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Is it possible to deduce the number of protons from the mass

  1. Apr 27, 2015 #1
    ...number?

    If you are give say Lead, 208Pb. What is the 208? and is it possible to deduce the number of protons Z from it with no other information?

    Haven't done chemistry yearts so any help would be appreciated.
     
  2. jcsd
  3. Apr 27, 2015 #2

    Borek

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    Nope. Number of protons is defined by the Pb symbol. 208 is sum of number of protons and neutrons (number of nucleons), and doesn't tell anything about how many of each there is.

    It is also not uncommon to have several isotopes of different elements all having the same total number of nucleons, these are called isobars.
     
  4. Apr 27, 2015 #3
    Many thanks for that, the lecturer seemed to assume we knew, no other info was given in class. So youre reply is reassuring thanks!

    One more thing if you have a free minute, for Pb 206, he said that there are no energetically favourable decay modes, can this be deduced from the 206 value? or would more information be required? or it is common knowledge and something to memorise?
     
  5. Apr 27, 2015 #4

    Borek

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    It is not a chemistry question - I will move the thread to nuclear physics. 206 is definitely not enough, but perhaps there are nucleus models that allow predicting decay modes from known Z and A.
     
  6. Apr 27, 2015 #5
    Ok. Many thanks for your replies!
     
  7. Apr 27, 2015 #6

    DaveC426913

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    Am I misreading something here?

    If you have a mass composed of Pb208, as the OP specified, then all the atoms contain 82 protons and 126 neutrons.

    If they didn't, they would either not be the 208 isotope, or they would not be lead at all..
     
  8. Apr 27, 2015 #7
    Did you deduce the bolded statement from the Pb208? Is there any chance that you could explain how?
     
  9. Apr 27, 2015 #8

    DaveC426913

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    Yes.

    (My 'shut up and keep your head down' alarms are going off in my head, so it is against my better judgement that I'm challenging Borek, but) it seems to me that
    - you specified the mass contains entirely Pb208
    - lead contains 82 protons, so the rest (126) are neutrons.

    Here it is, listed in the isotopes of lead chart:
    http://en.wikipedia.org/wiki/Isotopes_of_lead

    208Pb 82 126 207.9766521
     
  10. Apr 27, 2015 #9

    Borek

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    Definitely there is some misunderstanding, could be my English failed.

    As far as I understand the original question asked "is it possible to deduce number of protons from a single 208 number" - and then the answer is "no". At the same time when you are given entire "208Pb" information number of protons is given by the identity of the element, given by its symbol (and I believe that's what I wrote).
     
  11. Apr 27, 2015 #10

    DaveC426913

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    That's what I thought you were saying, yes. Given only 208, you do not know which element it is, and thus the ratio of protons/neutrons.

    But the OP did specify Pb208. Which is enough to deduce the ratio.
     
  12. Apr 27, 2015 #11

    mfb

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    You have to look up the masses of possible decay products to see if the decays are possible. It is possible to estimate them theoretically but to do that precisely is way beyond the scope of homework problems. Alternatively, look up directly if the nucleus is stable.
     
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