Homepage Forums Education Why doesn't the electron crash into the proton

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  • #1066
    AvatarRezwan
    Member

    Hi Folks,
    Do you have a nice, coherent answer to this question that we can add to our Learning Center? I’ve started the post already. Thought we’d hash out the answer here.

    Q:

    Why doesn’t the Electron crash into the proton?
    We know in an H atom the e- is attracted to the + charge of the proton.
    And it wants to get down to the “0 level” orbit.
    But what makes that level zero – or why does e- stop going down?
    Is there another force that counteracts the force of charge trying to pull them together?

    #9502
    BreakableBreakable
    Keymaster

    As there are 4 fundamental foces
    http://en.wikipedia.org/wiki/Fundamental_interaction

    I think the weak force is responsible for preventing the merging of electrons and protons, but I am not fully sure.

    Edit:On the other hand here is a long description with no clear answer:
    http://www.physicsforums.com/showthread.php?t=104715

    #9503
    Avatartheanphibian
    Member

    The reaction is not energetically favorable. Free neutrons decay into a proton and an electron with a half life of about 15 minutes. So it could hit, but what would it do? There is not enough energy in the system to turn it into a neutron.

    This does occur with electron capture, but only in nuclei that are proton-rich. However, the mechanics for why the electron isn’t always interacting directly with the nucleus is quantum and more puzzling.

    Why doesn’t the mater around us fall apart? If all we had were the 4 forces, matter would fall apart very quickly, but we have materials that hold together. That is due to the fact that the wave-nature of the electron prevents it from existing within the nucleus. The electron is very light (and probably the most fundamental type of particle in the universe) and because of this lightness, the wave nature of it exerts a very significant “outward pressure”, if you will.

    Regarding hydrogen, it’s not hard to believe looking at a S orbital. The P orbitals are comparatively mind-blowing. P orbitals of electrons plow straight through the nucleus. Do not deceive yourself – this is physical. At any given moment, the electron has a definable probability of being within the nucleus, but the size of the nucleus as well as a few other factors make this somewhat improbable. It only matters when a reaction within the nucleus is energetically favorable, like in electron capture. In that case, the probability of the electron being in the nucleus is what drives the rate at which the reaction occurs.

    For Hydrogen, the reaction is not permitted and never occurs, although the electron certainly spends some time within the structure of the proton itself. But it also spends some time on the other side of the universe too.

    #9556
    AvatarArvid
    Participant

    I’ve had people ask me this question, too. It’s amazing that the public is still fixated on the Bohr atom, which lasted 12 years over 85 years ago, but there it is. That’s the answer everybody has in their mind—the electron is whirling around really fast like a miniature Solar System, so it doesn’t crash into the nucleus.

    I don’t think it’s possible to avoid quantum theory altogether. Just remind people that energy—light, in particular—comes in discrete packets, proportional to the frequency. Then just tell them that particles, too, have a wavelength associated with them inversely proportional to their momentum. It may be hard to understand or visualize, but it’s an observed fact.

    Then the question of “Why doesn’t the electron crash into the proton?” answers itself—it did! You don’t need to talk about anything more complicated than a ground-state hydrogen atom, because after that it gets messy in a hurry, but in the hydrogen atom the electron is sitting exactly on top of the proton—it just can’t be localized nearly as closely because of its much longer wavelength.

    Then if you think that worked, you can remind them that that “cloud” that represents the electron is really just a graph of the probability of finding it if you look for it, and it could be inside the nucleus at any given moment—it’s just impossible to say. If they haven’t run away yet, tell them that if you gave the electron enough energy that you could say for sure it was in the nucleus, it would be far, far away very soon, because it would be moving too fast for the nucleus to hang onto it.

    I find if you take it step by step like that, most people seem to grasp the underlying point. Whether it sticks long-term or not, I don’t know. That pesky Bohr atom is still with us—but that’s a minor annoyance compared with the mystical mumbo-jumbo called the “Copenhagen Interpretation”!

    #9558
    AvatarRezwan
    Member

    Thanks Arvid! Now we’re getting somewhere.

