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Not Homework, But can somone explain

  1. Nov 28, 2006 #1
    This is a support question in my package. It says 2 different iron ions,
    Fe2+ and Fe3+ are accelerate by the same potential and then sent through the uniform magnetic field of a mass spectrometer

    which ion moves faster? and why?

    Equation v= Sqrt(2qV/m) where q is magnitude of charge of the ion and V is the potential difference aross the plate

    i would think that Fe 3+ would move faster because it has a larger magnitude of charge than Fe 2+

    but i could be totally wrong, can someone explain to me?
  2. jcsd
  3. Nov 28, 2006 #2


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    Staff: Mentor

    Is there any difference in mass?
  4. Nov 28, 2006 #3
    im not sure, i thought that they weighed the same ?? is one heavier than the other? i thought it would depend more so on the magnitude of charge than the weight.. but please help me understand this
  5. Nov 28, 2006 #4


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    Let's make sure.

    What's the difference between Fe2+ and Fe3+ ? Do you know what the 2+ or 3+ represents (alternatively, how do you make a positive ion starting from a neutral atom)?
  6. Nov 28, 2006 #5
    yes. valence it means that one has 2 valence electrons and the other has 3 valence electrons, considering this would that mean that Fe3+ has one extra proton making it weigh more?
  7. Nov 28, 2006 #6


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    Close, but you're making a small mistake. When you make an ion out of a neutral atom, you do not change anything in the nucleus (protons, neutrons). All you do is add or remove electrons.

    Starting from a neutral Fe-atom, removing two electrons gives you Fe2+. Pluck out another electron, and you have Fe3+.

    So the only difference in mass between those two ions, is the mass of a single electron! And, as you know, the electron's mass is tiny compared to the mass of the atom, so the dominant effect, as you correctly guessed earlier, is the different charges on the two ions.

    So your first guess was, in fact, correct! But it's important that the method applied also be correct.
    Last edited: Nov 28, 2006
  8. Nov 28, 2006 #7
    thank you very much.. all the help was appreciated
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