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The electric field of an atom is puzzling to me

  1. Jan 1, 2014 #1

    bobie

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    I hope someone can make me understand this:

    an electron radiates in all directions, when it is circling around a proton its electric field should radiate both inside and outside the orbit , but it doesn't : an atom of helium He2 is neutral and the protons neutralize also the outgoing field of the 2 electrons.
    That is a bit odd but I can understand so far.
    But, if we add an electron and get lithium: the inner shell He2 is not neutral any longer and the third electron gets electrostatic attraction.

    How does it work?

    Thanks
     
  2. jcsd
  3. Jan 1, 2014 #2

    SteamKing

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    Wait, what?
    No, you do not get lithium by adding an electron to helium. The chemical properties of an atom are determined by its atomic number, the number of protons in the nucleus.

    The number of electrons is equal to the number of protons in a neutrally charged atom. Atoms which have more or fewer electrons than protons are called ions, and the atom has a net positive or negative charge depending on whether the number of electrons is less than or greater than the number of protons in the nucleus.
     
  4. Jan 1, 2014 #3

    bobie

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    Yes, that's right, so the third proton's field overcomes the 2-electron barrier?
    And, why in the helium atom the positive and negative field cancel out?
    And why the second electron of helium gets a lesser attraction ?
     
  5. Jan 1, 2014 #4

    Simon Bridge

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    No it doesn't - only accelerating charges radiate. The electric field of an electron extends in all directions.

    The word "orbit" used to describe how electrons are bound to nuclei should not be taken literally - the electron is not thought of as moving in a circle about the nucleus.

    When the electron is bound to an atom, it is in a "stationary state" i.e. a non-radiating one.
    The electron charge behaves, under most situations, as if it is distributed about the nucleus like a cloud.

    The electric field of the electron goes in all directions even when it is bound to an atom.
    However, the electric field of the proton also goes in all directions.
    The total electric field of one proton and one electron is very close to zero - viewed from a long way away.

    If you have a point charge of +q surrounded by a solid sphere of total charge -q, the electric field outside the sphere is going to be zero. You can work it out for yourself.
    However - this is an oversimplification - the electron charge is not always distributed evenly.

    Adding an electron to Helium makes a helium ion - not lithium.
    To get another element, you have to add protons.

    Thus neutral lithium has three protons and three electrons.

    The electrons only shield the nucleus when seen from afar. Which is most situations.
    Some atoms can have more electrons than they have protons. This is not "most situations".
    Because the proton electric field goes everywhere, it is not always screened from nearby charges.
    The presence of the electrons does not use up the positive charge from the nucleus but they do make the "outer" electron more weakly attracted than the inner ones.

    From a long way away, the already weak attraction is so weak that quite small energies can overcome the attraction.

    Careful though: this is very hand-wavey.
    The details come from quantum mechanics.
     
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