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Homework Help: Specific charge. Don't understand how to convert to Coulombs

  1. Apr 3, 2009 #1
    1. The problem statement, all variables and given/known data
    An ion of a magnesium isotope has A=24 and Z=12. Find the specific charge

    2. Relevant equations

    3. The attempt at a solution

    I know that the charge of a proton in Coulombs is [tex]1.60\times10^{-19}[/tex]. From the information given, I know that I have 12 protons and [tex] 24-12=12 [/tex] neutrons. Neutrons have no charge so I can ignore them. Therefore my charge is [tex] 12 \times 1.60\times10^{-19} = 1.92\times10^{-18} [/tex]

    This is very wrong. My answer should be [tex] +3.2\times10^{-19} [/tex]

    I know that the mass of a proton is [tex]1.67\times10^{-27} [/tex] which is equal to a neutron. From the information given I have 24 nucleons and so a mass of [tex] 24\times1.67\times10^{-27} = 4.008\times10^{-26}[/tex].

    This is very wrong. My answer should [tex]3.98\times10^{-26}[/tex]

    I realise that electrons have charge and mass too, and in a stable atom, equal the number of protons. Persumably, if this is an ion, it's lost/gained some electrons and so is not stable. So I don't know how to tell how many electrons there are in this atom.

    Thank you very much for reading. I hope someone can sort out this confusion!

    Last edited: Apr 3, 2009
  2. jcsd
  3. Apr 3, 2009 #2
    This list of common ions should help you to find the number of electrons in Magnesium ion.
    http://www.rose.edu/faculty/Aslagle/ion%20list.pdf" [Broken]
    Last edited by a moderator: May 4, 2017
  4. Apr 3, 2009 #3
    I'm sorry but that hasn't helped me. I don't understand what means what in that document. This is a very basic A level physics question. Ions haven't been introduced. I only knew what I mentioned in the previous post from GCSE studies.
    Last edited by a moderator: May 4, 2017
  5. Apr 3, 2009 #4
    An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge.
    http://en.wikipedia.org/wiki/Ion" [Broken]

    Magnesium ion = Mg2+, which means that it has lost two electrons or has +2 (positive) charge.
    Last edited by a moderator: May 4, 2017
  6. Apr 3, 2009 #5
    Would I be right in thinking that without being given how many electrons have been gained/lost, this question can not be answered without a further understanding of ions? If so I might have wasted some time here as the question was an example in my book and the charge and mass were given. I just didn't understand where the numbers came from.

    If this is the case I'm terribly sorry for wasting your time.

  7. Apr 3, 2009 #6
    Oh I see, so with that knowledge you arrive at the answer by 2 x 1.60-19[\sup] = 3.2 x10-19[\sup].

    Is this how it's done?
    Last edited by a moderator: May 4, 2017
  8. Apr 3, 2009 #7
    No matter :smile:

    Yes, this is right.
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