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Force between bonded atoms

  1. Feb 21, 2016 #1
    In a covalent bonded molecule, what is the force responsible for the molecule staying together. To my knowledge it's to do with electrons of opposite spins and being at a lower energy with valence shells filled but what is the force itself that pulls a second atom along if the atom it's bonded to is pulled?
     
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  3. Feb 21, 2016 #2

    mfb

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    The electromagnetic interaction. Pull one nucleus, and you stretch the electron orbital a bit, the electron gets a slightly higher wavefunction amplitude between the two atoms and follows the pulled nucleus partially, that also lets the second nucleus follow.
    That is a very classical description, but it works surprisingly well.
     
  4. Feb 21, 2016 #3

    DrClaude

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    H2+ is a counterexample to that. The bond comes in the first place because the electron(s) forming the bond get to interact with the other nucleus, and the orbital spreads over both nuclei, lower the electron's energy. The presence of electron density in between the nuclei also helps reduce their Coulomb repulsion.
     
  5. Feb 21, 2016 #4

    mfb

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    I know that this is a common description, but I really don't like it. The force between the protons is not altered at all. You just get an attractive force between protons and electron(s) in addition.
     
  6. Feb 21, 2016 #5
    So now I think about it, and please correct me or point out if it's a silly comment but would this be a result of the magnetic part of the electromagnetic force?
     
  7. Feb 21, 2016 #6

    mfb

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    I don't think it is useful to split it into components here, but it is mainly the electric part. Nothing moves very fast here.
     
  8. Feb 21, 2016 #7
    In that case, I'm still not quite understanding how the bonded electrons enable each other to follow one another due to the electromagnetic force.
     
  9. Feb 21, 2016 #8

    mfb

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    Which part of post 2 is unclear?
     
  10. Feb 21, 2016 #9
    The idea that a bond between two negative charges consists of electromagnetic interaction.
     
  11. Feb 21, 2016 #10

    DrClaude

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    The bond is not between two electrons. The bond is between the atoms, due to the attraction of electrons and nuclei.
     
  12. Feb 22, 2016 #11

    DrDu

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    Covalent bonding is a quantum mechanical effect. In a bond, the electrons have more space to move than in the field of a single atom. By the uncertainty principle, this reduces their momentum and thence also their kinetic energy.
     
  13. Feb 22, 2016 #12
    So how does the electromagnetic force come into this explanation sorry, if that's the force responsible?
     
  14. Feb 22, 2016 #13

    mfb

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    The electromagnetic force is the interaction that holds everything together. It lets a nucleus attract electrons (and electrons attract nuclei).
     
  15. Feb 22, 2016 #14
    So would it be valid, to an extent to think of it in this sequence? Electron orbitals overlap forming larger area of electron density. Both nuclei are simultaneously attracted to this area holding the molecule together.
     
  16. Feb 22, 2016 #15

    Borek

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    As a first approximation it sounds OK to me. Note, that it doesn't require any magnetic part, just electrostatic attraction.
     
  17. Feb 22, 2016 #16
    I realise that now sorry, the guess at magnetic involvement was more a wild guess because the word spin was mentioned.
     
  18. Feb 23, 2016 #17

    DrDu

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    Of course it is. The attraction by the nuclei forms the basin in which the electrons are moving.
     
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