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Question on why metal ions are remain stationary in the metal body

  1. Feb 7, 2013 #1
    When a metal gets polarized it looks like this: http://postimage.org/image/bkdw8ttgn/

    However, only the electron moves towards the positive ion. Why is this so? As both experience an equal force despite the different accelerations, shouldn't both the positive ion and negative electron come together?
     
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  3. Feb 8, 2013 #2

    Simon Bridge

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    Because the electron is so much less massive than the rest of the atom - and the atoms are all bound to each other in a lattice. It is like, when you jump, technically the Earth also moves - but you don't take that into account in calculations do you?
     
  4. Feb 8, 2013 #3
    Oh so technically the ion also moves, but it's because the electron's mass is so small that it movement is practically nothing? But actually is there another force holding the ions there?

    Because when i was thinking about this, I thought of a positive metal rod http://postimage.org/image/k5xmanti1/ so if i follow my first statement where the ion also moves, won't the positive charges get repelled away?

    Thanks so much for the reply
     
  5. Feb 8, 2013 #4

    DrDu

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    It is not easy to explain the reason in few sentences.
    With some abstraction, you can regard a metal as composed of ionic cores and electrons interacting only through coulombic forces.
    The density of electrons and ions is equal (assuming singly charged ions).
    What is not equal is the ratio of kinetic energy to coulombic energy. While the mean Coulombic energy is quite equal for electrons and ion cores, the kinetic energies are vastly larger for the electrons than for the ions due to the different mass.
    Now it is known that when the kinetic energy becomes small, a gas of charged particles will condense into a lattice (the Wigner lattice). This happens for the ionic cores but not for the electrons at metallic densities.
    This is a phase transition similar to the gas-solid transition.
    The ionic cores cannot move freely any more but only as a whole.
     
  6. Feb 8, 2013 #5
    Oh, so the metal ions experience attraction among themselves that keep them from escaping? But what if I had a negatively charged metal object, in this case won't they be able to escape the whole metal object?

    So when I have a positively charged metal, the force holding the repulsion is that attractive force among themselves so actually those ions would have no net force?

    But actually, when i attract a positive charge like this: http://postimage.org/image/9xoxxfnpb/ [Broken] so when we apply Coulomb's Law here, we would find the net force of the entire object so if we calculate the acceleration it would be (A1+A2-Lattice pull)/total mass of object.

    So to match the same acceleration, would the ion experience still a net force forward, however, it is such that A1-lattice pull/mass of ion. However, this doesn't seem mathematically sound? Also, this also doesn't really make sense to me because when i have a polarized metal, and i remove the charged object, the electrons will just flow back to the positive ions. However, in this case http://postimage.org/image/9xoxxfnpb/ [Broken] the positive ion experiences a net force. So I'm pretty confused about this as well.

    Thanks for the help :)
     
    Last edited by a moderator: May 6, 2017
  7. Feb 8, 2013 #6

    Drakkith

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    Yes, the nuclei, the ions, are bound to each other through metallic bonds.
    http://en.wikipedia.org/wiki/Metallic_bond

    Not really. To do so you would have to apply so much force that you effectively rip the metal apart and ionize it. And such extreme negative charge would also rip apart your other object.

    When you positively charge an object you remove a tiny tiny fraction of its electrons from it. There are still plenty there to keep the ions bound to each other.

    I don't think you count the "lattice pull" since you are moving the entire object, not just part of it.
     
  8. Feb 9, 2013 #7
    But even if the number of electrons that is removed when a metal is positively charged is very small compared to the total number of positive ions/electrons present, won't the overall amount of repulsion experienced within the metal still be greater than the attraction (since attraction is now between cations and fewer electrons)? Then if whats holding the metal together is only the attractive forces, shouldn't the metal not be able to "stay" together?

    Thanks for the help :smile:
     
  9. Feb 9, 2013 #8

    Simon Bridge

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    After reading that link about metallic bonding, have a go working out how many electrons would have to be removed before the bonding breaks down.

    You seem to have this mental picture of a metallic bond being similar to an ionic bond.
     
  10. Mar 7, 2013 #9
    Hi sorry for the super late reply..

    Actually i was thinking of it like this: I have 5 positive ions in a sheet of metal. So overall there is 5 more positive charges than negative charges. So around them due to the electrons and positive nucleus are being covered by each other (effectively causing everything around them to be neutral), the only Coulomb's Force experienced by them is the repulsion by the other positive charges so shouldn't they get ripped out?

    Thanks so much for the help guys :)
     
  11. Mar 7, 2013 #10

    Drakkith

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    Even when charged positively, a piece of metal still has enough electrons shared between all atoms to maintain their bonds. The key here is that the atoms BOND to each other by sharing electrons. When they bond they drop to a lower energy state, which means that we need to add energy somehow to break them out of their bonds. Removing a few electrons from your metal only adds a little bit of energy to each atom, far less than what is required to break the bonds.

    It just isn't as simple as saying, "these 5 ions are positively charged and should repel each other". Even if you charge the metal, the attractive force between each atom is far greater than the overall repulsion. If we were to keep removing electrons, and thus charging the metal more and more positive, I would expect it to reach a point eventually where the repulsion is right on the verge of equaling the attractive force, and after that the metal would fly apart. However, this probably ignores all kinds of quantum effects that matter, so I can't say for certain what would happen.
     
  12. Mar 7, 2013 #11
    Oh could you elaborate on the attraction part? Because I'm thinking if i have an electron to attract it there, there would also be a positive charge so as a whole they don't have any effect on the positive charges.

    Thanks so much for understanding :) I just can't seem to get this
     
  13. Mar 7, 2013 #12

    Drakkith

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    Remember that atoms have all of their positive charge in the nucleus. The electrons occupy the "electron cloud" or "orbitals" or whatever you want to call them. Without delving into Quantum Mechanics let's just say that since the electrons are not static, you can have electrons shared between atoms in bonds, resulting in an attraction between them, even though both atoms have a neutral charge overall. In a metallic bond, not only do you have electrons shared between neighboring atoms, they are actually shared throughout the entire material!

    A more accurate description would involve talk of electron orbital energy levels, quantum spin, and a few other factors that are fairly in depth.

    I hope that helps a bit. I'm afraid I cannot figure out how to explain this in an "intuitive" way.
     
  14. Mar 7, 2013 #13

    Simon Bridge

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    The covalent bond is a different mechanism to what you are thinking about. The details involve quantum mechanics. I don't know how much detail to use because I don't know your level of understanding. Did you read the links about metallic bonding? Also read about "band structure of solids" and "covalent bond". See if you can rephrase your question in terms of what you learn: you should discover your question answered.

    Bottom line: the mental picture you have been using is too simple to cope with the subject. Any explanation you get here will not be in terms of ions attracting and repelling.
     
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