quantum Mechanics which is very non-intuitive, but says that the electron can only exist in discrete energy levels. The lowest allowed energy states still have the electron in orbit and energy is only radiated when electrons jump from higher energy states to lower energy states. It's a little mind boggling really.
  4. Wendy


    Mar 24, 2008
    What!?! No Quantum Physics? No energy levels and matching photon absorbtion and emission?!?

    Actually I wrote the above before I read all of Steve's post. The world of the very small is very strange, almost a different universe, but the classic model is good for getting the point across. There are a lot of concepts in electronics that are the same.
  5. someonesdad

    Senior Member

    Jul 7, 2009
    Jake, it is a very good question and was one that resulted in lots of famous work about 80-100 years ago. The basic problem was that classical electromagnetism predicted that, because of the Coloumb attraction, the electron "orbiting" the nucleus should spiral into the nucleus, combining with the proton(s). The reason it would spiral in is because an accelerated charge radiates electromagnetic energy. That energy had to come from somewhere and the only real candidates were the electron's kinetic and potential energy.

    But, as you've pointed out, the problem is that atoms don't behave that way. That means that the classical electromagnetism-based model was wrong.

    One of the pivotal observations came from Neils Bohr who basically reasoned, "OK, atoms aren't behaving like our classical models would predict. So we'll build a model that uses the assumption that the energy levels of the electron are quantized and can only take on certain discrete values. The electron can jump to a higher energy state (requiring an external input of energy) or drop from a higher level to a lower level (giving off some energy)."

    This was radical thinking because it wasn't coming from a more fundamental theory. But the key was that Bohr was able to use these assumptions and make a model that did a better job of explaining experimental facts, especially spectral information on the hydrogen atom (if I recall correctly, the Bohr model of the hydrogen atom was capable of yielding the Balmer series -- somebody who took their physics classes within the last few decades will probably remember better than I do...).

    Bohr's work along with Planck's study of blackbody radiation and Einstein's study of the photoelectric effect were the beginnings of modern quantum theory.

    Two takeaways are:

    1. The assumptions may seem strange, but they've proven useful in building models that are good at making predictions.
    2. Your mechanical, Newtonian view of the world (e.g., basketballs, planets, etc.) is simply inadequate when applied to the world at the scale of atoms and below.
    You don't say if you're a student or not, but if you are, you can learn a lot by taking an undergraduate course in atomic physics. Or, just pick up almost any book on atomic physics and you'll have lots to think about. If you visit a library, there will be many useful older magazine articles; I'd suggest starting looking through some old Scientific American copies.
  6. bertus


    Apr 5, 2008
  7. Jake1234

    Thread Starter Member

    Oct 14, 2007
    Thank you everyone for such prompt replies... I really appreciated the input. This stuff is really fascinating to me.

    I took a chemistry course in college and re-read everyone's post to try and understand them and it started to jog my memory about electrons being at discrete energy levels and expending energy to travel to these levels.

    I haven't taken another chemistry class since, because I'm currently and undergraduate computer engineer and computer science major and science is not a very strict requirement unfortunately.

    I started to remember the Bohr model and the s, p, etc. shells and other things, but I'm still a little foggy about it so I'll have to re-read my chemistry or later chapters in my physics books.

    Thank you everyone for providing your knowledge, I respect it greatly.