1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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!

    Charlie.
     
    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.

    Charlie.
     
  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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook