Mass Defect Problem

  1. 1. For astatine 21885At (atomic mass = 218.008684 u) obtain each of the following:

    (a) The mass defect in atomic mass units
    wrong check mark u
    (b) the binding energy (in MeV)
    (c) the binding energy per nucleon (in MeV)

    2. Shown in atempt

    3. ok, I don't see how I'm wrong here:


    218 - 85 = 133 = # of neutrons
    85(1.6726 x 10^-27 Kg) + 133(1.6749 x 10^-27 Kg) = 3.649327 x 10^-25 Kg

    (3.649327 x 10^-25 Kg) / (1.6605 x 10^-27 Kg) = 219.772779 u

    219.772779 u -218.008684 u = 1.764095 u.

    This is not correct. What am I doing wrong?
  2. jcsd
  3. Meir Achuz

    Meir Achuz 2,076
    Science Advisor
    Homework Helper
    Gold Member

    I think you have to include the electrons.
    The numbers are easier if you keep it all in MeV instead of kg.
  4. mass defect takes place in the nucleus though
  5. You do include the electrons..

    mass defect = (z*massproton+n*massneutron)-(matom-z*masselectron)
    Last edited by a moderator: Mar 5, 2007
  6. use units..

    binding energy = massdefect*C^2

    1 unit is approx = 931,494 MeV use that its easier
  7. but remember to use the right atom weight not the one in the periodic table, cause its an average of the percentage of the isotopes in nature (or something like that)
  8. ok i'll try that but my book says nothing about including electrons
  9. ok its right thanks

    no idea why my book left that well spent
  10. you should study in Denmark.. we get the books for free.. until University (college?)
  11. ok I'm getting it equals 34.8 MeV and this isn't right
    what am I doing wrong here.

    Again, my book shows how to convert from kg in J then into eV then into MeV.

    And I had to use u because that was the required unit in the first part of the problem
  12. As do we in America, I'm in college.
  13. all right.. the first result was in units and correct right?

    take it and and make it into kg by: 1 u = 1,6605655*10^-27kg

    and then make it into joule by: x kg * (3*10^8)^2 this is just c squared

    and make the joule into eV by: 1eV = 1,602189*10^-19 J

    and of course into MeV by mega means 10^6
    Last edited by a moderator: Mar 5, 2007
  14. thats what I did....

    would you mind running it quickly and seeing if you get a number other than 34.8 MeV?
  15. the correct answer in u was 1.810725 u for the mass defect
  16. i did the following:
    1.810725 u x 1.6605 x 10^-27 Kg = 3.00671 x 10^-27 Kg

    3.00671 x 10^-27 Kg x (c^2) = 5.5715 x 10^-12 J

    5.5715 x 10^-12 J / 1.6 x 10^-19 J = 34821826.4 eV = 34.82 MeV
  17. If i do like this i get:

    1.810725 u * 931,494 MeV/u = 1686,68 MeV

    don't know if thats the correct answer :S
  18. i also get this

    1.810725 u x 1.6605 x 10^-27 Kg = 3.00671 x 10^-27 Kg

    but then i get 2,70614*10^-10 J

    and then of course: 1,68903 *10^9 eV = 1689,03

    almost the same as the other result, but used more decimals with this one.,. gotta be right
  19. I'll write it with all the decimals:

    1,810725*1,6695655*10^-27 = 3,00683*10^-27 kg

    3,0068274649875*10^-27*(3*10^8)^2 = 2,70614*10^-10 J

    (2,7061447184888*10^-10)/(1,602189*10^-19) = 1,68903*10^9 eV = 1689,03 Mev
  20. ok its correct, I'm going back through it all to see what we did differently.
  21. hmm, rerunning it i get the 2.7 x 10^-7 Joules too, not sure how i got the other number the first time.... maybe I wrote it down wrong
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