    I don’t know about the public being fixated on the Bohr model. I think this is more the problem that rises with the concepts of “positive”, “negative” and “particles”. I suppose a question behind this is: why isn’t everything a neutron? No one is questioning the observed factness of these things, or the precise measurements and descriptive equations. It’s more of the “why” of things, and a need for a better way to visualize the relationships at work with these things we call protons, electrons, neutrons. The “wave” and “cloud” concepts help a lot.

    The “positive” and “negative” concepts make it difficult to understand. We’re told that protons, if they were to obey their electrical charge urges, would fly apart, but a “strong force” can bind them together. It’s natural to think then that if electrons and protons were to obey the electrical urges, they would bind into a neutron, so some force must be keeping them apart. Of course there’s an equation somewhere that shows how much energy is required for a neutron to become an electron and proton and vice versa. But now your cloud concept shows that the electron is right on top of the proton – so that makes it neutron-esque at certain moments.

    So the meta question is, “what is charge”?

    #9563
    Avatarvansig
    Member

    Rezwan wrote:
    So the meta question is, “what is charge”?

    i have yet to see a better answer than
    “charge is one component of ‘spooky’ action at a distance”

    #9571
    BreakableBreakable
    Keymaster

    theanphibian wrote: The reaction is not energetically favorable. Free neutrons decay into a proton and an electron with a half life of about 15 minutes. So it could hit, but what would it do? There is not enough energy in the system to turn it into a neutron.

    Ok, this does it for me. Even with a Bohr model

    #9572
    AvatarArvid
    Participant

    Well, that’s most of what I meant by decrying the Bohr atom, was this picture of the proton in particular, as a hard, positively-charged billiard ball, that must be made of positively-charged parts that would fly apart if given a chance.

    Here’s a picture that helps me visualize the whole thing—I can’t pretend it’s justifiable from a quantum-mechanical standpoint. Think of trying to stuff some cotton batting into your couch cushion. It’s really good stuff—so fluffy that it forms a cloud 40 or 50 miles across on this scale—but I’ve got big hands, so I just keep squeezing and squeezing and squeezing until I’ve got it compressed so small that it’ll fit inside the cushion, so I jam it in and zip it up. Problem is, just like compressing a spring, I’ve stored a tremendous amount of energy in it by squeezing it so tight, and it’ll just burst right back out.

    So why doesn’t it just combine with the proton to form a neutron? I’ve undoubtedly given it enough energy for it to do so. I think you have to look one layer deeper and realize that the proton is composed of three quarks, staying fairly close together in the center of what we call the “radius” of the proton. Just like the atom, most of the proton is empty space too. The quarks are kept from wandering apart by the color force, but since it has three different kinds of charge, it’s even harder to explain than electric charge, so just take it as read.

    It helps me to think of the hydrogen atom as formed of 4 quarks—3 colored quarks that form the proton, and one “colorless” quark that’s only held to them by the electric force, so it’s not bound nearly as tightly. If I force it into proximity with the other quarks, some “magic” has to happen to change the consistency of my cotton batting so it’ll stay inside my couch cushion. We call this magic the weak force. It can change the identity of the electron into a down quark and therefore the proton into a neutron. If you sit around waiting for magic to happen, though, it might take a while—most times when you force an electron into a proton it won’t.

    Well, now we’ve mentioned the color force and the weak force, so it appears that we’ve made the problem worse. Maybe it’s elephants all the way down! It may help people to realize that there’s more than one kind of charge, though. Gravitational charge is called “mass” and there’s only one kind. Color charge has three kinds. It’s just a fact of life at the quantum level that if we add enough energy to do something like turn a proton and electron into a neutron, we may just create new particles out of the vacuum that can do unexpected things. The world is a much more interesting place than Lord Kelvin thought it was!

    #9576
    Avatarjamesr
    Member

    For anyone wanting to know any more I heartily recommend Feynman’s “QED – The Strange Theory of Light and Matter”

    The full standard model of the all the quarks, leptons and Bosons still has a few flaws (maybe the Higgs will help – I doubt it though), but at the normal energies of electrons interacting in chemistry and dense plasma physics, Quantum Electrodynamics is all you could ever need.

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